Sharks of Onslow County

Tag: salt marsh ecology

Blue Land Crabs Near Onslow County: What to Watch For and What Their Presence Could Change

The Hole at the Grass Line

At the soundside edge of Topsail Island, a hole in the ground is not unusual. Marsh grass gives way to higher soil, fiddler crabs disappear when footsteps approach, and small brown crabs move beneath boards, leaves, and anything else that holds a little shade. A burrow at the edge of a coastal yard may belong to an animal that has lived along this shore far longer than the yard has.

That is why a hole alone would be poor evidence of a blue land crab.

The animal itself would be harder to overlook. A mature blue land crab, Cardisoma guanhumi, can grow to nearly six inches across its shell, with legs that make it appear larger still. An adult male is often blue-gray and carries one powerful claw, but females may be pale gray or white, while younger crabs can be orange, brown, or purple. From a distance, the uneven claws can resemble those of a fiddler crab, except the entire animal has been enlarged to a scale unfamiliar in a Carolina backyard (Hostetler et al., 2025).

Until recently, it was also unfamiliar along this part of the coast. The blue land crab’s historically documented range in the continental United States followed the Gulf of Mexico and southern Florida, with Vero Beach long recognized as its northern Atlantic limit (Scott et al., 2023). Occasional South Carolina records began in 2008, followed by 53 verified sightings there during a concentrated period of public reporting in fall 2022 (Scott et al., 2023). North Carolina confirmed its first blue land crab in summer 2023. By August 2026, 23 sightings had been confirmed statewide and another 34 reports remained unverified. Two of the confirmed records this year came from Emerald Isle (NC DEQ, 2026).

Those numbers do not yet describe a population. They describe a question appearing in yards, along roads, and beside the marsh: whether a warm-water crab is occasionally reaching North Carolina or beginning to live here.

A Crab That Does Not Stay in One World

The blue land crab is neither a displaced Atlantic blue crab nor simply a giant fiddler. The shared color and oversized claw describe what it looks like, not its closest relatives. It belongs to Gecarcinidae, the land-crab family, whose members have moved much of adult life above the tide. Their gills still do the breathing, even in air, and those gills must remain damp. The crab can leave the water; it cannot leave water behind (Marin & Tiunov, 2023).

That need for moisture keeps the crab between two worlds. Adults may live above the reach of ordinary tides—in coastal forests, brushy ground, grasslands, drainage edges, and even yards—but they return to damp shelter when the day becomes hot or when a newly molted shell leaves them vulnerable. Many blue land crabs can live near one another without sharing that refuge; each normally keeps a burrow of its own (Herreid & Gifford, 1963; Moraes-Costa & Schwamborn, 2018).

That higher position separates the blue land crab from many crabs living in the flooded marsh below it, but not completely. Its life still returns to the estuary. After mating, a female carries hundreds of thousands of eggs beneath her abdomen and travels to salt or brackish water to release them (Hostetler et al., 2025). The larvae drift through several planktonic stages before settling and returning to land as small crabs. Warm water is particularly important during this development, while salinity determines which parts of an estuary can carry the larvae successfully (Costlow & Bookhout, 1968a, 1968b).

An adult can therefore live beneath shrubs beside a yard while its offspring begin among fishes and plankton. The species belongs to the coastal upland and the estuary at the same time, and that divided life helps explain how an animal tied to one burrow can appear far beyond the range where its adults were previously known.

How a Southern Crab Reached This Shore

Blue land crabs are not likely to have walked from Florida to the Carolinas. Adults forage close to their burrows and show strong attachment to a small home area. Females make longer reproductive movements toward water, but those are local journeys between upland burrows and larval-release sites rather than migrations along hundreds of miles of coastline (Moraes-Costa & Schwamborn, 2018).

Long-distance movement is more plausible during the larval stage. Once released, larvae spend weeks in coastal water before returning to land, allowing currents to carry them beyond the place where they hatched (Hostetler et al., 2023). A widening pattern of sightings from Florida through Georgia and South Carolina would be consistent with natural range expansion as warming water and milder cold periods make northern development and survival more possible (Costlow & Bookhout, 1968b; NC DEQ, 2026; Scott et al., 2023). Human transport, including movement with vessels or materials, remains another possibility because the present records do not yet reveal how the crabs arrived (NC DEQ, 2026).

Warmer conditions in Florida would not simply make the crabs decide to leave. The more relevant change would occur at the northern boundary, where conditions that once killed larvae or prevented overwintering may no longer do so as reliably (Costlow & Bookhout, 1968b). Warming can open habitat ahead of a species without pushing adults out of the habitat behind it.

It is tempting to imagine worsening conditions in Florida sending blue land crabs north, but that does not fit particularly well with how the adults live. Coastal development, altered groundwater, harvesting, and the loss of mangrove-edge habitat have reduced populations in parts of Puerto Rico and Brazil. When the ground around an adult’s burrow is destroyed, however, the crab may move only a short distance or disappear with the habitat. It does not begin a walk to North Carolina (Govender et al., 2008; Moraes-Costa & Schwamborn, 2018).

Development may matter after the much smaller, drifting stage has already reached a new shore. A canal edge, drainage ditch, watered lawn, or patch of coastal vegetation can offer the damp ground a young crab needs as it leaves the estuary. Human-altered places may therefore help connect suitable patches without being the reason the crab traveled north in the first place (Riascos et al., 2024).

So is the blue land crab native to North Carolina, or is it invasive? Neither label fits neatly yet. The species is native to warmer parts of the western Atlantic and Gulf of Mexico, including Florida, but North Carolina lies beyond its historically recognized Atlantic range in the United States. If larvae are reaching the Carolinas on coastal currents and surviving because conditions have become more favorable, then the crab’s natural range may be expanding. If crabs arrived with boats or transported materials, that would be an introduction. The sightings cannot yet tell us which occurred (Scott et al., 2023; NC DEQ, 2026).

Either way, arrival alone does not make a species invasive. The blue land crab would have to establish a reproducing population, continue spreading, and cause ecological or economic harm. North Carolina has confirmed crabs, but it does not yet have evidence of an established, harmful population (NC DEQ, 2026).

Emerald Isle Is Not Far Away

There were two confirmed blue land crab sightings in Emerald Isle this year, out of 23 confirmed cases across North Carolina (NC DEQ, 2026). This is what makes the Emerald Isle records matter here. The confirmed crabs were not found along some distant part of the North Carolina coast. They were found on Bogue Banks, northeast of Onslow County and only several dozen coastal miles from North Topsail Beach.

Between Emerald Isle and Topsail are connected sounds, tidal creeks, inlets, marshes, developed islands, and the Intracoastal Waterway. A crab confirmed on Bogue Banks does not prove that one is already living around North Topsail, Surf City, or the soundside of Topsail Island. It does place the animal within a coastal landscape that continues toward all three.

That does not mean currents are carrying crabs neatly from Emerald Isle toward Topsail, and the statewide sightings should not be read as a marching line. Some animals may represent separate arrivals; others may have gone unnoticed for years. The map brings the question closer. Only repeated observations can show where the crab has actually found a place—and which members of the existing crab community are already there.

The Crabs Already Here

Onslow County’s marsh edge is not waiting empty for a land crab to arrive. Several native crabs divide the same transition from tidal creek to high ground, although each uses a somewhat different part of it.

Comparison of the blue land crab to other crabs seen in our Onslow County area. | Image credit: SC Department of Natural Resources

At low tide, the easiest to notice are often fiddlers. Atlantic marsh fiddler crabs, Minuca pugnax, favor muddier marsh sediment, while Atlantic sand fiddlers, Leptuca pugilator, are more common across sandy flats and the higher portions of the marsh. Red-jointed fiddler crabs, Minuca minax, appear where the water is fresher. Marsh and sand fiddlers have long been documented in North Carolina salt marshes, gathering into the familiar patches that vanish into small holes when a person approaches (O’Connor, 1993).

They sift algae, microbes, and bits of organic material from the surface, leaving tiny feeding marks across exposed sediment. Even a male carrying an oversized claw has a body only a fraction of the width of an adult blue land crab.

Closer to the cordgrass, purple marsh crabs, Sesarma reticulatum, move along muddy creek banks and feed heavily on the grass, including stems and tissues belowground. At ordinary numbers they are part of the marsh’s long-established community. Where their numbers become unusually high, their feeding can strip away vegetation and leave sediment exposed (Bertness et al., 2014; Wittyngham et al., 2024).

At the upper edge, the small brown crab beneath a board or patch of leaves may be a squareback marsh crab, Armases cinereum. Of the native crabs, it is the most likely to meet a blue land crab where marsh gives way to the yard. It moves above the high-water line, sometimes crossing lawns or climbing garage walls, and eats fallen leaves, living plants, and small invertebrates. Its menu is broad in much the same way as a blue land crab’s, although the squareback itself is far smaller (Buck et al., 2003).

