Sharks of Onslow County

Category: Mollusc

Reading the Stories Written in Onslow County Shells

Walk almost any beach in Onslow County after the tide has gone out, and before long you’ll find yourself looking down.

A tiny orange shell catches your eye. Just inches away lies another, almost perfectly white. Nearby, a third carries shades of lavender and gray. An old, thick oblong shell rests beside them. A smooth round shell glistens in the afternoon sun while the heavy spiral of another waits half buried in the sand.

It feels like someone scattered an artist’s palette of Onslow County shells across the beach.

You notice how they are all so different and wonder why there are so many different colors.

It’s a simple question, but the answer reaches far beyond color.

Every shell on this beach is the product of millions of years of evolution solving the challenges of this particular coastline. Their colors, shapes, textures, thickness, and even the places we find them are all clues left behind by the animals that built them (Vermeij, 1974).

The shell isn’t simply something an animal lived in.

It is a record of how it survived.

And long after the animal is gone, the coast continues adding to that story.

The First Author

Every shell begins with a living animal.

Before a shell becomes a beach treasure, it is the home of a living animal. Hidden beneath the shell’s edge, the mantle quietly writes the shell one layer at a time throughout the animal’s life. | Image credit: A. Mitchell

Hidden just beneath the edge of the shell is a thin layer of tissue called the mantle. If the shell were a book, the mantle would be its author. Throughout the animal’s life, it slowly writes the shell, depositing layer upon layer of calcium carbonate, proteins, pigments, and minerals (Marin, 2012; Lowenstam & Weiner, 1989). As the animal grows, the shell grows with it.

A mollusk’s mantle doesn’t build a shell all at once. As the mantle grows, it continuously deposits new layers of calcium carbonate along the shell’s edge. Small differences in growth over time produce the incredible variety of shell shapes we find along the Carolina coast. | Figure credit: D. Cossins

The process is remarkably slow. The process is remarkably slow. A shell is never built all at once. Instead, it is written over days, months, and often years. Growth may speed up when food is plentiful, slow during winter or periods of stress, and even record repairs after surviving an attack from a predator (Marin, 2012). Every new layer becomes part of the shell’s permanent record.

Those layers preserve more than the shell itself. In many species, they preserve part of the animal’s history. While shells do not record age as neatly as the annual rings of a tree, scientists can often read growth bands to estimate an animal’s age, identify periods of environmental stress, and even reconstruct past water temperatures, salinity, and other environmental conditions from the chemistry locked inside the shell (Caudle et al., 1981).

Those orange, purple, yellow, white, and brown colors are not painted onto the shell after it forms.

The mantle creates them.

The remarkable variety of colors and patterns found in coquina clams begins with the mantle. As the shell grows, pigments deposited layer by layer create the unique combinations that make each shell part of its own story. | Image credit: S. Bland

As it deposits each new layer of shell, it also deposits tiny amounts of pigments. Different species produce different pigments, while genetics determine much of the pattern. Nutrition, water chemistry, and the environment can influence exactly how those colors are expressed, making every shell as individual as the life that built it. Before the shell ever reaches the beach, much of its color has already been written into its structure (Marin, 2012).

Sometimes the shell also records when conditions are less than ideal. Pollution, disease, and changing ocean chemistry can all influence how well a mollusk builds its shell. Ocean acidification, for example, can make it more difficult for some species to produce thick, durable shells, leaving behind thinner walls or irregular growth. The shell becomes more than protection. It preserves clues about the world the animal experienced while it was alive (Marin, 2012).

But color is only one chapter of the story.

The shell’s shape, thickness, texture, strength, and even the way it grows reveal something even more important.

They reveal the problems the animal had to solve.

Different Coastlines Ask Different Questions

At first glance, shells seem wonderfully diverse.

Look a little closer, and a pattern begins to emerge.

No two coastlines ask life to survive in exactly the same way.

A rocky Pacific shoreline presents different challenges than a tropical coral reef. The cold waters of New England ask different questions than the warm Gulf of Mexico. Even along the Atlantic Coast, the barrier islands, estuaries, marshes, and surf zones of coastal North Carolina create conditions unlike almost anywhere else (Riggs et al., 1995).

No two coastlines ask life to survive in exactly the same way. A shell-covered beach in La Jolla, California (left) reflects a different geologic history, wave energy, sediments, and marine communities than the shell-rich shores of Onslow County, North Carolina (right). Over millions of years, those different conditions shaped different evolutionary solutions. | Image credit: TraipseAndTiptoe (left); and T. Ruff (right)

Different predators.

Different sediments.

Different tides.

Different temperatures.

Different food.

Different problems.

Over millions of years, evolution answered those questions with different shells.

What washes onto our beaches is not random.

It is the collection of species whose solutions worked here (Vermeij, 1974).

Those questions have not remained the same. Over the past 66 million years, North Carolina’s shoreline has advanced and retreated countless times as sea levels rose and fell, rivers shifted course, climates warmed and cooled, and ancient oceans repeatedly flooded what is now dry land (Moslow & Heron, 1981; Riggs et al., 1995). Some shell designs disappeared as habitats changed. Others changed surprisingly little because the solutions they evolved continued to work.

Many of the shells we collect today belong to lineages that stretch back millions of years. Although the species themselves may have changed, the challenges of burrowing into sand, clinging to hard surfaces, escaping predators, or hunting beneath the seafloor have remained remarkably familiar. Every shell scattered across our beaches represents another evolutionary solution that succeeded on this coastline (Vermeij, 1974).

One Mineral. Many Solutions.

Every shell you’ll find on our beaches begins with the same basic building material: calcium carbonate.

It is one of the most common minerals used by living organisms, and yet from this single material evolution has produced an astonishing variety of designs (Lowenstam & Weiner, 1989; Marin, 2012).

