Sharks of Onslow County

Category: Marsh Grasses

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 Shapes the Tide Leaves Behind: Circles, Spirals, and the Mathematics of a Living Coast

Patterns in Nature Along the Coast

Each March 14, mathematicians celebrate π — the constant that links the circumference of a circle to its diameter. But Pi Day in nature appears everywhere along the coast: in boundaries that curve back upon itself, in ripples spreading across still water, in the rounded mouth of a burrow, in the arcs traced by a turning tide. Along the coast, these circles and spirals reveal patterns in nature that emerge so often they begin to feel less like abstract mathematics and more like a language written into sand and water. The shoreline is not calculating anything deliberately, yet the same relationships appear again and again as tides move sediment, organisms grow, and currents redistribute energy. What looks at first like scattered shapes — a curved creek channel, a ring of crab pellets, the fivefold symmetry of a sea star — gradually reveals itself as part of a larger pattern. The coast is full of geometry, briefly visible each time the water recedes.

The Creek Writes in Curves

A tidal creek bends around the marsh edge behind Surf City, where vegetation and sediment redirect the flow of draining water. These shifting boundaries gradually guide channels into widening curves that reappear with each tide. | Photo credit: A. Mitchell

At the creek mouths behind Topsail Island, the marsh edge redraws itself each time the tide drains away. Water retreats through narrow runnels that refuse straight lines, bending around grass hummocks and soft ridges, leaving a fan of nested arcs etched into exposed mud. The channels widen as velocity drops, sediment settling in fractions that record the rate of energy loss, so the surface becomes a temporary map of fluid negotiation.

These curves appear wherever moving water gradually redistributes energy rather than releasing it abruptly. In tidal landscapes, vegetation and sediment interact with flow in feedback loops that reshape channels over time, producing curved drainage networks whose geometry reflects both plant resistance and water momentum (Kirwan & Murray, 2007; Temmerman et al., 2007; Murray & Paola, 1994). Across river basins and tidal creeks alike, these evolving paths often approach widening spiral-like patterns as flow repeatedly adjusts to the boundaries around it (Rodriguez-Iturbe & Rinaldo, 1998).

Foam left behind by the falling tide sometimes dries into thin white filaments that trace these curves for a few quiet minutes before collapsing, a temporary record of motion fixed long enough to be read.

The creek does not preserve a single spiral. Each tide erases and redraws the same proportional tendency. The form emerges not from design but from the repeated redistribution of energy through water and sediment.

Geometry in the Grass

Dense stands of Spartina alterniflora divide space through repeating stem spacing. This structure slows water movement and traps suspended sediment, linking plant growth to the gradual elevation of the marsh surface. | Photo credit: A. Mitchell

Along the marsh margin, stems of Spartina alterniflora divide space through incremental adjustment. Leaves diverge from one another at angles that reduce overlap, distributing light capture through the canopy in repeating offsets that resemble packing patterns seen throughout plant growth.

Experiments in plant development show that when new structures arise under simple inhibitory fields, spiral-like arrangements often emerge as stable growth solutions (Douady & Couder, 1996). These patterns are widely recognized in plant morphology, where spacing between leaves or stems tends to distribute light and nutrients efficiently through the canopy (Niklas, 1997).

In salt marshes, this spacing carries ecological consequences beyond plant structure. Vegetation alters local water flow, slowing currents and promoting the deposition of suspended sediments that gradually elevate the marsh surface (Bouma et al., 2009; Fagherazzi et al., 2013; Leonard & Luther, 1995).

Mud crab burrows often appear in clusters whose spacing echoes the density of surrounding vegetation, each opening maintaining just enough distance to avoid collapse into the next.

Spiral shell growth of the periwinkle snail follows a repeating geometric expansion, allowing the animal to grow while maintaining the same overall shape. | Photo credit: A. Mitchell

Marsh periwinkles climb these stems in staggered lines that mirror the spacing of the leaves, their positions shifting with the tide yet repeatedly settling into the same angular arrangement.

Across the marsh platform, geometry quietly mediates the relationship between plant growth and landscape formation.

