There's plenty of life around and under the rocks

Posted: Sunday, June 10, 2007

It's sometimes hard to find a good place for a walk during mud season.

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Trails at lower elevations often turn to bogs laced with slippery roots, while higher stretches are still buried in snow.

For anyone who doesn't want to go squelching through mud or plodding on snowshoes, good hiking options are limited.

If mud and snow hold no appeal, consider a beach hike. Beaches of sand and gravel make for easy walking, but they're biological deserts compared to rocky beaches.

And we have lots of rocky beaches around here.

On a good low tide, a dazzling array of fascinating critters can be found among the rocks in the intertidal zone.

Some, such as barnacles and mussels, perch on top of the rocks. Others, including sea anemones, often cling under rocky overhangs Still others, such as sea stars, wedge themselves into crevices at low tide.

Many species live under rocks but can be seen by gently lifting a rock (and ever-so-gently replacing it after taking a look).

Turning over rocks on a recent beach walk, we found two kinds of sea cucumbers, a few hermit crabs hunkered down in borrowed snail shells, chitons and a few limpets clinging tightly to the rocks.

There were also numerous buried clams, with just their breathing siphons extended from the substrate; bunches of snail eggs; clusters of prickly little sea urchins;, some baby king crabs; several kinds of worms and three kinds of fish.

A sizable male sculpin was guarding a big batch of eggs plastered to the underside of a rock. The big-headed, large-mouthed sculpin is a contrast with the slim, elongate, small-mouthed gunnels and pricklebacks that commonly hide under rocks when the tide is out.

Local folks sometimes mistakenly call them blennies, which aren't found here.

Some of the inhabitants of rocky beaches are carnivores and obtain their prey in several different ways. Here are just a few examples.

Sea stars are a common and familiar denizen of these shores. Most of them have five or six arms, each with numerous tube-feet on the underside.

When a sea star preys on a clam, it can pull the shell open, using those tube feet. Even a tiny crack between the two parts of the shell is enough, because sea stars have an extrusible stomach.

They can put their membranous stomachs where the food is and digest the clam inside its own shell.

Sea anemones close up when the tide is out and hang limply from the rocks. But when covered by the tide, they send out their tentacles and feed by capturing small prey - crabs, mollusks, even tiny fish.

The tentacles bear stinging cells that fire when they contact the prey and secrete nerve toxin that incapacitates the prey. Their food is then swallowed whole.

The large snails on the beach are predatory too. Snails have a tongue-like rasp that is used to scrape food particles. Some snails such as small, common periwinkles use the rasp to scrape algae from rocks.

Most of the predatory snails, however, use their tongues to drill holes in the shells of barnacles, clams, or other snails. The drilling is aided by acidic secretions and generally takes several hours.

The flesh of the prey is then rasped into fragments and consumed.

Under many of the rocks are ribbon worms, often looking rather like discarded shoelaces in brilliant red and orange hues. Their limp body shape can change from compact and slug-like to long, very thin strings.

Despite their innocuous appearance, they are voracious predators of worms and crustaceans. A tube near the front of their gut can be extended and wrapped around the prey. The prey is subdued by stabs of tiny, sharp spikes and by poisons secreted from the tube, and then drawn back into the mouth.

One species of ribbon worm attacks eggs on gravid female crabs in order to lay its own eggs there, but a heavy infestation of the worms can kill all the unhatched embryos.

If you have a vivid imagination, think about these predators from the point of view of the prey.

Imagine sitting inside your house, hearing that rasp coming slowly but inexorably closer, and finally shredding you to bits.

Imagine being covered by a filmy, membranous stomach and digested from the outside in. Or think of being wrapped in the coils of a ribbon worm feeding apparatus and being stabbed and poisoned while getting pulled back into the worm's mouth.

You could scare yourself to death! Just be glad that they are much smaller than you. Whether or not the invertebrate prey know "fear" is unanswerable. But they do have escape reactions.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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