An eastern oyster (left) and hard clam (right) are built primarily from the same mineral—calcium carbonate. Yet evolution shaped that shared material into two very different solutions for surviving along the Carolina coast. | Image credit: Original creator unknown

Some shells become thick fortresses.

Others become lightweight burrowing tools.

Some protected animals that never move more than a few inches in their lives.

Others belong to predators that spent their days hunting beneath the sand.

The mineral stayed the same.

The problems did not.

Each shell scattered across our beaches represents a different solution to surviving along the Carolina coast.

Oysters

Few animals have shaped North Carolina’s coast more than the eastern oyster (Crassostrea virginica).

An oyster’s greatest challenge is that it cannot run.

Once it settles as a young larva, it cements itself permanently to a hard surface. Every predator, every storm, every changing tide must be faced exactly where it stands. Escape is no longer an option (Grabowski & Peterson, 2007).

Its solution was to build.

Layer upon layer, the mantle produces an irregular shell that grows thicker and stronger over time. Those rough edges, deep ridges, and uneven shapes are not imperfections. They strengthen the shell and help neighboring oysters lock together into reefs far stronger than any one oyster could build alone (Marin, 2012).

That reef becomes one of the most important habitats along our coast. Small fish hide among the crevices. Juvenile shrimp and crabs find shelter between the shells. Countless worms, anemones, barnacles, and other invertebrates settle on its surface, creating an entire community built upon generations of oysters (Beck et al., 2011; Grabowski & Peterson, 2007).

An eastern oyster reef begins one shell at a time. As generations of oysters cement themselves to one another, they create living reefs that shelter fish, crabs, shrimp, worms, and countless other organisms along the Carolina coast. | Image credit: J. Utrup

People often describe oysters as filtering the water.

They do.

But filtering water is simply how an oyster feeds itself.

The reef—the ecosystem we admire—is what emerges from millions of oysters solving the same survival problem together (Grabowski & Peterson, 2007).

The shell reflects that strategy.

Not speed.

Not camouflage.

Permanence.

Coquina Clams

Walk a few hundred yards toward the surf, and the questions change.

Here the sand never stops moving.

Every incoming wave buries.

Every outgoing wave uncovers.

An oyster’s strategy would fail here.

The coquina clam (Donax variabilis) answered a different problem.

Its shell is small, smooth, lightweight, and remarkably varied in color. Instead of resisting the waves, it moves with them, burrowing into wet sand almost as quickly as each wave retreats. The same shifting surf that would bury many animals has become the rhythm that guides its entire life (Baird, 1960; Manning, 2003).

Those endless combinations of oranges, whites, yellows, purples, grays, and intricate patterns are among the most colorful shells found on our beaches. While their exact patterns are largely determined by genetics, that incredible variety may also help break up the outline of individual clams against the constantly changing mosaic of wet sand, shell fragments, and reflected sunlight.

No two coquina clams are exactly alike. The mantle deposits pigments into each new layer of shell, creating the remarkable variety of colors and patterns that make these tiny surf clams among the most recognizable treasures on Carolina beaches. | Image credit: A. Thamodharan

Coquinas don’t build habitat the way oysters do.

Instead, they move energy through the surf.

They filter microscopic algae and organic matter from the water before becoming food for fish, cownose rays, ghost crabs, gulls, and the flocks of sanderlings that race along the edge of the waves. During migration, entire flocks may depend on these tiny clams to fuel journeys spanning thousands of miles (Baird, 1960; Manning, 2003).

The shell reflects that role. Light enough to move with the surf. Strong enough to survive being tumbled by waves. Small enough to disappear beneath the sand in seconds.

Not because evolution intended to feed birds.

But because surviving here required an entirely different solution.

Moon Snails

Not every shell on our beaches belonged to prey.

Some belonged to hunters.

Moon snails (Neverita duplicata) spend much of their lives hidden beneath the sand, slowly searching for buried clams and other shellfish. Their large, muscular foot does most of the digging while their smooth, rounded shell slips easily through loose sediment without catching on sand or shell fragments (Grant, 2024; Witherington & Witherington, 2011).

The smooth, rounded shell of an Atlantic moon snail is more than beautiful. Its shape allows the animal to move easily through loose sand as it searches beneath the seafloor for buried clams and other shellfish. | Image credit: mattkeene1, iNaturalist

Finding prey is only half the challenge.

Opening another shell is the difficult part.

Rather than smashing their prey, moon snails use a remarkable combination of chemistry and patience. Holding the shell securely with their foot, they slowly rasp away the calcium carbonate with a rough tongue called a radula while releasing acidic secretions that soften the shell beneath. Hours later, all that remains is a nearly perfect circular hole (Grant, 2024).

Beachcombers often find these drilled shells without realizing they are looking at the evidence of one mollusk successfully hunting another.

The shell reflects that hidden lifestyle. Rounded instead of angular. Smooth instead of heavily sculptured.

Built not for resisting crashing waves, but for quietly moving through the sand in search of its next meal.

Whelks and Conchs

Some shells tell the story of animals that stayed in one place.

Others belonged to animals that never stopped moving.

Whelks and conchs are active travelers, spending much of their lives crawling across sandy bottoms, oyster reefs, and shallow estuaries in search of food. Their heavy spiral shells protect a surprisingly muscular animal capable of covering far more ground than most people realize (Magalhaes, 1948; Walker et al., 2008).

The long spiral of a lightning whelk shell provided room for a surprisingly muscular animal. Built for an active life spent searching the seafloor, the shell offered both protection from predators and space for the powerful foot and feeding structures that made whelks successful hunters and scavengers. | Image credit: gilbertgrant, iNaturalist

For many whelks, the challenge isn’t finding food.