Spheres at the Mouth of a Burrow

A mud crab burrow at the edge of marsh vegetation marks the boundary between sand, grass, and moving water where patterns of spacing emerge. | Photo credit: A. Mitchell

Along the upper edge of the beach where grasses begin to anchor the sand, small clusters of rounded pellets often surround the entrances to crab burrows. At first glance they resemble scattered grains or fragments of dry sediment, but kneeling close reveals a more deliberate pattern.

Each pellet forms as damp sand excavated from underground tunnels passes through the crab’s mouthparts before being pushed back to the surface (Lucrezi et al., 2009). As the grains are rolled and compressed together, they settle into rounded shapes before drying in the coastal wind.

Among all possible forms loose material might take, the sphere encloses volume while minimizing surface area — a principle known as the isoperimetric property. When damp sand is compacted from many directions, the grains naturally settle toward this configuration.

The crab does not deliberately engineer spheres; the physics of granular material does the work. Similar rounding appears wherever particles compress together, from bubbles forming in foam to droplets condensing in clouds.

Around the burrow entrance, the pellets accumulate in loose arcs or clustered rings marking the repeated path of excavation. Studies of mud and ghost crab burrowing show that these excavated pellets form characteristic surface patterns around burrow openings as crabs repeatedly transport sediment from their tunnels (Lim & Diong, 2003; Chan et al., 2006).

Within hours the pellets dry and crumble back into ordinary sand. By the next tide the pattern may vanish entirely, erased by waves or shifting grains. Yet while they last, these small spheres record the intersection of animal behavior, sediment physics, and geometry.

Fivefold Bodies in the Wrack

Sand dollars show pentaradial symmetry — a five-part body plan shared by many echinoderms. The familiar white “sand dollar” is the skeleton left behind after the animal dies. Living sand dollars are gray or brown and covered in tiny moving spines that allow them to feed and move through the sand. In North Carolina, collecting live sand dollars is illegal; only empty tests found on the beach may be taken.| Image credit: Suzanne Campbell-O’Rahilly

Along the wrack line, sea stars rest without a preferred direction, their five arms distributing contact evenly across wet sand. Pentaradial symmetry divides the body into five equal sectors, stabilizing locomotion and feeding while allowing regeneration to proceed without disrupting balance (Beadle, 1989).

A broken sea star missing an arm still preserves the angle of the remaining four. The body reorganizes around absence without abandoning its underlying symmetry.

Sand dollars flatten this same geometry into a disk etched with five petal-like openings across the shell surface. These structures guide water across respiratory tissues while reinforcing the skeleton against bending forces generated by waves and sediment movement (Ellers & Telford, 1992; Mooi & David, 1998; Telford, 1981).

In shallow swash zones, freshly uncovered sand dollars often rotate edgewise until resistance equalizes, their circular outlines turning slowly with each pulse of water.

The etched flower is neither ornament nor accident. It records the intersection of circulation and structural strength — a geometry recalculated as abrasion reshapes the shell and burial depth shifts with each surge.

Across many biological systems, similar proportional relationships appear when living structures must distribute forces or transport materials efficiently through tissue networks (Ball, 1999).

Structure Where Sand Breaks

Hard structure embedded in soft sediment creates pockets where currents slow and animals find shelter, turning smooth bottoms into complex habitat. | Photo credit: A. Mitchell

Beneath the surface where oyster shells, coquina fragments, and storm-scattered debris interrupt the sand, the bottom shifts from smooth sediment to broken relief. In these pockets of structure, octopuses occupy cavities narrow enough to seal with the mantle.

Field observations show that octopus dens occur most frequently within crevice-rich substrates where structural complexity provides refuge and leverage for movement and defense (Anderson et al., 2002). Small fish hover near the edges of these openings, maintaining circular perimeters that expand and contract with the reach of a hidden arm. Juvenile sheepshead pick along shell ridges in repeating passes, their feeding paths tracing arcs that mirror the curvature of the structure beneath them.

Within these shelters, the eight arms of an octopus function as semi-independent mechanical units whose forces combine into coordinated motion (Mather & O’Dor, 1991). Much of this control occurs locally within the arms themselves, allowing rapid adjustment as the animal navigates complex surfaces.