Other familiar crabs belong to neighboring settings. Atlantic mud crabs such as Panopeus herbstii remain among oyster shells, rocks, and submerged structure, where their heavy claws open oysters, clams, barnacles, and snails. Ghost crabs, Ocypode quadrata, occupy the open beach and dune sand on the ocean side. Either may be close in geographic distance to a blue land crab, but neither uses the soundside grass-to-yard boundary in quite the same way.

Each of these crabs already belongs to a particular part of the shore. A blue land crab would not arrive in an empty niche, but it would not duplicate any one of them either. To see where the differences matter, we have to look below the opening.

The Architecture Beneath the Opening

For a burrowing crab, the tunnel is more than a hiding place. It is a shelter from heat and predators, a humid chamber that protects the gills, and a safe place to molt. Its shape reflects the problem the crab needs it to solve, which is why burrows built only a few yards apart can belong to very different lives.

A blue land crab begins on higher ground with a comparatively broad entrance, then turns downward toward moisture. There is no single blueprint hidden below it. Burrows examined with a fiber-optic camera in Puerto Rico included short passages that descended at a shallow angle, longer tunnels that leveled before dropping sharply, and deep curves resembling an inverted S. Forked and corkscrew forms occurred less often. The route varied with the site, but the essential destination was usually the same: a humid refuge at or near groundwater, occupied by one crab rather than a colony sharing one chamber (Moraes-Costa & Schwamborn, 2018; Sample & Albrecht, 2016).

The blue land crab’s burrow will begin their entrance with a steep angle that levels off until a sharp downward turn to reach the water table. | Image credit: Sample & Albrecht, 2016

An Atlantic sand fiddler builds at another scale and for another rhythm. Its narrow shaft descends through sand exposed between tides, and a deeper breeding burrow may extend farther than the temporary refuge used while feeding. Across a flat, many small entrances can stand close together because each belongs to a small crab defending little more than the ground immediately around it. Although the shafts provide shelter and moist air, they remain much narrower than the passage required by an adult blue land crab (Christy, 1982). A fiddler opening would therefore be a poor ready-made home for a mature Cardisoma.

A 3D cast and schematic show the burrow formation of an Atlantic fiddler crab. | Image Credit: di Virgilio & Riberio, 2013

The purple marsh crab offers a different contrast. Its burrows occupy muddy, rooted creek banks and can become part of a connected network with several openings. When those networks become dense, especially where vegetation has already thinned, excavation and cordgrass feeding can loosen the bank and make sediment easier for moving water to carry away (Bertness et al., 2014; Farron et al., 2020). A blue land crab generally begins above ordinary tidal flooding and maintains a more solitary route toward groundwater. Its burrow should not be assumed to undercut a creek bank in the same way simply because both animals dig.

A ghost crab may also leave one obvious opening and a mound of newly moved sand, but its address is usually the open ocean beach or foredune. Its sloping or curving refuge is built in loose, comparatively dry beach sand, not at the moist soundside boundary between marsh and yard.

Ghost crab burrow structure from a 3D cast on a sandy beach | Image credit: Shinoda et al., 2019

The squareback marsh crab makes an even clearer distinction. It commonly slips beneath leaves, logs, rocks, boards, and other cover at the upper marsh edge instead of constructing the large groundwater-reaching tunnel associated with an adult blue land crab. Mud crabs and Atlantic blue crabs do not make permanent terrestrial burrows at all. One uses submerged crevices; the other may bury itself temporarily in bottom sediment.

The squareback marsh crab may use rocks, leaves or other surface materials to hide beneath instead of a burrow. | Image credit: critterbliss, iNaturalist

Burrow reuse deserves equal care. Smaller blue land crabs have been found in openings much larger than their bodies, which is consistent with their moving into burrows abandoned by larger blue land crabs (Carmona-Suárez & Guerra-Castro, 2012). For a smaller crab, an empty tunnel would mean less digging, less time exposed aboveground, and immediate access to a refuge that already holds moisture. Those are likely advantages rather than benefits tested directly.

The other crabs along a Carolina shore do not appear to offer the same shortcut. Their shelters are too narrow, too low in the marsh, too dry, or not true burrows at all. Nothing currently shows that blue land crabs routinely take over and enlarge the burrows of another crab species.

The digging leaves more than a hole. Soil brought up from below is spread across the surface, while leaves, scraps of food, old molts, and waste may collect inside. In places where blue land crabs are established, the soil within their burrows differs from the ground beside them in its mixture of sand and silt, acidity, organic matter, and nutrients including nitrogen, magnesium, and potassium (Quintero-Torres et al., 2018).

The open tunnel also allows air and water to reach soil that had been sealed underground. Across many kinds of coastal crabs, burrowing tends to loosen sediment and change the chemical work carried out there by microbes. How much changes—and whether the effect helps or harms the plants above—depends upon the crab, the vegetation, and where the burrow sits (Rinehart et al., 2024).

Water salinity is shown in a burrowed marsh (a), a straight burrowed marsh (b), a sloped marsh (c ), and a marsh at the high tide stage and how salinity is distributed in the marsh as influenced by burrows. | Image credit: Yin et al., 2023

That does not mean each hole ventilates an entire marsh. Oxygen entering a tunnel may change only a thin layer of soil along its walls. Around fiddler burrows, that reach has been far smaller than the familiar phrase “aerates the soil” might suggest (Michaels & Zieman, 2013). A single high-ground blue land crab tunnel is not a new tidal creek either. Rain, groundwater, or an unusually high tide may enter it and change the wetness immediately around the opening, but blue land crab burrows have not been shown to redirect tidal flow through a maritime forest.

Roots present the same uncertainty. Digging can break fine roots and loosen the soil holding them, while the extra space and shifted nutrients around a tunnel may create new conditions for roots and soil life. Blue land crabs also clip and eat plants aboveground, so a damaged plant would not tell us whether feeding, digging, or both were responsible.

One occupied hole may change little beyond a small patch of ground. If the same digging were repeated across many burrows, those patches could begin to influence which plants take hold, how firmly their roots bind the edge, and how water moves through the soil. That possibility is one reason burrows are worth mapping. It is not evidence that those changes are already happening at North Carolina sighting sites.

At first, the tunnel keeps one crab alive. Only when that work is repeated across a place does it begin to become part of the landscape.

From the surface, however, most of that architecture remains hidden.

What a Burrow Can—and Cannot—Tell Us

A mature blue land crab may leave an opening three to five inches wide. Freshly excavated soil may collect around the entrance, and plant pieces or tracks may be visible nearby. These clues become more persuasive when the hole sits above normal tidal flooding in shaded, moist ground and a very large crab has been seen retreating into it (Hostetler et al., 2025).

It is still not an identification. A smaller blue land crab may occupy an older, larger burrow, while erosion can widen the entrance to a much smaller tunnel. Marsh soil also collapses, roots leave openings, rodents dig, and water continually reshapes the grass line. Size alone can mislead, particularly in a photograph with nothing nearby for scale.

The setting and the pattern offer better clues. Many small holes spread across exposed mud or sand point toward fiddlers. Several openings cut through a rooted creek bank fit purple marsh crabs. One hole high on the dry ocean beach is more likely a ghost crab’s. Squarebacks are often found beneath something rather than beside a large mound of freshly dug soil. The strongest match for the blue land crab would bring several details together: a broad opening on higher soundside ground, signs of recent digging, and a crab far larger than the native marsh species.

That is why a useful photograph includes more than the hole. The crab, entrance, moved soil, surrounding plants, nearby water, and something familiar for scale allow the scene to be read together. There is no need to dig, probe, or flood the tunnel. Without the animal, a burrow can identify a place worth watching, but not the species living beneath it.

What the New Crab Would Eat

The large claw may make a blue land crab look like a hunter arriving to eat every smaller crab in its path. Most of what it gathers, however, is plant material: fallen leaves, fruit, berries, flowers, tender shoots, grasses, and seeds. It may carry that food home and pull some of it underground. Insects, carrion, feces, and other animal matter add to the diet when available, and blue land crabs sometimes eat one another (Gifford, 1962; Herreid, 1963; Moraes-Costa & Schwamborn, 2018).

That does not place native crabs entirely off the menu. A blue land crab may seize a small animal it can catch, but regular hunting of fiddlers, squarebacks, or purple marsh crabs is unlikely to be its main influence. The more important changes would probably begin with the plants and fallen leaves it removes, the ground it excavates, and the space it keeps around its burrow.