It’s getting through another shell.

Using a long, extendable feeding tube called a proboscis, many species pry apart or drill into clams, oysters, and other shellfish. Others scavenge animals that have already died, recycling nutrients that would otherwise remain locked away on the seafloor. Along the way, they become prey themselves for larger fish, rays, sea turtles, and even other whelks (Magalhaes, 1948; Askin et al., 2022).

Their shells reflect that roaming lifestyle. Thick walls help defend against predators while the long spiral provides room for a muscular body that can withdraw deeply into the shell when threatened. Even the wide opening allows that powerful foot to extend far enough for steady movement across shifting bottoms.

The shell isn’t simply a home.

It is armor carried wherever the animal goes.

Scotch Bonnet

North Carolina’s state shell rarely washes ashore as often as oysters or coquinas, making each discovery feel a little more special.

The Scotch bonnet (Phalium granulatum) spends much of its life offshore on sandy bottoms where waves are gentler than those crashing onto the beach. There, it hunts worms and other small invertebrates hidden beneath the sediment (Grant, 2024; Witherington & Witherington, 2011).

Its shell reflects a different set of priorities.

Instead of thick ridges or heavy armor, the Scotch bonnet carries a smooth, rounded shell with delicate markings that blend surprisingly well among sand, shell fragments, and scattered gravel. The shell protects the animal while remaining compact enough for a life spent moving slowly across the seafloor (Grant, 2024; Witherington & Witherington, 2011).

North Carolina’s state shell, the Scotch bonnet, reflects a quieter life beyond the breakers. Its smooth, rounded shell protects a predator that spends much of its life moving across sandy seafloors in search of marine worms and other small invertebrates. | Image credit: gilbertgrant, iNaturalist

For many beachcombers, finding a Scotch bonnet feels like finding a rare treasure.

For the animal that built it, the shell was simply another successful solution to living in a quieter part of North Carolina’s coastal waters. One lived beyond the breakers where survival depends less on enduring crashing surf and more on navigating a different world beneath the waves.

Scallops

Not every shell relies on strength.

Some rely on surprise.

Unlike oysters, scallops – the Atlantic bay scallop (Argopecten irradians), Atlantic calico scallop (Argopecten gibbus), and the Atlantic sea scallop (Placopecten magellanicus) – never permanently attach themselves to the bottom. Although they often rest quietly on the seafloor, they can escape danger by doing something few other shellfish can.

They swim (Grant, 2024).

When threatened by sea stars, crabs, or other predators, a scallop rapidly opens and snaps its shell shut. Each clap forces a jet of water from the hinge, propelling the animal through the water in a series of short bursts (Grant, 2024).

The broad, fan-shaped shell makes this possible. Strong muscles close the shell with remarkable force while the evenly shaped valves help direct each burst of water.

The broad, fan-shaped shell of an Atlantic bay scallop reflects one of the most unusual escape strategies among shellfish. Rather than relying solely on armor, scallops can rapidly clap their shells together, jetting water from the hinge to swim away from predators. | Image credit: lmcconachie, iNaturalist

Scallops also possess dozens of tiny blue eyes along the edge of their mantle. Individually, each eye forms only a simple image, but together they help detect movement and approaching predators long before contact is made (Grant, 2024; Palmer et al., 2017).

Their shell reflects a life balanced between resting quietly on the bottom and escaping at precisely the right moment.

Sometimes survival isn’t about building thicker armor.

It’s about knowing when to leave.

Augers

Some predators chase.

Others wait.

Eastern augers (Neoterebra dislocata) spend much of their lives buried beneath the sand with only a small portion of their bodies exposed. From this hidden position they search for marine worms moving through the sediment (Grant, 2024; Witherington & Witherington, 2011).

Their shells are long, narrow, and remarkably slender. That shape isn’t simply beautiful.

The long, tapered shell of an auger snail reflects a life spent beneath the sand. Its slender shape allows the animal to move through loose sediment with little resistance as it hunts marine worms hidden below the surface. | Image credit: gmskupien, iNaturalist

It allows the animal to slip easily into the sand while occupying very little space as it burrows. Instead of pushing aside large amounts of sediment, the shell moves through it with surprisingly little resistance (Grant, 2024; Witherington & Witherington, 2011).

Many augers also possess venom that helps subdue their prey before swallowing it whole. Their role is quiet and rarely seen, yet they help regulate populations of worms living beneath the surface while becoming prey for larger animals in turn (Grant, 2024; Witherington & Witherington, 2011).

Like so many shells on our beaches, the auger’s design reflects an animal most people never realize is there.

Hard Clams (Quahogs)

Not every shell has to choose between staying still and constantly moving.

Hard clams, or quahogs (Mercenaria mercenaria), do both.

Most of their lives are spent buried beneath the estuary bottom with only their siphons reaching the surface. Hidden beneath the sand, they filter microscopic algae and organic matter from the surrounding water while remaining safely out of sight of many predators. But if conditions change, they can slowly pull themselves through the sediment in search of a better place to live (MacKenzie & Tarnowski, 2018).

Their shell reflects that balance.

The thick shell of a hard clam, or quahog, reflects a life spent buried beneath the sand. Strong, tightly closing valves protect the animal from predators while allowing it to remain hidden as it filters microscopic food from the water. | Image credit: rimcdon, iNaturalist

Unlike the thin shell of a coquina built for speed or the irregular fortress of an oyster reef, a hard clam carries a thick, rounded shell designed to withstand pressure from above. Crabs, rays, and whelks all present different dangers, and the clam’s tightly closing valves provide its best defense against them (MacKenzie & Tarnowski, 2018).

