an invertebrate extravaganza (with some vertebrates too)

When the tide goes out, it’s time for a natural history treasure hunt. This year, both May and June brought really low tides (more than minus-four feet) at more or less reasonable hours of the day. Of course, we had to go looking for weird and wonderful creatures that might be visible. We went to two likely spots, one in May and the other in June. At one of them, we were supervised closely by a pair of watchful black oystercatchers.

Here are a few of our ‘treasures’. All of them were viewed in place and sometimes photographed, or they were carefully replaced where they were found.

Numerous tiny sculpins scuttled for cover as soon as their pools were disturbed or even at the approach of the terrestrial monsters that cast long shadows. Hiding under small boulders were cockscomb pricklebacks, crescent gunnels, and a pale, thin fish called a gravel-diver. These little fishes are sometimes called eels, which they are not, or blennies, although they are not even very closely related to true blennies. They are reported to eat a variety of small invertebrates.

Sunflower sea stars, with their soft surfaces and multiple arms, were plentiful at one site; they came in all sizes from about four inches to perhaps twenty inches in diameter. They usually feed by swallowing their prey whole, and they eat almost anything, including sea urchins, clams, snails, other sea stars, and mussels. The common five-armed sea stars displayed an astounding array of colors: gray, olive, bright and dull orange, brilliant purple, turquoise, and tan. There were lots of little ones of this species, about half an inch across, and these didn’t seem to show such a wide variety of color. These sea stars can use their tube feet to pry molluscs open or lift them off the rocks. They typically feed by everting part of the stomach over the prey and digesting it in place. Despite their very crunchy nature, they are preyed upon by large gulls, big sunflower sea stars, and large crabs. A special treat was finding a few brittle stars, mottled in maroon and green. They are detritus feeders, preyed upon by some fishes and diving ducks.

There were at least five kinds of sea cucumbers, a big purple one, medium orange ones, small white ones, smaller translucent ones, and thousands of the very small black ‘tar spot’ cucumbers. Sea cucumbers typically breathe through their hind ends—pulling sea water through the anus into a set of branched respiratory tubules connected to the hind gut. They feed on organic detritus mopped up from the substrate or captured in the water column. I presume there is a mechanism for keeping digestive products out of the respiratory system! Their predators include several kinds of sea stars, some fishes, and sea otters.

Worms came in several guises. My favorite, one I’d never seen before, was an intertidal gillworm, buried in mud under a rock: bright red, with feeding tentacles at the front end and many thin filaments along the side that serve as gills for breathing. Of course, there also were other polychaete (meaning ‘many-bristled’) worms of several types, with their numerous body extensions containing various kinds of stiff bristles that may help in locomotion, and the extensions also assist in respiration. Many polychaetes feed by extending a tubular, muscular proboscis, usually armed with teeth, to grab their invertebrate prey. Polychaetes are the favored prey of ribbon worms, which can change their shape from elongate, skinny ribbons to stubby slug-like forms. Ribbon worms subdue their prey by stabbing with a sharp stylet and injecting a neurotoxin, then pulling it in to digest.

The common ‘black katy’ chitons came in all sizes, and so did the more colorful lined chiton. We found one hairy chiton, with its frill of ‘hairs’ all around the edge. Chitons are basically grazers, eating algae and little invertebrates that are stuck to the rocks (baby barnacles, sponges, and so on). Chitons are prey of sea urchins, some sea stars, black oystercatchers, harlequin ducks, and river otters, among others. I once found a pile of plates from a chiton on top of one of the mountain ridges, perhaps indicating that a raven had pilfered one from another predator.

Lined chiton. Photo by Pam Bergeson

There were hermit crabs of all sizes, hundreds of urchins, the usual big green and red anemones, and the smaller green burrowing anemones that somehow squeezed themselves into impossibly small crevices. Small periwinkle snails abounded in some places; they are grazers. And there were a few larger, carnivorous snails known as whelks, which can drill into other shelled creatures and slurp out the innards. Just imagine being a blue mussel and feeling that big snail rasping through your shell! Ah, but sometimes even those sedentary mussels can fight back, by ensnaring the attacking whelk in byssal threads, which are usually used to attach mussels to the rocks but can be diverted to repel invaders.

A nice find was an alga that turned out, upon investigation, to be two algae. A dark, filamentous alga bore odd, warty, oval bubbles or sacs on its fronds. Those sacs didn’t belong to that alga; they were another alga altogether, one that lives epiphytically, attached to other kinds of algae. It is called ‘studded sea balloons.’ A new one for me!

A treasure hunt, indeed. I’m basically a terrestrial ecologist, so a visit to the intertidal zone is always both fun and educational.