The amount of competition would differ from crab to crab. Fiddlers graze the thin film of algae, microbes, and organic particles coating the marsh surface, so they share relatively little food with a crab gathering fruit and leaves above the high-tide line. Squareback and purple marsh crabs share more of that plant-based menu, although they forage at different scales and in different parts of the marsh. If blue land crabs remain, the strongest overlap may be over shaded ground, damp refuge, and nearby plant food rather than over a particular prey animal.

What Established Populations Tell Us

Sharing food and high ground does not make displacement inevitable. Where blue land crabs have lived for generations, other crabs continue to live beside them by using different levels of the shore. Along the Brazilian coast, for example, the mangrove crab, Ucides cordatus, digs into softer ground between the tides, while Cardisoma lives higher in firmer, sandier soil. Their ranges overlap without their burrows occupying precisely the same ground (Firmo et al., 2012).

North Carolina has a different community, but the same separation by water, soil, and elevation is already familiar here. The squareback comes closest to overlapping with a blue land crab at the upper edge. Even there, one defended Cardisoma burrow might cost a squareback a feeding spot without removing squarebacks from the wider marsh.

The first change people noticed might therefore be simpler than one crab replacing another: a much larger animal at the grass line, fresh soil beside an entrance, fallen fruit or leaves drawn toward it, and tender plants clipped within a short walk of the burrow.

That picture could change if the crabs became numerous. Many defended burrows would divide more of the narrow band of damp high ground, increasing competition for shelter and food. Their digging would repeat the small soil changes described above, while their feeding could influence which seeds and seedlings survive and how quickly fallen leaves disappear. Those broader forest effects are known across several land-crab species rather than from a newly established Cardisoma population, but they show why abundance matters more than the mere arrival of one large crab (Lindquist et al., 2009).

Predators would respond only if the crabs became dependable prey. Large birds and mammals eat blue land crabs within their established range, while eggs and larvae released into the estuary become food for aquatic animals (Firmo et al., 2012; Hostetler et al., 2025). One adult crossing a Carolina yard offers one unusual meal. A recurring population could give raccoons or coastal birds a reason to search the marsh edge and could place many larvae into the estuarine food web.

This is where two kinds of change can look like the same thing. Digging alters the ground directly, making the crab an ecosystem engineer (Rinehart et al., 2024). A trophic cascade would have to continue through feeding relationships: fewer surviving seedlings changing the vegetation, for example, or predators drawn to land crabs changing their pressure on other prey (Lindquist et al., 2009).

Blue land crabs are capable of beginning both pathways. What the North Carolina sightings cannot yet tell us is whether enough crabs are arriving, surviving, and reproducing for the work of individuals to become the work of a population.

When One Sighting Becomes a Place

One dated photograph gives the map a place. Another observation weeks or months later begins to show whether the crab stayed. As those records accumulate, they can reveal when the crabs are active, whether they appear after rain, which kinds of ground they use, and whether they return across seasons. Crabs of different sizes at one site may have survived across more than one year, while juveniles, egg-bearing females, or repeated movements toward the water would offer stronger evidence that the species is completing more of its life cycle here.

The same map can show where the larger ecological questions belong. One wandering adult does not tell us that native crabs are being displaced or that a trophic cascade has begun. Several occupied burrows, plant damage, younger crabs, and repeated sightings at the same place would give biologists a reason to compare its vegetation, soil, native crabs, and predators with nearby places where Cardisoma is absent.

Anyone who sees a possible blue land crab can photograph it from a safe distance and submit the image, date, and location through the Blue Land Crab Sightings reporting. A photograph showing the animal, its approximate scale, and the surrounding habitat is more useful than trying to catch or relocate it.

That distinction matters for anyone standing at the grass line. A three-inch hole is not an identification, clipped leaves are not proof of ecological harm, and a large crab is not automatically an invasive species. Each is a piece of context. Together, photographed and reported without catching or relocating the animal, those pieces can show whether North Carolina is receiving occasional southern visitors or adding a new resident to the narrow landscape between the forest and the sea.

The coast will answer that slowly, one place at a time.

References

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Buck, T. L., Breed, G. A., Pennings, S. C., Chase, M. E., Zimmer, M., & Carefoot, T. H. (2003). Diet choice in an omnivorous salt-marsh crab: Different food types, body size, and habitat complexity. Journal of Experimental Marine Biology and Ecology, 292(1), 103-116. https://doi.org/10.1016/s0022-0981(03)00146-1

Carmona-Suárez, C. A., & Guerra-Castro, E. (2015). Comparison of three quick methods to estimate crab size in the land crabs <i>Cardisoma guanhumi</i> Latreille, 1825 and <i>Ucides cordatus</i> (Crustacea: Brachyura: Gecarcinidae and Ucididae). Revista de Biología Tropical, 60, 139. https://doi.org/10.15517/rbt.v60i0.19854

Christy, J. H. (1982). Burrow structure and use in the sand fiddler crab, Uca pugilator (Bosc). Animal Behaviour, 30(3), 687-694. https://doi.org/10.1016/s0003-3472(82)80139-5

Costlow, Jr., J. D., & Bookhout, C. G. (1968a). The complete larval development of the land-crab, Cardisoma guanhumi Latreille in the laboratory (Brachyura, Gecarcinidae). Crustaceana, 15(3), 259-270.

Costlow, Jr., J. D., & Bookhout, C. G. (1968b). The effect of environmental factors on development of the land-grab,cardisoma guanhumiLatreille. American Zoologist, 8(3), 399-410. https://doi.org/10.1093/icb/8.3.399

Farron, S., Hughes, Z., FitzGerald, D., & Strom, K. (2020). The impacts of bioturbation by common marsh crabs on sediment erodibility: A laboratory flume investigation. Estuarine, Coastal and Shelf Science, 238, 106710. https://doi.org/10.1016/j.ecss.2020.106710

Firmo, A., Tognella, M. M., Silva, S. R., Barboza, R. R., & Alves, R. (2012). Capture and commercialization of blue land crabs (“guaiamum”) Cardisoma guanhumi (Lattreille, 1825) along the coast of Bahia state, Brazil: An ethnoecological approach. Journal of Ethnobiology and Ethnomedicine, 8(1). https://doi.org/10.1186/1746-4269-8-12

Gifford, C. A. (1962). Some observations on the general biology of the land crab, cardisoma guanhumi (Latreille), in South Florida. The Biological Bulletin, 123(1), 207-223. https://doi.org/10.2307/1539516

Govender, Y., Sabat, A. M., & Cuevas, E. (2008). Effects of land-use/land-cover changes on land crab,Cardisoma guanhumi, abundance in Puerto Rico. Journal of Tropical Ecology, 24(4), 417-423. https://doi.org/10.1017/s0266467408005130

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Herreid, C. F., & Gifford, C. A. (1963). The burrow habitat of the land crab, Cardisoma guanhumi (Latreille). Ecology, 44(4), 773-775. https://doi.org/10.2307/1933027

Hostetler, M. E., Mazzottii, F. J., & Taylor, A. K. (2025, October 28). Blue land crab (Cardisoma guanhumi) (WEC 30). Department of Wildlife Ecology and Conservation, UF/IFAS Extension. https://ask.ifas.ufl.edu/publication/UW013

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Michaels, R. E., & Zieman, J. C. (2013). Fiddler crab (Uca spp.) burrows have little effect on surrounding sediment oxygen concentrations. Journal of Experimental Marine Biology and Ecology, 448, 104-113. https://doi.org/10.1016/j.jembe.2013.06.020

Moraes-Costa, D., & Schwamborn, R. (2018). Site fidelity and population structure of blue land crabs (Cardisoma guanhumi Latreille, 1825) in a restricted-access mangrove area, analyzed using PIT tags. Helgoland Marine Research, 72(1). https://doi.org/10.1186/s10152-017-0504-0

NC Dept. of Environmental Quality. (2026, August 26). Please report blue land crab sightings. https://www.deq.nc.gov/news/press-releases/2026/08/26/please-report-blue-land-crab-sightings

O’Connor, N. (1993). Settlement and recruitment of the fiddler crabs Uca pugnax and U. pugilator in a North Carolina, USA; salt marsh. Marine Ecology Progress Series, 93, 227-234. https://doi.org/10.3354/meps093227

Quintero-Torres, E., Chacón, N., & López-Sánchez, B. (2018). The ecosystem engineering role of the neotropical crab Cardisoma guanhumi on mangrove soil properties. Wetlands Ecology and Management, 26(5), 993-1000. https://doi.org/10.1007/s11273-018-9618-7

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Sample, S., & Albrecht, M. (2016). Determination of the burrow shapes of Cardisoma guanhumi on Vieques, Puerto Rico. Journal of Coastal Life Medicine, 4(2), 94-97. https://doi.org/10.12980/jclm.4.2016j5-245

Scott, E., Kendrick, M., Kingsley-Smith, P., James, M., Lemeris, J., Weeks, E., & Sasson, D. (2023). Using public sightings to document the widespread distribution of the non-endemic blue land crab, Cardisoma guanhumi, in South Carolina. Southeastern Naturalist, 22(4). https://doi.org/10.1656/058.022.0403

Wittyngham, S. S., Johnson, D. S., Chen, Y., & Kirwan, M. L. (2024). A grazing crab drives saltmarsh carbon storage and recovery. Ecology, 105(9). https://doi.org/10.1002/ecy.4385

September 5, 2026
The Atlantic Mud Crab and the Marsh Periwinkle Snail

Along the edge of a North Carolina salt marsh, some of the easiest animals to find are also among the easiest to overlook.