That shell also records an unusually long life. Some North Carolina hard clams live for decades, adding new growth each year as the mantle slowly deposits another layer of shell. Scientists can often estimate a clam’s age by studying those growth bands, much as foresters study the rings of a tree, although the story written in a shell is often more complicated than counting one band for every year (Caudle et al., 1981).

Like oysters, hard clams improve water quality simply by feeding.

Their shell reflects patience.

Rather than escaping danger, it protects an animal that survives by remaining hidden beneath the bottom.

Cockles

Some shells solve a different problem altogether.

Instead of burrowing deeply and staying put, cockles, like the giant Atlantic cockle (Dinocardium robustum), live much closer to the surface where waves, shifting sand, and hungry predators are constant challenges (Grant, 2024; Douglass, 1989).

Their answer is written in a shell unlike almost any other.

The deeply ribbed shell of an Atlantic cockle is more than decoration. Its strong ribs add strength without excessive weight, helping the animal withstand shifting sand and the pounding energy of the surf while it lives just beneath the surface. | Image credit: tceaton, iNaturalist

Strong, radiating ribs run from the hinge to the shell’s edge, strengthening the shell without making it excessively heavy. Those ridges act much like the folds pressed into a piece of cardboard, adding remarkable strength while using relatively little additional material. The rounded shape also helps distribute pressure from predators trying to crush the shell (Grant, 2024; Douglass, 1989).

Although cockles can burrow, many rely on quick movements near the sediment surface, even using their muscular foot to hop short distances when threatened.

The shell reflects that lifestyle. Not as smooth as a coquina. Not as heavy as a hard clam.

Instead, it balances strength with mobility in the constantly changing surf and shallow subtidal sands (Grant, 2024; Douglass, 1989).

Lion’s Paw Scallop

Few shells stop beachcombers in their tracks quite like a lion’s paw scallop (Nodipecten nodosus).

With its brilliant orange, red, or deep coral colors and bold knobby ribs, it hardly resembles the quieter shells scattered around it.

The bold ribs and flared spines of a lion’s paw scallop are more than striking. They strengthen the shell while helping it blend among oysters, rocks, and shell-covered bottoms, where this scallop spends its life. | Image credit: ahoppermann, iNaturalist

Its appearance reflects where it lives.

Unlike bay scallops that spend much of their lives on relatively shallow bottoms, lion’s paws are usually found farther offshore in deeper water where they rest among shell bottom, reefs, and hard substrates. Their heavy ribs strengthen the shell while making it more difficult for predators to crush (Rupp et al., 2005).

Like other scallops, lion’s paws can swim by rapidly clapping their shells together, jetting water from the hinge to escape danger. But unlike the delicate bay scallop, the lion’s paw invests more heavily in protection than speed, reflecting the different challenges of life in deeper coastal waters (Grant, 2024; Rupp et al., 2005).

Finding one washed onto an Onslow County beach usually means the ocean has done some of the traveling for it. Storms, strong currents, and changing tides occasionally carry these offshore shells landward, where they become one of the most treasured discoveries a beachcomber can make (Grant, 2024; Rupp et al., 2005).

The shell reflects both rarity and resilience.

Not because the animal evolved to become a collector’s prize.

But because its offshore home demanded a different solution than the shells living closer to shore.

The Coast Becomes the Second Author

The mantle stops writing the moment the animal dies.

The Carolina coast does not.

What remains is no longer simply a shell. It becomes part of an entirely different story, one written by waves, tides, storms, sunlight, other living organisms, and time itself (Kidwell & Bosence, 1991).

In many ways, the shell begins a second life.

Every tide carries it somewhere new. It may tumble through the surf for years before finally washing ashore. It may become buried beneath shifting sand, exposed again by the next storm, or carried into an estuary where the water, chemistry, and even the color of the bottom are entirely different from where the animal once lived (Kidwell & Bosence, 1991).

The shell that catches your eye today may have traveled miles from the habitat where it was first written.

When the Coast Changes the Color

Not every color you see was created by the animal.

Many are added later.

A bright shell left exposed on the open beach gradually fades as sunlight breaks down the pigments once produced by the mantle. Over time, oranges soften, purples become pale, and richly patterned shells may bleach almost completely white (Kidwell & Bosence, 1991).

Carry that same shell into a quiet salt marsh, and the story changes.

The dark mud beneath the marsh is rich in decaying plant material, bacteria, and low-oxygen sediments. As shells rest there, they often become stained shades of gray, brown, or nearly black. Beachcombers are sometimes surprised to find black shells scattered along an estuary, but the color usually comes from the marsh itself rather than the animal that built the shell.

Elsewhere, iron-rich sediments may leave rusty orange stains. Thin films of algae can tint shells green, while mineral deposits and other chemical reactions slowly alter their appearance over months or years (Kidwell & Bosence, 1991).

The mantle wrote the original color into both of these lion’s paw shells. After the animals died, the Carolina coast continued writing the story. Sunlight, marsh sediments, algae, minerals, and other marine organisms gradually changed their appearance long after the shells were formed. | Image credit: nikole14, iNaturalist

Sometimes those changes tell us almost as much as the shell itself.

A bright, colorful shell may have spent little time exposed after the animal died.

A heavily bleached shell may have rolled through the surf for years.

A blackened shell may have rested quietly in marsh mud before tides carried it back onto the beach.

The coast has been editing the story.

Every Scar Has a Story

Color is only one way the coastline leaves its mark.

Look closely and you’ll often find scars, chips, holes, and rough edges that formed long after the animal was gone.

Rolling waves grind shells against sand and one another, slowly rounding sharp edges until they become smooth enough to fit comfortably in the palm of your hand. Storms break larger shells into fragments before carrying them into entirely different habitats (Kidwell & Bosence, 1991).

