Swashbuckling Life

[HenryC]

From Florida Wildlife, May/Jun '06

Is it just me?   Or are Coquina and sandbugs not as common as they used to be?


A sandy beach is a desert, even along the fertile Florida coast.  Take a walk along the edge of the sea and at first glance it appears devoid of life.  There is no cover, no food, only the piles of weed and driftwood infrequently washed up on it.  The mounds of crushed shells and the occasional carcass or body fragment of some dead creature are the only obvious evidences of life. The waves pound ceaselessly against the hard sand and move it about, constantly building and tearing it down, sculpting the eternal grains and sorting the debris of the shore into neat piles of similarly sized fragments.  The narrow strip between high and low tides is bounded on one side by the lifeless sand and on the other by a turbulent and restless surf.  It is an ephemeral and temporary place, like the dunes of the desert, but it also appears as sterile and eternal as the bare rocks of alpine cliffs.

But like any desert, there is life there, close under the surface.  Beneath the wet sand, in the thin film of salt water that cushions each sand grain from its countless neighbors are endless billions of microscopic creatures. Each wave brings a fresh load of plankton, the tiny plants and animals that are the base of the marine food chain and that float on the surface of the sea, a thin broth of living things upon which every creature in the sea ultimately depends, even us.  In this harsh environment, the larger creatures, those we can see with our unaided eye, stay hidden.  They hide beneath the sand, prowling ceaselessly as they feed on each other and hide from attack by predators from water, air and land.  At the very upper edge of the sand, where the earth meets the atmosphere,and where the constant wave action alternately exposes and submerges that boundary is a thin film of salty, foamy water.  There is life there too, and if you look carefully you can see it and reach out and touch it.

This unyielding environment, the meeting place of earth, sea and sky, has been colonized by two entirely different creatures, each evolved to thrive here and nowhere else.  They are both small animals, roughly an inch long, but they betray their presence to even the most casual beachcomber by their numbers.  They live in colonies of hundreds, even thousands, and they are in constant motion up and down the swash ( the narrow strip of sand alternately covered and exposed by the endless surf). There is constant activity here as they continuously reposition themselves to exploit the nutrient-rich waters that flow back and forth across the glistening sand. They come and go with the seasons and the tides; sometimes the beach is totally devoid of them, at other times they form patches several yards across and several yards apart where they crowd the sand so closely packed that you can feel them tunneling about your bare feet as you pause in the ankle deep water rushing back to sea. They are both totally harmless creatures, they won't hurt you and you won't hurt them if you pick them up and take a closer look at them.  Just put them back when you're done, they deserve to live too. They are the mole crab and the coquina clam.

The mole crab (Emerita talpoida) is a little crustacean shaped more or less like a Volkswagen beetle.  It's round white carapace is just the right shape to absorb the pounding of the surf and tunnel into the wet sand when it is loosened by rushing water.  Underneath the shell, its many legs dig out the soup of water and grains and it slides backwards into the sand until only its face and two brushy antennae are left exposed, facing the sea.  As the water from each wave returns to the surf, the antennae deploy and entangle the minute micro-organisms suspended in the thin film of seaward flow.  The antennae are then retracted and the food particles removed by their mouth parts. Every wave is a meal, and as the water flows back the shallow stream splits at the exposed head of each critter, making tiny little V's in the slip stream, the sharp ends pointing towards the land. When the tide or the wave action changes the shape or position of the swash, the mole crabs shake themselves loose and let the waves carry them to a new spot where the water depth will be just right for feeding.  Somehow, they manage to do this all at once so that what appears to be a plain stretch of sand and water suddenly explodes into hundreds of little white bodies scurrying about, swimming furiously backwards and submerging themselves at the same time into the sand, ready for the next wave.  Whether they do this because they all sense simultaneously that it is time, or whether they somehow coordinate their actions by some kind of signal, no one knows, but the result is the same.  By moving together, they dazzle and confuse predators and even the sharpest and most alert seagull will only have time to catch one of them.  Many feed on the mole crabs, birds constantly walk the beaches for them and those unlucky enough to get washed out  will find the near-shore waters well patrolled by a variety of fish ready to take the stragglers.  Ghost crabs dart out from the dunes and strike without warning.  Fishermen call them "sand bugs" and find them ideal bait for snook, whiting, and channel bass. The swash is a deadly place.

The swash is also the home of the coquina (Donax variabilis), a bivalve mollusc about half the size of your thumbnail.  But like their crustacean colleagues, they make up in numbers what they lack in size. They inhabit Florida beaches by the billion, so many that their dead shells, sorted and collected and compacted by the forces of coastal geology, form layers of rock that were quarried by the Spaniards to build their forts.  Each individual clam is roughly the shape of a right triangle, the two valves joined along the hypotenuse so tightly that you need a penknife to pry it open.  The shells are hard and glossy, and beautifully decorated in multicolored stripes and patterns.  No two are the same but all are exquisitely lovely.  When they die and the soft tissues inside are devoured by scavengers the shells remain attached and opened like butterflies on the sand.  Eventually, the scouring action of wave and sand polishes away the enamel, the sun bleaches the pigmentation and they surrender their beauty to the sea.  The two shells break apart and they collect in piles on the beach, and eventually, in layers beneath the sand. 

Coquina also feed on the plankton washed up on the swash but they gather their food by straining it from the water directly.  A long siphon brings in the plankton-rich water, and another exhausts it back into the stream.  Like the mole crab, they also must constantly reposition themselves in the optimum spot which they accomplish by releasing their hold on the sand by retracting a fleshy foot protruding from their shell.  As the motion of the water moves them either up or down the beach, they dig into the soft sand with their foot and bury themselves so only the siphons protrude above the surface.  It is a delight to watch as a large wave rolls back to sea leaving hundreds of little multi-colored porcelain jewels frantically burying into the soft, wet sand with a pulsing, bobbing motion.  They have to be fast, they too are constantly threatened by being stranded out in deep water or higher up on the beach, away from food and vulnerable to predators.  Despite it's hard shells, the coquina are also relentlessly hunted.  Some fish and birds specialize in cracking them open, as do both marine and land crabs;  predatory snails burrow through the soft sand to attack them from below.  Beachcombers dig them up by the bucketful to brew a delicious chowder.  On the beach, everyone's hungry and everyone's on the menu.

Many creatures are adapted to life in the swash, but on Florida beaches, these two are the most obvious and easily spotted.  They are particularly fascinating because both have developed to exploit the same environment, in the same way, although they are distinctly different creatures, a mollusc and a crustacean, with completely different anatomical plans.  As a result, their life styles and habits are very similar.

Both the coquina and the mole crab have complex life cycles, releasing their eggs and sperm into the sea where they are fertilized and the larvae then drift with the plankton until they are washed up, by luck, onto an appropriate beach.  The female mole crabs, in season, can be recognized by the clusters of orange eggs under their shells; they are also larger than the males.  Be especially gentle with these, they are the future.