As currents pass through these cavities, suspended particles settle into protected depressions, feeding microbial films that alter oxygen exchange and nutrient cycling along the bottom boundary. Structural geometry therefore governs not only animal behavior but also the micro-distribution of material across the seafloor.

Spirals Carried Offshore

As a thin sheet of water drains across the sand, it splits into branching paths that curve and merge before disappearing. These temporary channels briefly record how moving water redistributes energy along the shoreline. | Photo credit: A. Mitchell

Outside the inlet bars, a drifting boat leaves a wake that separates into tightening vortices. Each eddy contracts as it rotates, conserving angular momentum while turbulence redistributes energy through surrounding water.

Similar rotating structures form within rip currents, where narrow jets of water moving seaward generate circulation cells that trap plankton and suspended particles (Feddersen, 2014; MacMahan et al., 2006; Thorpe, 2005).

Fluid motion often organizes into spiraling paths under these conditions, reflecting the conservation of momentum within rotating systems (Longuet-Higgins, 1969; Peregrine, 1976).

Foam left behind by receding breakers sometimes curls into arcs that briefly echo shell fragments scattered across the wash.

Schools of baitfish caught at the margins of these rotations may briefly organize into crescent formations before the structure dissolves.

Incoming waves arrive in layered packets because slightly offset frequencies overlap and reinforce one another. When multiple rhythms travel through the same body of water, their interaction produces envelopes of larger motion surrounding smaller oscillations (Longuet-Higgins, 1969).

From the deck of a small boat these envelopes pass as broad rises containing finer pulses, a hierarchy of motion that continuously reshapes sandbars and sediment pathways along the coast.

Circles the Water Keeps

A fish briefly touching the surface sends expanding rings across the water, one of the simplest expressions of circular motion in nature. | Photo credit: A. Mitchell

At creek mouths and along nearshore bars, circles appear and vanish faster than the eye can catalogue them. These expanding rings are among the simplest patterns in nature, appearing whenever energy spreads outward through still water.

A ripple expands from a falling drop, its edge widening until it meets another wave and dissolves into interference. The distance around that circle always exceeds the span across it by the same proportion — the constant mathematicians call π.

Circular motion governs more than surface ripples. Tidal creeks bend into loops where erosion and sediment deposition redistribute its momentum along the channel edges that gradually produce curved meanders (Phillips, 1977; Temmerman et al., 2007; Seminara, 2006).

Within these bends, suspended sediment slows and settles, forming point bars that redirect flow during the next tidal cycle.

Offshore, rotating eddies may close into temporary rings that trap plankton and organic particles before dissolving again (MacMahan et al., 2006).

The circle becomes a moving boundary that regulates exchange while it lasts.

Proportion in a Moving Margin

Sunlight reflecting across shallow ripples reveals the repeating wave patterns that constantly reshape coastal sand flats. | Photo credit: A. Mitchell

Across marsh edge, wrack line, and nearshore water, similar patterns recur because natural systems governed by energy exchange tend to converge toward stable configurations.

Spiral drainage, fivefold symmetry, clustered leaf spacing, rotating vortices, and circular ripples represent different expressions of the same negotiation between force and structure.

Across biological and physical systems, recurring proportional relationships often emerge because they minimize energetic cost while maintaining stability (Ball, 1999; Cross & Hohenberg, 1993; Rodriguez-Iturbe & Rinaldo, 1998).

As sediment accumulates or erodes and vegetation thickens or thins, these geometric tendencies alter water residence time, root exposure, and nutrient retention within the marsh (Fagherazzi et al., 2013).

Each tide crosses the boundary again.

And each time it does, the coast recalculates its proportions.

References

Anderson, R. C., Wood, J. B., & Byrne, R. A. (2002). Octopus senescence: The beginning of the end. Journal of Applied Animal Welfare Science, 5(4), 275-283. https://doi.org/10.1207 /s15327604jaws0504_02

Ball, P. (1999). The self-made tapestry: Pattern formation in nature. Oxford University Press. https://avalonlibrary.net/ebooks/Philip%20Ball%20-%20The %20Self-Made%20Tapestry%20-%20Pattern%20Formation%20in%20Nature.pdf

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March 13, 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.

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January 10, 2026