Barnacles cement themselves onto abandoned shells. Bryozoans spread across their surfaces like delicate lace. Boring sponges and marine worms slowly tunnel through the calcium carbonate, creating tiny chambers where other organisms eventually move in (Kidwell & Bosence, 1991).

Even after death, the shell continues providing shelter. Some become homes for hermit crabs. Others become attachment sites for young oysters beginning reefs of their own. Small fish hide among broken shell piles while countless microscopic organisms colonize every available surface (Kidwell & Bosence, 1991) .

A shell’s story doesn’t end when the animal dies. Empty shells become homes for hermit crabs, extending their usefulness long after the original builder is gone. | Image credit: A. Mitchell

The shell no longer protects the animal that built it.

Instead, it begins supporting an entirely new community.

Eventually, Even Shells Become the Beach

The Carolina coast wastes very little.

As shells continue breaking apart, the fragments become smaller and smaller until many are no longer recognizable as shells at all.

They become part of the sand (Moslow & Heron, 1981; Riggs et al., 1995).

On many North Carolina beaches, tiny pieces of shell are mixed with quartz grains carried from the Appalachian Mountains by rivers over millions of years. Every handful of sand is a mixture of geology and biology, mountains and oceans, living animals and ancient rock (Moslow & Heron, 1981; Riggs et al., 1995).

Viewed under magnification, beach sand reveals countless tiny shell fragments mixed among mineral grains. Many of the shells we collect today will eventually weather into pieces so small they become part of the next generation of Carolina beaches. | Image credit: A. Mitchell

Even after the shell disappears, it remains part of the coastline.

Its story simply changes again.

Every Shell Has Two Authors

We often collect shells because they are beautiful.

Beauty is usually the first thing we notice. But it is rarely the most interesting thing they have to offer.

Every shell begins with a living animal whose mantle slowly writes a record of its life—its growth, its habitat, its challenges, and the evolutionary solution that allowed it to survive.

After the animal dies, the Carolina coast picks up the story.

Sunlight softens the colors. Marshes stain them. Waves polish them. Other animals bore through them, build upon them, and make them part of their own lives.

What you hold in your hand is no longer simply the remains of a mollusk.

It is a story with two authors.

One wrote the shell. The other never stopped editing it.

The next time you find a shell along the beaches of Onslow County, look beyond its color.

Notice the thickness. The curves. The ridges. The scars. The stains. The tiny holes.

Each one is a clue.

Each one is another sentence in a story that began with a living animal and continues with every tide that reaches our shore.

Every shell scattered across the beach carries two stories: one written by the animal that built it and another written by the Carolina coast long after the animal was gone. Every tide leaves behind another collection of those stories, waiting to be discovered. | Image credit: A. Thamodharan

References

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Williams, A. B., & Porter, H. J. (1971). A ten-year study of Meroplankton in North Carolina estuaries: Occurrence of Postmetamorphal bivalves. Chesapeake Science, 12(1), 26. https://doi.org/10.2307/1350499

Witherington, B., & Witherington, D. (2011). Seashells of Georgia and the Carolinas. Pineapple Press.

July 18, 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

Beadle, S. C. (1989). Ontogenetic regulatory mechanisms, heterochrony, and eccentricity in dendrasterid sand dollars. Paleobiology, 15(3), 205-222. https://doi.org/10.1017/s0094837300009428

Bouma, T. J., Friedrichs, M., Van Wesenbeeck, B. K., Temmerman, S., Graf, G., & Herman, P. M. (2009). Density‐dependent linkage of scale‐dependent feedbacks: A flume study on the intertidal macrophyte Spartina anglica. Oikos, 118(2), 260-268. https://doi.org/10.1111/j.1600-0706.2008.16892.x

Chan, B. K., Chan, K. K., & Leung, P. C. (2006). Burrow architecture of the ghost crab Ocypode ceratophthalma on a sandy shore in Hong Kong. Hydrobiologia, 560(1), 43-49. https://doi.org/10.1007/s10750-005-1088-2

Cross, M. C., & Hohenberg, P. C. (1993). Pattern formation outside of equilibrium. Reviews of Modern Physics, 65(3), 851-1112. https://doi.org/10.1103/revmodphys.65.851

Douady, S., & Couder, Y. (1996). Phyllotaxis as a dynamical self organizing process part II: The spontaneous formation of a periodicity and the coexistence of spiral and whorled patterns. Journal of Theoretical Biology, 178(3), 275-294. https://doi.org/10.1006/jtbi .1996.0025

Fagherazzi, S., Mariotti, G., Wiberg, P., & McGlathery, K. (2013). Marsh collapse does not require sea level rise. Oceanography, 26(3), 70-77. https://doi.org/10.5670/oceanog.2013.47

Feddersen, F. (2014). The generation of Surfzone eddies in a strong alongshore current. Journal of Physical Oceanography, 44(2), 600-617. https://doi.org/10.1175/jpo-d-13-051.1

Kirwan, M. L., & Murray, A. B. (2007). A coupled geomorphic and ecological model of tidal marsh evolution. Proceedings of the National Academy of Sciences, 104(15), 6118-6122. https://doi.org/10.1073/pnas.0700958104

Leonard, L. A., & Luther, M. E. (1995). Flow hydrodynamics in tidal marsh canopies. Limnology and Oceanography, 40(8), 1474-1484. https://doi.org/10.4319 /lo.1995.40.8.1474

Lim, S. S., Tantichodok, P., & Yong, A. Y. (2011). Comparison of burrow morphology of juvenile and young adult Ocypode ceratophthalmus from sai Kaew, Thailand. Journal of Crustacean Biology, 31(1), 59-65. https://doi.org/10.1651/10 -3314.1

Longuet-Higgens, M. S. (1969). On the joint distribution of wave periods and heights. Journal of Marine Research, 27, 1-16.

