Ocean Watch: Flotsam crabs ride where the waves go

During one of my recent beach walks, I found a hefty piece of bark about the size of my hand. The rough surface of this former tree trunk provided the perfect habitat for one of my favorite offshore animals. And sure enough, when I looked at the bumpy underside of the bark, a little crab with the formal name of Planes was hanging on for dear life.

Planes might not sound right for a crab, but for a scientific name, it works. (It’s not bad for airplanes, either.) Pronounced PLAH-nes, the name comes from Greek, meaning a wanderer or rover. And roving is exactly what the world’s three little Planes species do. They climb aboard whatever they find that floats and then ride atop the world’s oceans eating whatever comes their way.

One species usually found in the Atlantic Ocean is called the Columbus or gulfweed crab. This is likely what Christopher Columbus discussed in his journal when he wrote that during voyages to the New World, explorers “saw much more weed appearing, like herbs from rivers, in which they found a live crab. … They thought these crabs were certain signs of land.”

Good guess, but not so. These crabs are true offshore animals that have nothing to do with land. The “herbs” the sailors saw was sargassum, a seaweed that grows around the world. One species of sargassum is famous for floating free in great masses offshore in the Atlantic.

The other two Planes species, called flotsam crabs, are most often found in the Pacific and Indian oceans. During strong tradewinds such as we’ve had, it’s common to find flotsam crabs shipwrecked on Hawaii’s windward beaches.

All three of the drifter species have a back shell about a half-inch long and a half-inch wide. Some individuals are blue, matching the color of the ocean they live in, and others are various shades of brown, blending in with their rafts.

In latching onto anything that floats, the crabs can be found on driftwood, seaweed, plastic, pumice, feathers, tarballs or floating jellyfish. While snorkeling off Zanzibar last year, I found a flotsam crab on a rubber slipper. The crabs also hitch rides on the backs of sea turtles, eating the turtle’s parasites.

As other marine species grow on a raft, an ecosystem forms, the animals eating algae and each other, as well as floating plants and animals that the raft passes. In addition to the flotsam crab, my piece of bark had gooseneck barnacles and bryozoans (“moss” animals) stuck to it.

After several of us beach walkers admired the little crab and its fellow travelers, Craig heaved the bark past the surf break to see whether the side-shore current might carry the tiny village back to sea. To our dismay, when the bark hit the water, the crab broke loose.

Our flotsam crab, however, was a surprisingly good swimmer and paddled like mad straight for home. When it reached the bark and scrambled aboard, we cheered, wishing good luck to the mariner and its fellow sailors.

May the drift be with them.

To reach Susan Scott, go to susanscott.net and click on “Contact” at the top of her home page.