Wild Neighbors: The Liminal Life of the Sea Roach

By Joe Eaton
Thursday June 05, 2008 - 10:18:00 AM
A western sea roach on the beach.
By Ron Sullivan
A western sea roach on the beach.

Last week I wrote about an encounter with migrant phalaropes at Hayward Regional Shoreline. On that same day, we wound up having lunch at the edge of the Bay, on a narrow beach littered with driftwood and miscellaneous flotsam and jetsam, including an abandoned doll.  

Out of the corner of my eye, I saw something small-maybe an inch long—and dark scurry across the sand and under a driftlog. It could have passed for a cockroach except for my impression that it had too many legs. 

The birding was slow at that point, and I was curious enough to keep watching for more activity. Pretty soon I was seeing these odd creatures everywhere. I had been right about the legs: they had to be some kind of crustacean, not an insect. They had long antennae and some kind of forked apparatus on their rear ends. In the course of scuttling from shelter to shelter, a smaller individual bumped into a larger one and grappled it briefly before moving on.  

Then we discovered Slab City. The concrete slabs at the edge of the water were teeming with these things, whatever they were. We noticed not only size differences but color differences: some were decidedly darker.  

Back home, I tracked the creatures down in J. Duane Sept’s Beachcomber’s Guide to Seashore Life of California. They were western sea roaches (Ligia occidentalis), AKA rock lice or sea slaters. Classified as crustaceans-relatives of the crabs and shrimp-they were more specifically isopods, which seems to mean “equal feet.” I’m not sure what the size differences signify, but the color variation had to do with exposure to sunlight. Mostly active at night, sea roaches become darker by day.  

Ed Ricketts wrote about them in Between Pacific Tides, noting that a Japanese relative demonstrates “the beginnings of social order”: “The animals move back and forth, in a more or less orderly procession, along apparently established routes from shelter among upper boulders at high tide down to the lower beach to feed. The procession is led by the older members of the tribe. The casual impression gained from watching our own Ligia is that of an aimless, disorganized rabble, dispersing in various directions.” 

We talk about the invasion of the land by aquatic animals-marine scorpions venturing out of the shallows, fishlike amphibians waddling onto an ancient shore—as if it were something that happened once and for all millions of years ago. But in fact it’s still going on, species by species. In their own way, fiddler crabs, mudskipper fish, and other littoral creatures are making their own accommodation to the land. And so is the sea roach. 

Although it spends most of its time on the beach, the sea roach still breathes with gills. Since those gills require moisture, the creature is still tied to the sea—at least to the shallows and tidepools. On a regular basis, it has to return to the water and wet down its rear end, where the gills are located.  

Many isopods have gone whole hog for the terrestrial lifestyle. The garden-variety sowbug has evolved pseudotracheae like those of insects and arachnids that allow it to breathe air. But about half the members of the ten-thousand strong Order Isopoda have not yet made it onto the beach. Some, in fact, thrive in the most extreme of marine environments.  

On the floor of the deep ocean, in total darkness and at chilling temperatures and enormous pressures, the isopod Bathynomus grows over a foot long. It’s a scavenger, relying on food—including the occasional whale carcass—that drifts down from surface waters. Bathynomus is an example of abyssal gigantism, the tendency for unusually large species to evolve at great depths, where predators are rare and food resources are stable. 

Other marine isopods live nearshore, like the Harford’s greedy isopod (Cirolana harfordi), the dermestid beetle of the sea: marine biologists use it to clean up fish skeletons for study. Still others, slender reef dwellers, cling to seaweed and snag passing morsels of food with long forelegs. 

The most bizarre of the lot is a parasitic isopod, Cymothoa exigua of the Gulf of California. There’s a photograph of one in Carl Zimmer’s splendid book Parasite Rex. This isopod invades the mouth of a fish and eats its tongue. Then, in a grotesque kind of partnership, it actually replaces the tongue: the fish uses the isopod to grip and swallow prey, of which the isopod presumably takes its cut. I don’t know how this works out for the fish in the long run.  

In comparison, the sea roach is pretty much benign. It has found a comfortable niche on our rocky (or rocky-equivalent) shorelines, and as long as it’s able to dip its butt in the water, it will do fine.