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.
In addition to the protection they may find while perched atop sea cucumbers, at least some brittle stars also gain feeding opportunities and efficient transportation from the relatively fast-moving bottom-feeders. As sea cucumbers cruise the ocean floor, sifting through sediment, they inevitably stir up edible bits of detritus and tiny organisms that generalist feeders like brittle stars can latch onto with their snake-like arms. By hitchhiking aboard such creatures, scientists think that the stars likely improve their odds of bumping into an easy meal. And because sea cucumbers travel quickly, brittle stars that stick with their hosts for long periods of time can travel much farther than they could otherwise. Scientists studying these types of relationships between species in New Caledonia, for example, found that brittle stars that hitched rides on sea cucumbers were able to find their way to entirely new patches of reef.
Savigny’s brittle star is hardly the only species known to set up shop atop or inside of a sea cucumber. These tube-shaped animals are often host to a veritable ecosystem of creatures, including clams that take up residence in cucumber throats, fish and crabs that colonize their anuses, and worms and snails that drink their bodily fluids. This tiny brittle star aboard its leopard-print cruise ship is yet another reminder of the critical ecological role that sea cucumbers play in coral reefs around the world—and a prime example of the fact that oft-overlooked species can be surprisingly important to the health of an entire community.
David Fleetham is one of the most published underwater photographers in the world, with more than two hundred magazine covers to date. His award-winning work has been displayed in museums around the globe. Fleetham began diving and photographing underwater in 1976, first in the cold, rich waters of British Columbia, and since 1986, in Hawaii. He is a founding member of The Ocean Artists Society.