Pacific Piggyback

Aboard a reef dweller that acts as a living cruise ship,

Photograph by David Fleetham

this tiny hitchhiker may receive much more than just a free ride from one patch of coral to another.

At the bottom of the sea, a lone brittle star slowly chassés across a pockmarked surface. What appears to be a weathered barn door or an abstract painting beneath the star’s tube feet is actually the dimpled skin of a sea cucumber, a creature as much as 10 times the size of its tiny hitchhiker—and a convenient conveyance, at least for the moment.

Both Savigny's brittle stars (Ophiactis savignyi) and leopard sea cucumbers (Bohadschia argus) are ubiquitous throughout the world’s tropical coral reefs and sandy-bottomed lagoons, including those here in Micronesia. Because they occupy many of the same habitats, the two bottom-dwelling sea creatures are bound to run into one another. Brittle stars sometimes take advantage of these meetings by climbing aboard their larger relatives and hitchhiking for however long it suits them. Although scientists are still studying the benefits of these free rides, Christopher Mah, a marine biologist at the Smithsonian Institution who specializes in echinoderms (the phylum that includes starfish, sea urchins, brittle stars, and sea cucumbers) says research has begun to reveal a few possible explanations.

In many cases, brittle stars may simply be seeking cover—and benefitting from their hosts’ various defense mechanisms, Mah says. Many sea cucumbers, for example, produce foul-tasting chemicals or have fleshy protrusions that help to ward off would-be predators. Others, including the leopard sea cucumber, shoot a web of sticky strings called Cuvierian tubules out of their anuses as a deterrent to predatory crabs and mollusks.