Delta Journal

by Bob Thomas

A walk down the beach where the water laps up on the sand, especially in Alabama and Florida where the water is clear and the sand is white, is relaxing but can be a bit biologically misleading.

From all appearances, there is no life at this interface between land and the sea. On the beach side, there are plenty of seagulls, ghost crabs, and the occasional mouse or lizard. Along the margins of the water one can see a variety of tiny fish that move rapidly with the sloshing water.
But what about the zone where the water washes over the sand, then disappears back into the sea. It is quite hard to walk on, and there seems to be nothing there, so it must be a mini-desert devoid of life.

Actually there is lots of organic matter down in the sand along this edge, and there are a number of animals that are adapted for living in this specific habitat, so much so that they are rarely encountered in adjacent areas.

But first, since we can walk on the sand and it is very hard and compact, how can critters live beneath the surface?

When sand in the surf zone is without water, it is very hard. When the surf skitters up the beach front, the sand still seems rather hard and easy to walk on.

The trick is something called thixotropy. That is, if the sand is covered with water and is agitated, it mobilizes.

But how do critters make a living in the surf using thixotropy? One of the most common denizens of the surf zone is the mole crab. This smooth, cylindrical crustacean sits under the sand, out of sight of predators, with its antennae lying on the beach front surface. As the surf rolls in and out, the antennae sift organic matter toward the crab’s mouth - a very efficient feeding mechanism, If there isn’t enough food available, the crab moves into the next incoming wave and disappears into the sand in a new place as the wave moves out.

If you can find the v-shaped antennae exposed in the retreating wave, just scoop your hand below the v and you will have a nice shiny mole crab in your hand.

A second common critter of the surf zone is the Variable Coquina, Donax variabilis, a small bivalve mollusk. They burrow in the thixotropic sands by extending their foot and rapidly disappearing.

One of the most obvious terrestrial species to take advantage of thixotropy is small birds, such as sanderlings and ruddy turnstones, who run down the beach front seemingly chasing the retreating wave, quickly peck at the sand, then run back up the beach as the next surf rolls in. By using thixotropy, these small birds are able to easily stick their beaks into the sand and retrieve small living creatures.

Here is an experiment for the next time you visit the beach. Walk into the water about mid-calf deep. You will note that you are not sinking into the sand. Now, vigorously wiggle your toes. You will begin to sink into the sand. This is thixotropy.

Think of the fun you are going to have telling people about thixotropy.

Also published in Nature Notes, The Times Picayune, August 18, 2009.