Small spiral shells cling to smooth cordgrass, sometimes only inches above the mud and sometimes surprisingly high along the stems. These are marsh periwinkles, Littoraria irrorata, and where they sit changes as the tide moves beneath them. Nearby, oyster shells collect along creek edges and among the vegetation. Beneath one of them may be an Atlantic mud crab, Panopeus herbstii, tucked into a space barely larger than its body.

Neither seems capable of influencing much beyond the few inches it occupies.

Yet the relationship between these two small animals reaches into the grass around them, the sediment beneath them, and eventually the ability of a salt marsh to maintain itself as water levels change.

Both are also indicator species. Their abundance, where they occur, and how they respond to the conditions around them can offer clues about the health of the marsh they share (Fowler & Kingsley-Smith, 2015; Rietl et al., 2018).

For someone walking a boardwalk, paddling a tidal creek, or looking across the marsh on the way to the beach, that makes them particularly interesting. Much of what keeps a salt marsh functioning happens underground, underwater, or so gradually that we never see it happening.

These two animals give us something we can actually watch.

The Snails on the Grass

The marsh periwinkle spends much of its life following the tide up and down the cordgrass.

Marsh periwinkle, Littoraria irrorata. Small enough to fit in the palm of a hand, this marsh snail spends much of its life moving up and down cordgrass with the tide. | Image credit: A. Mitchell

At lower tides, the snails move down the stems and closer to the marsh surface to feed. When the tide comes back in, fish, crabs, and other aquatic predators come with it. The periwinkles respond by climbing higher on the grass, putting distance between themselves and the hunters now swimming below.

So when you see dozens of little snails perched high on the cordgrass beside a flooded marsh, they did not simply choose a nice place to sit.

They have moved upstairs.

As the tide rises, salt marsh periwinkles climb higher on the cordgrass, staying above the water and farther from predators moving through the flooded marsh. | Image credit: andyjones, iNaturalist

That daily trip between the lower grass and the safety of the upper stems is shaped partly by predators. Where those predators are more common, periwinkles change where they spend their time and may even develop differently under that continued pressure (Rietl et al., 2018).

When the water falls again, the little climbers come back down.

As the tide falls, periwinkles move back down through the cordgrass toward the marsh surface, where much of their feeding takes place. | Image credit: JunoMoon, iNaturalist

And this is where their job in the marsh becomes especially interesting.

Periwinkles graze across the surface of smooth cordgrass, Spartina alterniflora. One easy way to picture them is as tiny lawnmowers moving through the marsh. They do not clip every blade equally, though. They often favor older, yellowing, or dead cordgrass, helping work through plant material that is already on its way out (Klinges et al., 2025).

But their feeding is not as simple as taking a bite from a blade of grass.

Those scrapes leave small wounds on the cordgrass where fungi can grow. The snails later return to those areas and feed again, consuming the fungal growth along with plant material. When snail numbers and grazing become unusually high, those repeated wounds can contribute to heavier fungal growth and increasing damage to the cordgrass (Silliman & Newell, 2003).

Like a lawn, the important part is not whether the grass is ever cut.

It is how much.

Enough grazing is part of a functioning marsh. Periwinkles belong here, and their feeding is one of many interactions helping shape the plant community. But put too many little lawnmowers in the same patch and they can begin cutting faster than the grass can recover. Cordgrass becomes shorter and thinner, more plant tissue is wounded, and eventually patches can begin to disappear.

Studies in southeastern salt marshes have shown just how far that change can go when predators are removed and periwinkles become unusually abundant. Heavy grazing can contribute to substantial losses of Spartina rather than simply keeping its growth in check (Altieri et al., 2012).

Fewer snails are not automatically better, either. Grazers are part of the balance of the marsh, just as predators are. What matters is that no single part of the relationship overwhelms the others.

And one of the animals helping keep these little lawnmowers in check is usually not up on the grass at all.

It is waiting below.

Beneath the Shell

Atlantic mud crabs, Panopeus herbstii, are much easier to miss.

Atlantic mud crab, Panopeus herbstii. Small, heavily clawed, and easy to miss among shell and mud, this lower-marsh predator helps keep periwinkle grazers in check. | Image credit: nickifal, iNaturalist

If you have ever looked among oyster shells at the lower edge of a marsh creek, you have looked directly into the kind of neighborhood they prefer. Mud crabs tuck beneath shell and stones, move over muddy bottoms, and use shallow burrows along marsh edges. They live in estuaries where the water is never quite the same from one day to the next—saltier during some periods, fresher after heavy rain, warmer through summer and cooler through winter. Atlantic mud crabs are remarkably good at living within those changing conditions (Fowler & Kingsley-Smith, 2015).

Before any of that, however, a mud crab begins life somewhere very different.

A newly hatched crab does not look like the crab hiding beneath an oyster shell. It begins as a tiny larva drifting in the water, carried through the estuary while it develops. Eventually it must leave that drifting life behind and settle somewhere suitable.

Atlantic mud crab larva, Panopeus herbstii. At this stage, the future bottom-dwelling crab is tiny, transparent, and drifting through the estuary before settling into marsh and oyster-shell habitat. | Image credit: True et al., 2020

Finding that place is not entirely luck.

Young mud crabs can respond to signals in the environment that tell them they have reached the kind of habitat where adult crabs live. Oyster shell, rocks, other organisms, and even the thin films of microorganisms coating submerged surfaces can provide chemical and physical clues that encourage the larva to settle and begin the next stage of life (Andrews et al., 2001; Rodriguez & Epifanio, 2000).

Think about what that means for something small enough to drift with the current.

It does not have a map of the marsh.

Instead, the place itself begins to smell and feel right.

Once settled, the crab eventually becomes one of the predators moving through that habitat. Atlantic mud crabs eat oysters, clams, worms, small crustaceans, snails, and whatever other suitable prey they can capture.

And beneath those oyster shells, researchers have found evidence of one meal again and again.

Periwinkles.

In North Carolina salt marshes, researchers found about 80 percent of the Atlantic mud crab lairs they examined beneath live oyster shells. When they looked at prey remains associated with those lairs, marsh periwinkle shells were among the most common finds (Silliman et al., 2004).

Above the shell, the snail moves along the cordgrass.

Below it, the crab waits.

When Predator Meets Grazer

This is where two small animals begin influencing something much larger.

Periwinkles spend part of each tidal cycle higher on the cordgrass, away from aquatic predators. When the water drops and they move lower to feed, they become more accessible to the mud crabs below.

The crabs do not need to climb the grass and chase them.

They simply need to be there when the snails come back down.

Research has shown that Atlantic mud crabs can keep periwinkle numbers lower, and that mud crab predation increases when more periwinkles are available (Silliman et al., 2004). In practical terms, the predator helps keep all of those little lawnmowers from becoming too numerous.

That matters because the crab is indirectly influencing grass it never eats.

More mud-crab predation can mean fewer periwinkles grazing on the cordgrass. Fewer grazers can mean less pressure on the grass. When predators disappear and periwinkles become too abundant, the opposite can happen.

That chain reaction has a scientific name: a trophic cascade.

A change involving a predator works its way through the animals it eats and eventually affects something farther down the food web. In this case, the path is easy to follow:

mud crab → periwinkle → cordgrass.

Studies elsewhere in southeastern salt marshes have shown how dramatic that cascade can become. Where intensive recreational fishing reduced predator numbers, periwinkles became more abundant, grazing increased, and large areas of marsh vegetation were eventually lost (Altieri et al., 2012).

The Atlantic mud crab never takes a bite from the cordgrass.

Yet by eating an animal that does, it helps influence how much grass remains standing.

And keeping that grass standing turns out to matter far beyond the snail and crab.

An empty periwinkle shell may not stay empty for long. Hermit crabs often move into abandoned shells, turning yesterday’s snail home into today’s shelter. | Image credit: A. Mitchell

What the Grass Holds

A salt marsh has to maintain more than vegetation.

It has to maintain ground.

