Few scuba divers on the Belize Barrier Reef ever spot the tiny shrimp Typton carneus. This bright orange crustacean hides snugly inside the fingerlike chimneys of living fire sponges. Scientists have now discovered that the shrimp uses the sponge for more than just shelter. Many species of these tiny houseguests may also be feeding off their hosts - literally.
Scientists have only recently discovered sponge-inhabiting shrimp because many of these crustaceans, usually no more than 1.5 centimeters long, rarely leave the twisting tunnels built into sponges. "For a long time people didn't essentially know they were there," says J. Emmett Duffy, an ecologist at the College of William & Mary in Gloucester Point, Virginia, who was not involved in this study. But researchers have now cataloged a wide diversity of them; one species even forms antlike colonies in sponges.
While many have assumed that most of the many sponge-bound crustaceans were parasites that took bites out of the sponges that gave them shelter, scientists had only dug up evidence for the behavior in two species from a single shrimp lineage. To a get a look at just how widespread this parasitism is, a team of researchers first collected bright red fire sponges from Belize. Poking gingerly through the primitive animals' tissues, which ooze toxins that burn skin, they found T. carneus or a close relative in 34% of the samples.
The first clue to the devious nature of T. carneus lay in its claws, says study co-author Adam Petrusek, an ecologist at Charles University in Prague. Under a scanning electron microscope, he and his colleagues spotted a nasty-looking "tool" on the shrimp's second set of legs. "Rather than looking like the normal claws of crawfish or lobsters, it's more like scissors or shears," he says. Based on the wear and tear on the claws, the team concluded that the shrimp often were using these tools, so it was unlikely that the scrapes resulted from shrimp fights, not an especially common occurrence.
Just what these little real-life Edward Scissorhands were doing with their scissors became clear when the team opened up their stomachs. The shrimp, it seems, had been dining exclusively on sponge tissue, the researchers reported online this month in PLoS ONE. It's convenient: When these crustaceans get hungry, all they need to do is snip off a piece of sponge. T. carneus' close relative, T. distinctus, also has similar sets of claws and sponge tissue in its guts. From this census and others, the group discovered many more shrimp that likely nibble on sponges. Petrusek suspects that such parasitism is common among sponge-dwellers.
But as far as bad houseguests go, the shrimp may not be that rotten. Sponges efficiently regenerate their own tissue, so the tiny bites probably don't do that much harm. T. carneus may give a little something back to its hosts too, Petrusek adds. In addition to their scissors, male shrimp wield a single, hefty claw that could come in handy for fighting off interlopers. Similar species of shrimp defend their hosts against unrelated invertebrates, such as brittle stars, that eat sponges. And because T. carneus shrimp prefer to live in pairs, one male and one female per sponge, they may drive out other would-be squatters so the sponge won't be overrun.
Duffy agrees that the shrimp-sponge relationship falls into the "it's complicated" category: "The question remains open because we need to know what their effect on the host sponges is." If shrimp guard their sponges well and eat little, they could become sponge friends. But if the crustaceans gorge themselves and fall short on their defensive duties, they're more likely parasites. And because sponge-inhabiting shrimp are so diverse, their friend-or-foe status likely differs from species to species.