Prowling the intertidal

variety and mystery in a challenging habitat

I find it great fun to go out on a minus tide and prowl along the exposed intertidal zone. It’s a bit like a treasure hunt—seeing how many different kinds of invertebrates and little fishes I can find. Being a terrestrial ecologist, I often can’t put specific names on what I see, but the variety of colors and body form is always intriguing.

Many of the critters like to hide under rocks, so I have to turn over those rocks—very carefully, so as not to crush the animals. When I’m done inspecting what’s been exposed, I try to put each rock back exactly as it was, again without crushing anybody, to preserve them and their hiding places. On a recent low tide, I was dismayed to see that other searchers had often not replaced the turned rocks, leaving the various animals exposed to hot sun and predators.

Seeing all those intertidal invertebrates made me think about the many ways they have for eating, many of which have no counterparts among the vertebrates. Most vertebrates have jaws, sometimes with teeth; only a few lack jaws altogether (lampreys and hagfish). It’s very different, among the invertebrates.

In the intertidal zone, stand in one spot and contemplate the several styles of feeding used by the array of invertebrates there. Sea cucumbers filter small particles from the water using tentacles; barnacles do so with their legs. Sea stars evert their stomachs, either wrapping a prey item or inserting the stomach into a clam shell to digest the meat. Crabs mince and nibble their prey to bits, using pairs of sharp mouthparts. Snails of many sorts rasp their food with a file-like structure: predatory snails rasp a hole in the shells of mussels or clams or other snails, shredding the meaty contents, while the herbivorous ones graze by scraping algae off rocks and seaweeds. Ribbon worms, pile worms, and iridescent worms have a bulbous proboscis that can be extruded; armed with sharp hooks or daggers, the proboscis clamps onto the prey and pulls it back into the mouth of the predator. Detritus feeders, such as lugworms, vacuum up soft junk from the substrate. And that’s just a sample.

We often find several kinds of sea cucumbers, sea stars, snails, chitons, anemones, and so on, each time we go cruising the low tide line. Occasionally, we find something unusual or uncommon, such as the nudibranch (a shell-less mollusc) that eats barnacles. Recently, we found numerous odd lozenges of a jelly-like substance strewn over the sands; each little oval was about an inch long and contained rows of tiny eggs or embryos.

This was a big mystery for us, so I sent a photograph to a friendly local expert on marine invertebrates, who said that these were the cocoons of the Pacific lugworm. Lugworms can live at quite high densities in the sediment, each one in a J-shaped burrow. They feed on detritus from the surface of the sediments; in the process, they take in a quantity of dirt as well, which is eliminated in long thin coils (which observers often see on the surface). We happened to be on the beach during the reproductive season, and females had produced these cocoons of babies, each one attached by a thin string to the mother’s burrow. According to research reports, males produce packets of sperm, which get washed over the surface of the sands to a female’s burrow, where the packets break open, releasing the sperm to fertilize the female’s eggs. The eggs are brooded in cocoons by the female either in her burrow or, in the present case, tethered to her burrow. When the young emerge from the cocoon, they are little wormlets that forage over the sediments near the surface.

I’m looking forward to our next ‘treasure hunt’!

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