Smooth cordgrass helps slow the water as each tide moves across the marsh. When the water slows, tiny bits of sand, mud, and other material it was carrying can drop out and settle around the grass. Below the surface, the roots and underground stems help hold that ground together, while old plant material breaks down and becomes part of the marsh soil. Little by little, those layers add up and help the marsh keep its height above the water (Reed, 1995; Elsey-Quirk et al., 2011).

That elevation determines how long and how often the marsh floods.

A difference of only a few inches can change the amount of time vegetation spends underwater, the salinity it experiences, and which plants and animals can occupy a particular part of the marsh.

For a marsh facing rising relative sea level, maintaining elevation becomes increasingly important. If sediment deposition and organic-matter accumulation equal or exceed the rate at which the marsh is becoming submerged, vegetation may persist. If elevation cannot keep pace, flooding increases and vegetation can eventually be lost (Reed, 1995).

A salt marsh has a few ways to keep up as water levels rise. It can build upward as new sediment settles among the grass and as roots and other plant material become part of the soil. It can also slowly shift farther inland onto slightly higher ground—if that ground is still available. How well a marsh can do either depends on the place itself: how much sediment reaches it, how well the vegetation is growing, how strong the tides are, how much erosion is occurring, and how quickly local water levels are rising (Kirwan & Mudd, 2012; Fagherazzi et al., 2020).

The important point is that the biological and physical marsh are not separate systems.

The grass affects sediment.

The sediment affects elevation.

Elevation affects flooding.

Flooding affects the plants and animals.

And among the animals affecting that grass are a snail climbing its stems and a crab waiting below.

When the Water Rises Faster

Sea-level rise can feel abstract when we describe it as a number measured in millimeters or as a line on a map showing where water might be decades from now.

A salt marsh makes it easier to understand.

Marsh plants already live with water coming and going every day. They are built for it. But being covered during part of a normal tide is very different from spending more and more time underwater as the average water level rises.

The marsh has two important ways to respond.

It can build upward as sediment and plant material accumulate. Just up the New River at Camp Lejeune, that struggle to maintain marsh elevation can be seen on the ground.

And, where higher undeveloped ground remains beside it, the marsh can gradually move inland.

That second option matters enormously along a developed coastline.

If rising water makes the lowest edge of a marsh increasingly difficult for cordgrass to occupy, suitable marsh vegetation can begin establishing slightly farther inland. Over time, the position of the marsh shifts.

Unless something is already there.

A road does not move uphill for the marsh.

Neither does a house, parking lot, bulkhead, or other hardened shoreline.

When development occupies the higher ground immediately behind a marsh, rising water can begin pressing from one side while development prevents the marsh from moving on the other. Its room to adjust becomes smaller.

What can eventually disappear is not simply an undeveloped patch of grass that might otherwise have held another building.

NOAA’s Sea Level Rise Viewer lets you watch that landward movement—and where it can become constrained—as water levels change.

The marsh was already working.

Salt marshes provide nursery habitat for fishes and crustaceans, trap sediment, store carbon, help stabilize shorelines, and reduce some of the energy moving through shallow coastal water. Their ability to keep doing those things depends upon the marsh itself remaining high enough, vegetated enough, and connected enough to continue functioning (Fagherazzi et al., 2020).

That is one reason decisions about where and how we build along the coast matter beyond the property line.

A marsh does not have to contain a building to have a purpose.

And Then a Storm Arrives

Sea-level rise changes a marsh gradually.

A storm can rearrange parts of it in a day.

Anyone who lives along the North Carolina coast has seen what heavy rainfall alone can do to local water. Creeks swell. Freshwater pours into sounds and estuaries. Salinity drops. A tropical storm or hurricane can add storm surge, waves, erosion, and enough moving water to carry shell, sediment, wrack, and pieces of the marsh itself from one place to another.

Mud can be stripped away from one creek bank and deposited somewhere else.

Oyster shells that were buried may suddenly be exposed.

Shell that once provided hiding places may be covered.

Cordgrass can be flattened, torn loose, buried beneath wrack, or left standing in a slightly different landscape once the water recedes.

Estuary sediments can preserve that history. In North Carolina’s Albemarle estuarine system, sediments record both the slow changes occurring over decades and the sudden rearrangements associated with storms and other major events (Corbett et al., 2007).

For the Atlantic mud crab and periwinkle, those changes can rearrange the places where their relationship happens.

The periwinkle needs the cordgrass.

The mud crab relies on suitable water conditions, prey, and places such as oyster shell and shallow burrows for cover.

Change the grass, move the shell, freshen the water, or reshape the creek edge and the animals must respond to the new version of the marsh that remains.

That does not mean a healthy marsh should never change.

Quite the opposite.

Tides, storms, erosion, sediment deposition, rainfall, and shifting salinity have always been part of coastal marshes.

What matters is whether enough of the living and physical system remains intact for the marsh to adjust afterward.

Two Species That Tell a Larger Story

This is what makes the Atlantic mud crab and marsh periwinkle especially useful animals to know.

They are not simply inhabitants of the marsh. Both have been used as indicators of conditions within the ecosystems they occupy because their abundance, distribution, and responses are connected to environmental conditions and ecological relationships around them (Fowler & Kingsley-Smith, 2015; Rietl et al., 2018).

The periwinkle gives us one view.

It lives directly on the vegetation, responds to tidal flooding and predation, and can exert increasing pressure on cordgrass when predator–prey relationships change.

The Atlantic mud crab gives us another.

Its abundance is associated with habitat structure, prey availability, Spartina characteristics, oyster habitat, temperature, and salinity, while its predation helps influence the abundance of grazers such as the periwinkle (Silliman et al., 2004; Fowler & Kingsley-Smith, 2015).

Together, they give us something particularly interesting.

They allow us to see part of a trophic relationship that reaches from predator to grazer to plant and, through the plant, into processes affecting the physical marsh.

That does not mean finding ten snails instead of five provides a diagnosis of marsh health, or that turning over one oyster shell and finding no crab means something is wrong. Scientific monitoring requires repeated observations, measurements, and knowledge of local conditions.

But it does mean that these animals are worth noticing.

They can make processes that otherwise seem abstract visible.

Periwinkles scattered through the marsh vegetation offer a visible clue to the living community around them. Their numbers and where they occur can help reveal changes in the balance between grazing, predation, and cordgrass health. | Image credit: BioInteractive

Looking at the Marsh Differently

The next time you are beside the salt marsh along Surf City, look at the grass before you look across it.

Find the little spiral shells.

Notice whether the tide is in or out and where the periwinkles are sitting along the stems. Look lower, toward the mud and oyster shell around the creek edge, at the little spaces where a mud crab might disappear.

Then look at the marsh around them.

Is the cordgrass thick and continuous? Are there areas where the edge is eroding? Has a storm moved wrack or exposed new shell? Where does the water reach at high tide, and where does the ground begin rising toward houses, roads, or other development?

Those observations do not turn a beach walk into a scientific survey.

They do something simpler.

They let us recognize that the marsh is alive with relationships, and that some of the smallest animals in it can help us understand why the larger landscape looks the way it does.

Somewhere in that system, a periwinkle climbs higher as the tide comes in.

Below it, an Atlantic mud crab remains hidden among the shell.

Most people walking toward the beach will pass both without ever knowing they are there.

But once you know what connects them, they become two small signs of something much larger happening beneath your feet.

Across the soundside marshes of Surf City, water, vegetation, wildlife, and the developed shoreline exist side by side—each connected to what happens in the marsh between them. | Image credit: A. Mitchell

References

Altieri, A. H., Bertness, M. D., Coverdale, T. C., Herrmann, N. C., & Angelini, C. (2012). A trophic cascade triggers collapse of a salt-marsh ecosystem with intensive recreational fishing. Ecology, 93(6), 1402–1410. https://doi.org/10.1890/11-1314.1

Andrews, W. R., Targett, N. M., & Epifanio, C. E. (2001). Isolation and characterization of the metamorphic inducer of the common mud crab, Panopeus herbstii. Journal of Experimental Marine Biology and Ecology, 261(1), 121–134. https://doi.org/10.1016/S0022-0981(01)00268-4

Corbett, D. R., Vance, D., Letrick, E., Mallinson, D., & Culver, S. J. (2007). Decadal-scale sediment dynamics and environmental change in the Albemarle estuarine system, North Carolina. Estuarine, Coastal and Shelf Science, 71(3–4), 717–729. https://doi.org/10.1016/j.ecss.2006.09.024

Elsey-Quirk, T., Seliskar, D. M., Sommerfield, C. K., & Gallagher, J. L. (2011). Salt marsh carbon pool distribution in a Mid-Atlantic lagoon, USA: Sea level rise implications. Wetlands, 31(1), 87–99. https://doi.org/10.1007/s13157-010-0139-2

Fagherazzi, S., Mariotti, G., Leonardi, N., Canestrelli, A., Nardin, W., & Kearney, W. S. (2020). Salt marsh dynamics in a period of accelerated sea level rise. Journal of Geophysical Research: Earth Surface, 125(8). https://doi.org/10.1029/2019JF005200

Fowler, A., & Kingsley-Smith, P. (2015). Atlantic mud crab, Panopeus herbstii (H. Milne Edwards 1834). Supplemental Volume: Species of Special Conservation Concern, South Carolina State Wildlife Action Plan.