Lucrezi, S., Schlacher, T. A., & Walker, S. (2009). Monitoring human impacts on sandy shore ecosystems: A test of ghost crabs (Ocypode spp.) as biological indicators on an urban beach. Environmental Monitoring and Assessment, 152(1-4), 413-424. https://doi.org/10.1007/s10661-008-0326-2

MacMahan, J. H., Thornton, E. B., & Reniers, J. H. (2006). Rip current review. Coastal Engineering, 53(2-3), 191-208. https://doi.org/10.1016/j.coastaleng.2005.10.009

Mather, J., & O’Dor, R. (1991). Foraging strategies and predation risk shape the natural history of juvenile Octopus vulgaris. Bulletin of Marine Science, 49(1-2), 256-269.

Mooi, R., & David, B. (1998). Evolution within a bizarre phylum: Homologies of the first echinoderms. American Zoologist, 38(6), 965-974. https://doi.org/10.1093/icb/38.6.965

Murray, A. B., & Paola, C. (1994). A cellular model of braided rivers. Nature, 371(6492), 54-57. https://doi.org/10.1038/371054a0

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Rodriguez-Iturbe, I., Rinaldo, A., & Levy, O. (1998). Fractal river basins: Chance and self-organization. Physics Today, 51(7), 70-71. https://doi.org/10.1063/1.882305

Seminara, G. (2006). Meanders. Journal of Fluid Mechanics, 554, 271-297.

Telford, M. (1981). Structural analysis of the test of echinoids. Zoomorphology, 98, 93-110.

Temmerman, S., Bouma, T., Van de Koppel, J., Van der Wal, D., De Vries, M., & Herman, P. (2007). Vegetation causes channel erosion in a tidal landscape. Geology, 35(7), 631. https://doi.org/10.1130/g23502a.1

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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Valentine, J. F., & Duffy, J. E. (n.d.). The central role of grazing in seagrass ecology. Seagrasses: Biology, Ecology and Conservation, 463-501. https://doi.org/10.1007/1-4020-2983-7_20

Zhang, G., Bai, J., Jia, J., Wang, W., Wang, D., Zhao, Q., Wang, C., & Chen, G. (2023). Soil microbial communities regulate the threshold effect of salinity stress on SOM decomposition in coastal salt marshes. Fundamental Research, 3(6), 868-879. https://doi.org/10.1016/j.fmre.2023.02.024

January 10, 2026
The 12 Days of Estuary Christmas | New River Estuary

In the season of chilly tides and twinkling pier lights, the New River estuary doesn’t quiet down — it parties in its own salty way. So grab your cocoa, bundle up, and join us for a winter countdown of festive fins, feathers, and the ecological magic beneath the misty surface.

(Sing along if you dare — apologies in advance.)

Day 12: Twelve Dolphins Dancing

Bottlenose dolphins along the mid-Atlantic coast shift into cooperative foraging teams in the cooler months — synchronized movements that feel almost choreographed (Torres & Read, 2009). Their leaping, circling, and flipper-flicking tactics help herd fish just like dancers driving the story across a winter stage.

Cue underwater Nutcracker ballet.

Day 11: Eleven Stripers Schooling

Atlantic striped bass move into estuarine channels when the water cools, fueling popular winter fisheries (Boyd, 2011).

Cold water? Hot bite.

Day 10: Ten Blue Crabs Burrowing

Blue crabs overwinter right here — burrowed into sediment, metabolism slowed, waiting for spring, or when water temperatures rise above 9℃ (Glandon, Kilborn & Miller, 2019).

The ultimate cozy blanket fort.

Day 9: Nine Oysters Filtering

Oysters continue filtering water through the winter, though more slowly — still improving water quality and boosting biodiversity (Grabowski & Peterson, 2007).

Nature’s tiny elves never clock out.

Day 8: Eight Croakers Drumming

Atlantic croaker remain common in NC coastal waters during cooler months, shifting to deeper estuarine areas (Miller et al., 2003).

Rumble, rumble — underwater holiday percussion.

Day 7: Seven Specks Still Striking

Speckled seatrout stay active in winter, especially in deeper holes and marsh channels where prey concentrates and water temperatures remain above 7℃ (Ellis, Buckle & Hightower, 2017).

Even cold-blooded fish love a good holiday snack.

Day 6: Six Sharks Snow-Birding

Juvenile coastal sharks like sandbars and sharpnose depart estuaries in late fall, migrating offshore and southward (Bangley et al., 2018).

“See you after the thaw!”

Day 5: FIVE… OYS-TER REEFS!

Oyster reefs provide the essential winter housing market — structured refuge for juvenile fish, crustaceans, and invertebrates (Coen et al., 2007).

Deck the reefs with beds and breakfasts..

Day 4: Four Buffleheads Diving

These small sea ducks, buffleheads, arrive from the Arctic and forage in our coastal waters all winter long (Gauthier, 2014).

Feathered travelers escaping the Arctic freeze.

Day 3: Three Terrapins Burrowed

Diamondback terrapins overwinter in marsh sediments, lowering heart rate and waiting out the cold (Harden, Midway & Willard, 2015).

A brumation vacation.

Day 2: Two Menhaden Shoals

Atlantic menhaden form huge winter schools offshore and near inlet mouths, fueling predator energy budgets (Orth, 2023).

The estuary’s holiday punch bowl.

Day 1: And a Red Drum in the Mar-sh-Tree

Red drum remain year-round, feeding in creeks and marsh edges even in winter low-temp slow-motion (Bacheler et al., 2009).

Our coastal Christmas (and state) mascot.

