by Bob Thomas
It is easy to walk down a pretty sand beach, with waves sloshing back and forth, and imagine that there is little life along the edge of the sea. There is no vegetation growing – there is just sand.
Actually, the margin of the sea is alive with a host of organisms who have figured out how to exploit the unique environment and its seemingly invisible wealth of organic foods.
One of the masters is the mole crab, Emerita portoricensis. Not the large edible variety to which we are accustomed, this inch or so long critter looks more like a shiny bullet, being smooth shelled and tapered at both ends. They do not bite or pinch humans, and they cannot walk. Their limbs are adapted for burrowing and swimming.
Mole crabs make their living in what is called the surf zone where waves die as they move up the beach face, then slide back into the sea. When there is no water, the sand seems impenitrable. A person can walk across it and not sink. When the water washes across the sand, any slight aggitation easily separates sand particles and the sand becomes a thick fluid. This is called thixotropy, and allows access to the sand as a place to hide as well as a place to dine.
This is where the mole crab excells. As the water momentarily covers the sand, mole crabs appear in the water, swim about frantically looking for a new dining room, and disappear beneath the surface, leaving only their antennae exposed on the surface to funnel food particles into their mouths. If one squats and looks down the beach face, the antennae appear as little Vs along the sandy surface.
Mole crabs normally feed on plankton and other small organisms in the water, but they are also known to eat the toxically armed tentacles of Portuguese man o’ war, a jellyfish whose stings are very painful to humans..
Mole crabs are an excellent example of an animal that has perfectly adapted to living in a specialized environment. Be sure to look for them on your next visit to the beach.