Kirwan, M. L., & Mudd, S. M. (2012). Response of salt-marsh carbon accumulation to climate change. Nature, 489(7417), 550–553. https://doi.org/10.1038/nature11440

Klinges, D. H., Martin, C. W., & Roberts, B. J. (2025). Ecological associations of the coastal marsh periwinkle snail Littoraria irrorata: Field and laboratory evidence of vegetation habitat preferences. PeerJ, 13, e19071. https://doi.org/10.7717/peerj.19071

Reed, D. J. (1995). The response of coastal marshes to sea-level rise: Survival or submergence? Earth Surface Processes and Landforms, 20(1), 39–48. https://doi.org/10.1002/esp.3290200105

Rietl, A. J., Sorrentino, M. G., & Roberts, B. J. (2018). Spatial distribution and morphological responses to predation in the salt marsh periwinkle. Ecosphere, 9(6), e02316. https://doi.org/10.1002/ecs2.2316

Rodriguez, R., & Epifanio, C. E. (2000). Multiple cues for induction of metamorphosis in larvae of the common mud crab, Panopeus herbstii. Marine Ecology Progress Series, 195, 221–229. https://doi.org/10.3354/meps195221

Silliman, B. R., Layman, C. A., Geyer, K., & Zieman, J. C. (2004). Predation by the black-clawed mud crab, Panopeus herbstii, in Mid-Atlantic salt marshes: Further evidence for top-down control of marsh grass production. Estuaries, 27(2), 188–196. https://doi.org/10.1007/BF02803375

August 15, 2026
The Hidden City in the Grass

How seagrasses and marsh grasses—and the animals within them—build the marshes of Onslow County

In Onslow County’s estuarine marshes, the best time to understand how the landscape works is when the water pulls back. As tides drain from creeks and shallow flats, patterns begin to emerge—where water lingers, where it moves easily, and where it hesitates. These patterns are not random. They reflect the combined influence of plants, animals, and sediments continually reshaping the boundary between land and sea.

Like the microscopic shells of foraminifera preserved in sediment, marsh and seagrass communities record environmental conditions. But unlike the past locked in mud, these systems are alive, constantly negotiated by plants, grazers, predators, and microbes.

From permanently submerged seagrass beds to the highest marsh edge, each elevation zone in Onslow County is maintained not just by vegetation, but by species that actively regulate growth, chemistry, and water flow.

Subtidal shallows: seagrass beds maintained by grazers

In the shallow, light-penetrated waters of the New River Estuary and protected soundside areas, seagrass beds form underwater meadows that stabilize sediments and provide nursery habitat for fish and invertebrates. Species present or expected in Onslow County waters include eelgrass (Zostera marina), shoalgrass (Halodule wrightii), and widgeongrass (Ruppia maritima) (Mallin, 2000; Orth, 1984).

Seagrass blades rapidly accumulate epiphytic algae and microbial films. Without constant grazing, this layer can block light and suppress photosynthesis. Amphipods, isopods, and small gastropods act as continuous maintenance crews, grazing epiphytes and preventing them from overwhelming the plants themselves (Orth & van Montfrans, 1984; Valentine & Duffy, 2006).

Experimental studies show that when these grazers are removed, seagrass condition declines even under favorable light conditions, demonstrating that plant survival depends as much on animal activity as on physical environment (Duffy et al., 2015). Beneath the canopy, burrowing worms and bivalves recycle nutrients and oxygenate sediments, preventing organic matter from accumulating around roots (Orth, 1984).

In this zone, seagrass persists because grazers keep blades clean and sediments breathable—a cooperative system built on constant biological upkeep.

Gammarus mucronatus, a common amphipod grazer on eelgrass | Photo credit: E. A. Lazo-Wasem, Yale Peabody Museum, 2013.

The low marsh edge: cordgrass shaped by snails and crabs

At the daily-flooded edge of the marsh, smooth cordgrass (Spartina alterniflora) dominates. This narrow fringe marks the boundary between open water and marsh interior, where erosion pressure is highest and stability matters most.

Left: Healthy smooth cordgrass (Spartina alterniflora) line the estuary edge in Surf City, NC. | Photo credit: A. Mitchell, 2022. Right: Salt marsh die-off from grazing stress by marsh periwinkle snails and reduced predation by crabs, such as blue crabs, can create bare mudflats. | Photo credit: By Esuglia at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=65794096

Cordgrass growth here is tightly regulated by the marsh periwinkle snail (Littoraria irrorata). These snails climb grass stems to avoid inundation and graze directly on living tissue, often intensifying damage by facilitating fungal infection. At high densities, periwinkle grazing can dramatically reduce cordgrass height and biomass, effectively mowing the marsh edge (Silliman & Zieman, 2001).

Marsh periwinkle snails (Littoraria irrorata) are a common sight on cordgrass (Spartina alterniflora) in North Carolina | Photo credit: North Carolina Aquarium at Roanoke Island, 2018.

Unchecked grazing can destabilize the marsh platform—but periwinkles themselves are regulated by crabs, including blue crabs (Callinectes sapidus), fiddler crabs (Genus Uca), purple marsh crabs (Sesarma reticulatum), hermit crabs and other burrowing species. Crabs prey on snails, limiting grazing pressure and indirectly protecting cordgrass (Silliman et al., 2005).

Crabs also function as ecosystem engineers. Their burrows aerate sediments, relieve sulfide stress around plant roots, and improve tidal water movement through compacted soils (Bertness, 1985; Thomas & Blum, 2010). Where crabs are abundant, cordgrass grows taller and denser; where they are lost, marsh die-off can occur rapidly.

This zone persists through a trophic cascade: grass builds land, snails limit grass, and crabs keep the system in balance.

Mid-marsh: mussels and detritus processors reinforce the platform

Just upslope, where flooding becomes less frequent, plant communities shift toward mixtures that often include saltmeadow cordgrass (Spartina patens). Here, the ribbed mussel (Geukensia demissa) emerges as a key stabilizing force.

Saltmeadow cordgrass (Spartina patens) is an important marsh stabilizer that has higher productivity when it grows near ribbed mussel aggregations | Photo credit: Kristie Gianopulos

Ribbed mussels form dense clusters at the base of marsh vegetation, binding sediments with byssal threads and physically reinforcing marsh soils against erosion (Bertness, 1984). As filter-feeders, they concentrate nutrients by removing organic matter from tidal waters and depositing nitrogen-rich biodeposits directly into marsh sediments (Jordan & Valiela, 1982).

Ribbed mussels (Geukensia demissa) at the base of marsh vegetation | Photo credit: R. Bachand

Grasses growing near mussel aggregations exhibit higher productivity than those without mussels, demonstrating a strong facilitative relationship between animals and plants (Bertness, 1984). As vegetation senesces, detritivorous worms, insects, and microbial decomposers break down dead plant material, converting standing biomass into detritus that fuels food webs throughout the estuary (Mann, 1988).

The mid-marsh functions as a processing zone, reinforcing marsh structure while converting plant matter into usable energy.

High marsh: microbes that manage chemical stress

In the high marsh, dominated by black needlerush (Juncus roemerianus) and saltmeadow cordgrass (Spartina patens), flooding is limited to spring tides and storms. Prolonged exposure to air creates harsh soil conditions, including elevated salinity and sulfide accumulation.

Black needlerush grass (Juncus roemerianus) dominates the high marsh | Photo credit: ©Andy Newman

Here, microbial communities play a central role. Sulfate-reducing and sulfur-oxidizing bacteria regulate sulfide concentrations that would otherwise become toxic to plant roots, while microbial decomposition controls nutrient availability under fluctuating oxygen conditions (Howarth & Giblin, 1983).

Beneath the marsh surface, soil microbes regulate decomposition, carbon exchange, and chemical stress. Changes in salinity and flooding reshape microbial communities, influencing how marsh soils process organic matter and support vegetation across tidal elevations. | Image credit: Zhang et al., 2023.

Small soil invertebrates maintain pore spaces that allow brief pulses of oxygenated water to penetrate during flooding. Unlike the visibly engineered low marsh, the high marsh is stabilized largely through biogeochemical regulation rather than grazing or predation.

This zone endures because microbes quietly buffer plants against chemical extremes.