The Estuary Never Sleeps

Even as we wrap gifts and check lists twice, life beneath the cold surface hustles on — feeding, moving, filtering, and keeping the New River ecosystem healthy through the darkest season.

So here’s to the citizens of our winter waters —
May your tides be merry and bright!

References

Bacheler, N., Paramore, L., Buckel, J., & Hightower, J. (2009). Abiotic and biotic factors influence the habitat use of an estuarine fish. Marine Ecology Progress Series, 377, 263-277. https://doi.org/10.3354/meps07805

Bangley, C. W., Paramore, L., Dedman, S., & Rulifson, R. A. (2018). Delineation and mapping of coastal shark habitat within a shallow lagoonal Estuary. PLOS ONE, 13(4), e0195221. https://doi.org/10.1371/journal.pone.0195221

Boyd, J. B. (2011). Maturation, fecundity, and spawning frequency of the Albemarle/Roanoke striped bass stock (2011. 1510474) [Doctoral dissertation]. ProQuest Dissertations and Theses Global.

Coen, L., Brumbaugh, R., Bushek, D., Grizzle, R., Luckenbach, M., Posey, M., Powers, S., & Tolley, S. (2007). Ecosystem services related to oyster restoration. Marine Ecology Progress Series, 341, 303-307. https://doi.org/10.3354/meps341303

Ellis, T., Buckel, J., & Hightower, J. (2017). Winter severity influences spotted seatrout mortality in a southeast US estuarine system. Marine Ecology Progress Series, 564, 145-161. https://doi.org/10.3354/meps11985

Gauthier, G. (2014, July 14). Bufflehead – Bucephala albeola. Birds of the World – Cornell Lab of Ornithology. Retrieved November 29, 2025, from https://birdsoftheworld.org/bow/historic/bna/buffle/2.0/introduction

Glandon, H. L., Kilbourne, K. H., & Miller, T. J. (2019). Winter is (not) coming: Warming temperatures will affect the overwinter behavior and survival of blue crab. PLOS ONE, 14(7), e0219555. https://doi.org/10.1371/journal.pone.0219555

Grabowski, J. H., & Peterson, C. H. (2007). Restoring oyster reefs to recover ecosystem services. Theoretical Ecology Series, 281-298. https://doi.org/10.1016/s1875-306x(07)80017-7

Harden, L. A., Midway, S. R., & Williard, A. S. (2015). The blood biochemistry of overwintering diamondback terrapins (Malaclemys terrapin). Journal of Experimental Marine Biology and Ecology, 466, 34-41. https://doi.org/10.1016/j.jembe.2015.01.017

Mead, J. G., & Potter, C. W. (1995). Recognizing two populations off the bottlenose dolphin (Tursiops Truncatus) of the Atlantic coast of North America-Morphologic and Ecologic Considerations. https://repository.si.edu/server/api/core/bitstreams/9c563919-2b27-4ac4-bba1-92e7d090fd72/content

Orth, D. J. (2023). Fish, fishing and conservation. Blacksburg: Virginia Tech Department of Fish and Wildlife Conservation.Torres, L. G., & Read, A. J. (2009). Where to catch a fish? The influence of foraging tactics on the ecology of bottlenose dolphins (Tursiops truncatus) in Florida Bay, Florida. Marine Mammal Science, 25(4), 797-815. https://doi.org/10.1111/j.1748-7692.2009.00297.x

December 13, 2025
The Moonlit Assassin: The Stealth of the Atlantic Moon Snail
On a stroll along the damp sand of the tide line, just after dusk under the soft glow of the moon over Topsail or North Topsail Beach, you might spot a round, glossy shell that looks more like a polished marble than a living creature. This little orb belongs to the Atlantic moon snail or Shark Eye (Neverita duplicata) — smooth and innocent-looking by day, but by night the script flips. It becomes a stealthy predator disguised in plain sight, turning the quiet sand into a midnight hunting ground.

A Shell with a Secret

To the casual beachcomber, the moon snail’s shell looks harmless — a perfect spiral polished smooth by the tide. But that rounded shape is built for stealth and movement beneath the sand. Moon snails bury themselves under the flats and inlets of sandy shores, waiting to pounce. Their favorite prey? Soft-bodied bivalves — clams, coquinas, and other shelled animals that live partly buried in sand.

The moon snail doesn’t wait for prey to drift by; it actively hunts. Using its broad, muscular foot, it glides unseen beneath the surface, sensing vibrations of hidden clams. Once it finds a meal, it envelops the prey in its foot like a slow-moving trap — then the drilling begins.

A rough tongue called a radula rasps the surface while secreted acid softens the calcium shell. Hours later, a single round hole opens a doorway to dinner. Every drilled shell that washes up tells the same story. These beveled, countersunk holes are so distinctive that scientists can identify the species of moon snail just by the shape of the bore (Grey & Bounding, 2005; Dietl & Kelley, 2006).

That neat, circular hole — the hallmark of moon snail predation — is what beachcombers often discover without realizing the drama that unfolded beneath their feet.

Have you seen shells with a perfect hole in them? This is evidence of predation by moon snail.

Hunting Under the Sand

Unlike many beach creatures you spot on the surface, the moon snail is mostly hidden. During low tide, it may creep near the edge of the exposed flats; at other times, it lies just beneath the surface. When it finds a buried bivalve, it uses a combination of mechanical drilling (via the radula) and acidic secretions to bore through the shell until it can reach the soft tissues inside (Visaggi, Dietl, & Kelley, 2013).

In experimental trials, moon snails were observed to prefer drilling over suffocating their prey, even when the prey was buried at different sediment depths (Visaggi et al., 2013). That means whether the sand is shallow or deeper, the snail still goes for the drill rather than waiting for the prey to weaken.