From microbes in the soil to grasses at the surface, biological interactions drive marsh formation. Microbial processes govern decomposition and organic matter buildup, helping determine whether marsh platforms gain elevation, remain stable, or collapse | Image credit: Abbot, Quirk & Fultz, 2022.

The marsh–upland transition: keeping the boundary intact

At the uppermost margin of the marsh, tidal influence becomes intermittent and environmental stress shifts from salinity to erosion and freshwater input. Burrowing invertebrates increase soil permeability, allowing stormwater and tidal surges to infiltrate rather than scour the surface (Thomas & Blum, 2010).

A profile illustration . depicting the recommended transition of plant types from the edge of the salt marsh to the upland buffer. | Image credit: Massachusetts Office of Coastal Zone Management

Vegetation root networks stabilize soils exposed to drying and wave action, while animal burrows act as pressure-release pathways during extreme events. When these biological processes are disrupted—by shoreline hardening or vegetation removal—the marsh edge often collapses abruptly rather than adjusting gradually.

This boundary holds only as long as water can move through it.

Black, organic-rich peat exposed after storms marks the remains of an ancient salt marsh once buried beneath barrier sands. Its reappearance along North Topsail Beach records long-term shoreline change and marsh migration. Photo credit: Bill Tresnan, 2024.

A marsh built by interactions

Across all elevations in Onslow County marshes, the pattern is consistent:

Plants define the zones—but animals and microbes determine whether those zones endure.

Conceptual diagram of revised juvenile blue crab ontogenetic habitat shifts. Arrows depict transitions between habitats with increases in size. Arrow widths denote abundance contributions of individuals between habitats. | Image credit: Hyman et al., 2023

From grazers that keep seagrass blades clean, to crabs that hold the marsh edge together, to microbes that manage invisible chemical stress, the marsh is sustained by small organisms with outsized influence. Together, these interactions determine not just what lives in the marsh, but whether the marsh itself endures.

Purple marsh crabs (Sesarma reticulatum) moving together along the marsh edge on South Topsail Island, North Carolina. Their collective movement and feeding activity illustrate how small organisms play outsized roles in maintaining marsh structure. Photo credit: A. Mitchell, 2025.

References

Abbott, K. M., Quirk, T., & Fultz, L. M. (2022). Soil microbial community development across a 32-year coastal wetland restoration time series and the relative importance of environmental factors. Science of The Total Environment, 821, 153359. https://doi.org/10.1016/j.scitotenv.2022.153359

Bertness, M. D. (1984). Ribbed mussels and Spartina Alterniflora production in a New England salt marsh. Ecology, 65(6), 1794-1807. https://doi.org/10.2307/1937776

Bertness, M. D. (1985). Fiddler crab regulation of Spartina alterniflora production on a New England salt marsh. Ecology, 66(3), 1042-1055. https://doi.org/10.2307/1940564

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Howarth, R. W., & Giblin, A. (1983). Sulfate reduction in the salt marshes at Sapelo island, Georgia. Limnology and Oceanography, 28(1), 70-82. https://doi.org/10.4319/lo.1983.28.1.0070

Hyman, A. C., Chiu, G. S., Seebo, M. S., Smith, A., Saluta, G. G., Knick, K. E., & Lipcius, R. N. (2023). Model-based evaluation of critical nursery habitats for juvenile blue crabs through ontogeny: Abundance and survival in seagrass, salt marsh, and unstructured bottom. https://doi.org/10.1101/2023.07.20.549877

Jordan, T. E., & Valiela, I. (1982). A nitrogen budget of the ribbed mussel, Geukensia demissa, and its significance in nitrogen flow in a New England salt marsh. Limnology and Oceanography, 27(1), 75-90. https://doi.org/10.4319/lo.1982.27.1.0075

Mallin, M. A., Burkholder, J. M., Cahoon, L. B., & Posey, M. H. (2000). North and South Carolina coasts. Marine Pollution Bulletin, 41(1-6), 56-75. https://doi.org/10.1016/s0025-326x(00)00102-8

Mann, K. H. (1988). Production and use of detritus in various freshwater, estuarine, and coastal marine ecosystems. Limnology and Oceanography, 33(4part2), 910-930. https://doi.org/10.4319/lo.1988.33.4part2.0910

Orth, R. J., Heck, K. L., & Van Montfrans, J. (1984). Faunal communities in seagrass beds: A review of the influence of plant structure and prey characteristics on predator: Prey relationships. Estuaries, 7(4), 339. https://doi.org/10.2307/1351618

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January 10, 2026
Foraminifera: The Marsh’s Memory Keepers

What microscopic shells along Topsail and Surf City tell us about ancient seas, living marshes, and the future coastline

On a winter walk along the marsh edge in Topsail or Surf City, the landscape feels quiet. Cordgrass has faded to straw, tidal creeks run clear, and storm tides have pulled back layers of sediment that were hidden just months ago. Winter slows the marsh, but it also reveals it. Along exposed creek banks and tidal flats, the smallest residents of these ecosystems leave behind subtle traces — grains, spirals, and pin-sized shells that most people would mistake for sand.

These are the remains of foraminifera, key marsh indicators, and they carry a record far older than the marsh itself (Murray, 2006; Scott et al., 2001).

What Are Foraminifera?

Foraminifera, often called forams, are single-celled marine organisms — not animals, but protists — that live in oceans, estuaries, and salt marshes around the world (Murray, 2006). Despite their microscopic size, most foraminifera build protective shells, known as tests, made either from calcium carbonate or from tiny grains of sediment cemented together (Scott et al., 2001; Debenay & Guillou, 2002).

Different species occupy very specific zones within a marsh. Some live high in the intertidal, others closer to open water. Their distribution reflects precise environmental conditions such as salinity, tidal elevation, oxygen availability, and sediment type (Edwards et al., 2004; Culver & Horton, 2005). Because of this tight ecological coupling, foraminifera respond quickly when conditions change (Debenay & Guillou, 2002).

Peneropolis proteus is the one of three most dominant species of fossil foraminifera in the Onslow Bay area, occurring in about 15% of samples (Schnitker, 1971).

Why Winter Reveals the Record

In summer, marsh surfaces are busy and obscured. Dense vegetation, algae, burrowing organisms, and constant sediment mixing make it difficult to see what lies beneath. In winter, vegetation thins, biological activity slows, and storm tides rework creek edges and tidal flats. Fine sediments are redistributed, exposing layers that formed years, decades, or even centuries earlier (Scott et al., 2001; Gehrels, 1994).

Winter does not create this record — it simply makes it visible (Murray, 2006).

Size, Stability, and Ancient Seas

Some fossil foraminifera grew to the size of coins, while most living forms today are no larger than grains of sand (Murray, 2006). This contrast reflects the environments they evolved within. In ancient shallow seas, conditions were often warm, stable, and chemically consistent for long periods of time. Temperature, salinity, and carbonate availability changed slowly, allowing foraminifera to grow over many years, build thick and complex shells, and, in some cases, form partnerships with symbiotic algae — similar to the relationship between corals and the algae that live within their tissues — which provided an additional energy source through photosynthesis (Hallock, 1981; Murray, 2006). These systems favored persistence and size.

Over time, coastlines shifted and sea levels changed, giving rise to the highly dynamic estuaries and marshes we see today. In these modern environments, conditions can fluctuate over hours or seasons. Salinity rises and falls, oxygen levels vary, sediments are rearranged, and water chemistry responds quickly to storms and freshwater input (Debenay & Guillou, 2002; Culver & Horton, 2005). Under such variability, smaller foraminifera that grow rapidly and tolerate change are more likely to survive. Because foraminifera respond directly to these environmental conditions, even subtle shifts can reorganize their communities, altering shell size, composition, and diversity in ways that can persist in sediments long after the initial change has occurred (Edwards et al., 2004; Kemp et al., 2013).

Tiny Shells, Deep Time: How Marshes Remember

Foraminifera are among the most powerful tools scientists use to reconstruct ancient coastal ecosystems because the conditions they live in are permanently recorded in their shells. Individual species occupy narrow ecological ranges defined by salinity, tidal elevation, oxygen availability, temperature, and sediment type. Because of this specificity, the particular mix of foraminifera preserved in a layer of marsh sediment reflects the environmental conditions present when that layer formed.

When scientists extract sediment cores from marshes, they are not looking for isolated snapshots in time, but for transitions. As layers accumulate, changes in species composition, shifts between calcium-based shells and sediment-built shells, and variations in diversity reveal how marsh conditions evolved. These biological signals can indicate changes in flooding frequency, sediment stability, freshwater influence, and tidal reach — often aligning with known shifts in sea level or shoreline position.

What makes foraminifera especially valuable is that they record change continuously. Each generation reflects the conditions it experienced, leaving behind a layered biological archive that links past marshes to present ones — comparable to how sedimentary layers exposed in the Grand Canyon record changing environments over deep time.This continuity allows scientists to distinguish gradual environmental adjustment from more abrupt change and to assess whether modern conditions resemble states marshes have previously endured — or represent departures from historical patterns.

Quinqueloculina seminula (left) and Plancopsilina confusa (right) are the top three most dominant species of fossil foraminifera in the Onslow Bay area, each occurring in about 20% of samples (Schnitker, 1971).

What Lives in a Handful of Marsh Sand

If you scoop a small handful of sand or mud from a North Carolina marsh and let it dry, it looks ordinary—grains, bits of plant matter, flecks of shell. Where sediment cores reveal depth at the scale of decades and centuries, living marsh surfaces show that same pattern compressed into just a few centimeters. But research from the Outer Banks suggests that even this unremarkable material holds a surprisingly rich living community.

Foraminifera under biological microscope with sand.

In a detailed study of marsh sediments along the North Carolina coast, scientists examined not just which foraminifera were present, but which ones were alive at the time of sampling. What they found was not a thin layer of life resting at the surface, but a vertically structured community extending down into the sediment itself (Culver, 2005).

Some foraminifera lived right at the surface, where tides regularly wash over the marsh. Others occupied sediments a centimeter or more below, in darker, less oxygenated layers. In total, more than twenty species were documented living within marsh sediments, their distributions shaped by subtle differences in tidal flooding, salinity, and marsh elevation (Culver, 2005).

Not all species were equally widespread. A few, including Jadammina macrescens and Tiphotrocha comprimata, appeared across multiple sites and depths, suggesting a tolerance for changing marsh conditions. Many others were more selective, occurring only in certain zones or at particular depths. This means that even small changes in where you stand—closer to a tidal creek or higher on the marsh platform—can correspond to a different microscopic community beneath your feet (Culver, 2005).

Upper image: Jadammina macrescens under microscope.| Image credit: Parker, G. G., Phleger, et al. 1953. Cushman Found.Foram.Research Spec.Pub. (n.2): 15, pl.3,f.8.
Lower image: Tiphotrocha comprimata under microscope | Image credit: Hesemann, M., The Foraminifera.eu Database (2026). Accessed at http://www.foraminifera.eu.
https://doi.org/10.13140/RG.2.2.22727.11680/1.

As these organisms die, their shells remain. Layer by layer, those shells become part of the sediment, preserving a record of where tides reached, how often flooding occurred, and how stable the marsh surface was at that moment in time (Scott et al., 2001). What begins as a living community quietly becomes part of the marsh’s long-term record.

Although the Outer Banks are not identical to the marshes behind Topsail and Surf City, the pattern holds across North Carolina’s coast: foraminifera respond to local conditions at very small scales. Their presence, abundance, and depth within the sediment shift from place to place, reflecting the marsh’s relationship with water, salt, and time (Edwards et al., 2004; Culver & Horton, 2005).

Cibicidoides bradyi (horizontal scale bar = 200μm, vertical scale bar = 400μm) occur in less than 20 m at about 1% of samples in the Onslow County area (Schnitker, 1971).

For someone walking the marsh in winter, this means that the sand exposed along a creek bank carries more than the imprint of the last storm. It carries traces of countless tides before it—each one leaving behind shells small enough to escape notice, yet durable enough to remember.

What Changes in Foraminifera Mean for the Ecosystem

An example of how shifts in reef communities reflect shifts in foraminiferal communities below (Prazeres, Martínez-Colón & Hallock, 2020).

Foraminifera do not exist in isolation. They are part of the marsh food web, contributing to the transfer of energy and nutrients from microscopic primary producers to larger organisms (Murray, 2006). Many small invertebrates consume foraminifera directly, while others rely on the microbial communities and organic matter associated with their shells (Debenay & Guillou, 2002). In turn, these invertebrates support fish, crabs, and birds that depend on marsh productivity (Scott et al., 2001).

When foraminiferal communities shift, the effects can ripple outward. A decline in diversity or a move toward stress-tolerant species often reflects changes in sediment stability, oxygen availability, or salinity — conditions that also influence marsh plants, benthic invertebrates, and juvenile fish habitat (Culver & Horton, 2005; Edwards et al., 2004). In this way, changes in foraminifera can foreshadow broader ecological adjustments, even when the marsh surface still appears healthy (Debenay & Guillou, 2002).

Because foraminifera respond quickly to environmental change, they often register these shifts before larger organisms do. Their shells capture early signals of altered flooding patterns, reduced sediment input, or changing water chemistry (Gehrels, 1994; Kemp et al., 2013). What follows may be changes in plant community structure, altered nutrient cycling, or shifts in the species that use marshes as nursery grounds. Foraminifera do not cause these changes, but they reveal when the system’s internal balance begins to shift (Scott et al., 2001).

Reading Change in Living Marshes

Salt marshes are dynamic systems by nature. They grow, erode, migrate, and rebuild as sediment moves and sea level changes (Kemp et al., 2013). The challenge for scientists is distinguishing normal variability from directional change — shifts that push marshes beyond the conditions they have historically been able to tolerate. Foraminifera are especially useful in making that distinction because they respond quickly and directly to their surroundings (Debenay & Guillou, 2002).

When marsh conditions move outside typical ranges — whether through altered hydrology, changes in sediment supply, or shifts in salinity — foraminiferal communities reorganize. Species diversity may decline, stress-tolerant forms can become dominant, and assemblages tied to specific tidal elevations may disappear (Culver & Horton, 2005). These changes often occur before larger, more visible signs of stress appear, such as widespread plant die-off or shoreline erosion (Edwards et al., 2004). In this sense, foraminifera act as early responders, recording change while the marsh still appears intact at the surface (Scott et al., 2001).

Along the marshes behind Topsail and Surf City, this sensitivity gives foraminifera particular importance. They help establish local baselines for what healthy marsh conditions look like, provide context for interpreting present-day shifts, and preserve a record of the conditions that supported marsh stability in the past (Culver & Horton, 2005; Kemp et al., 2013). By linking modern observations to sedimentary records, foraminifera allow scientists to ask not only what is changing, but how quickly change is occurring and whether it remains within the range marshes have previously endured. Understanding marsh resilience in this way is not abstract or theoretical — it is grounded in the specific history and behavior of this coastline.

Salt marsh in Surf City, NC. | Photo credit: Mitchell (2026)

Closing

Standing at the marsh edge in winter, it is easy to miss the smallest details. Yet beneath the quiet surface, microscopic shells record centuries of change — how water moved, how shorelines shifted, and how marshes adapted (Murray, 2006). Foraminifera remind us that long before satellites or tide gauges, coastlines were already keeping their own records. All we have to do is learn how to read them.

References

Culver, S. J. (2005). Infaunal marsh foraminifera from the Outer Banks, North Carolina, U.S.A. The Journal of Foraminiferal Research, 35(2), 148-170. https://doi.org/10.2113/35.2.148

Debenay, J., & Guillou, J. (2002). Ecological transitions indicated by foraminiferal assemblages in paralic environments. Estuaries, 25(6), 1107-1120. https://doi.org/10.1007/bf02692208

Edwards, R., Wright, A., & Van de Plassche, O. (2004). Surface distributions of salt-marsh foraminifera from Connecticut, USA: Modern analogues for high-resolution sea level studies. Marine Micropaleontology, 51(1-2), 1-21. https://doi.org/10.1016/j.marmicro.2003.08.002

Gehrels, W. R., & Kemp, A. C. (2021). Salt marsh sediments as recorders of Holocene relative sea-level change. Salt Marshes, 225-256. https://doi.org/10.1017/9781316888933.011

Hallock, P. (1981). Algal symbiosis: A mathematical analysis. Marine Biology, 62(4), 249-255. https://doi.org/10.1007/bf00397691

Kemp, A. C., Horton, B. P., Vane, C. H., Berhhardt, C. E., Corbett, D. R., Engelhart, S. E., Anisfeld, S. C., Parnell, A. C., & Cahill, N. (2013). Sea-level change during the last 2500 years in New Jersey, USA. Quaternary Science Reviews, 81(2013), 90-104. https://www.whoi.edu/cms/files/Kemp2013QSR_170144.pdf

Murray, J. W. (2006). Ecology and applications of benthic foraminifera. Cambridge University Press.

Schnitker, D. (1971). Distribution of Foraminifera on the North Carolina Continental Shelf. Tulane Studies in Geology and Paleontology, 8(4), 169-215. https://journals.tulane.edu/tsgp/article/view/560

Scott, D. B., Medioli, F. S., & Schafer, C. T. (2001). Monitoring in coastal environments using foraminifera and Thecamoebian indicators. Cambridge University Press.

January 3, 2026