But that’s not all — stable-isotope studies, which measure chemical fingerprints of diet, show that the moon snail’s feeding habits may be more flexible than once thought. It appears to feed not only on animal prey but occasionally on other resources, hinting at omnivory (Casey, Fall, & Dietl, 2016). In other words: it’s a predator, yes — but one with a backup plan when food runs low.

Why It Matters (and Why It’s Cool)

1. Predator and Prey in One
The moon snail helps regulate bivalve populations, preventing a single species from dominating the sand flats. In doing so, it maintains habitat diversity. At the same time, it transfers energy upward in the food chain by becoming prey for larger predators — fitting neatly into the middle of the coastal food web.

2. Ecosystem Engineer
Burrowing, hunting, and leaving behind drilled shells change the physical landscape of the beach. Each empty shell, each “crime scene” hole, becomes part of the microhabitat for small invertebrates, microbes, and future shell-seekers. In this way, the moon snail contributes to sediment health and biodiversity.

3. The Hidden Drama Beneath Your Feet
From a beachcomber’s perspective, the moon snail is a perfect spooky-season protagonist. Picture it: a “moon” shell glinting under moonlight, a buried assassin beneath your feet, and a perfect little hole in a clam shell — the evidence of a tiny midnight crime.

The Perfect Crime

When a clam is finished, only the perfect circular hole remains — a tiny signature in the sand. Moon snails rarely suffocate their prey; they almost always drill, no matter how deep the clam is buried (Visaggi, Dietl, & Kelley, 2013). This ancient behavior has marked shells for millions of years — the same holes appear in fossils that predate the Carolina coastline.

And when food runs low, the story turns darker. In crowded flats where competition is fierce, moon snails have been observed turning their drills on each other. Cannibalism, rare in most mollusks, becomes a desperate survival strategy in the world beneath the sand (Gould, 2010).

Drilled Atlantic moon snail shell shows cannibalism | Photo credit: iNaturalist, plawrynpx

The Ripple Beneath the Sand

The moon snail’s creeping movements do more than tell tales of horror. Its trails of mucus — left behind as it slides through the sand — subtly transform the beach. That film of slime “primes” the sediment, helping microbes break down organic matter faster and releasing nutrients back into the ecosystem (Hannides & Aller, 2016).

Atlantic moon snail in the sand | Photo Credit: D. Bray

So while it’s a killer to clams, it’s also a quiet caretaker. Its hunting churns the sand, its slime fuels the microbes, and its discarded shells shelter the next wave of tiny life.

The Balance of the Hunt

Every act of predation keeps the beach in balance. Moon snails are secondary consumers, feeding on clams that filter plankton from the water. In doing so, they prevent any single species from overrunning the flats. And they, in turn, become prey for blue crabs, rays, and drum fish that patrol the shallows.

Each neat hole in a shell is a record of that balance — energy passed from clam to snail to crab to ocean. Nothing wasted. Everything connected.

For Beachwalkers in Onslow County

If you’re exploring the sands of Onslow County — Topsail Island, North Topsail, Surf City, or the New River Inlet — here’s what to watch for:
- Round, glossy shells washed up on the tide flat — the moon snail’s shell is distinct, almost like a little orb.
- Sand collars: doughnut-shaped, jelly-like structures found after storms or low tide. These are the moon snail’s egg masses, eerie “rings” that crumble when dry.
- Drilled bivalve shells: clams or coquinas with a neat circular hole — the aftermath of a moon snail meal.
- Time and place: Visit during low tide, especially after a storm that exposes more of the flat. The sand will tell its stories. Wear shoes, tread lightly, and remember — much of the life here remains hidden.
Atlantic moon snail sand collar | César Andrés Castillo (c) 2019

Final Thought

It’s easy to write off a pretty shell as just another beach find. But every shell, hole, and collar tells a story of life beneath the waves. The moon snail wears the moon’s name well — luminous, secretive, and always working in the dark. Its beauty hides its appetite; its predation hides its purpose.

In its own way, it keeps the beach breathing — a tiny engineer of life and death that turns sand, shell, and shadow into an endless cycle. The next time you find that round shell glinting in the surf, you’re not just finding a souvenir — you’re finding the ghost of a hunter in the sand.

References

Casey, M. M., Fall, L. M., & Dietl, G. P. (2016). You are what you eat: Stable isotopic evidence indicates that the naticid gastropod Neverita duplicata is an omnivore. Frontiers in Ecology and Evolution, 4, 125. https://doi.org/10.3389/fevo.2016.00125

Dietl, G. O., & Kelley, P. H. (2006). Can naticid gastropod predators be identified by the holes they drill? Ichnos, 13(2), 103–113. https://doi.org/10.1080/10420940600842979

Gould, E. S. (2010). Unexpected rates of cannibalism under competitive conditions by the naticid gastropod Neverita duplicata. Marine Biology, 157(10), 2341–2349. https://doi.org/10.1007/s00227-010-1505-8

Grey, M., & Bounding, E. G. (2005). Shape differences among bore holes drilled by three species of naticid gastropods. Palaeogeography, Palaeoclimatology, Palaeoecology, 221(3–4), 245–260. https://doi.org/10.1016/j.palaeo.2005.02.003

Hannides, A. K., & Aller, R. C. (2016). Priming effect of benthic gastropod mucus on sedimentary organic matter remineralization. Marine Ecology Progress Series, 545, 77–88. https://doi.org/10.3354/meps11587

Visaggi, C. C., Dietl, G. P., & Kelley, P. H. (2013). Testing the influence of sediment depth on drilling behaviour of Neverita duplicata (Gastropoda: Naticidae), with a review of alternative modes of predation by naticids. Journal of Molluscan Studies, 79(4), 310–322. https://doi.org/10.1093/mollus/eyt023
October 27, 2025