Sea Slug Inks Its Way to Safety

Abstract

SOCIETY FOR INTEGRATIVE AND COMPARATIVE BIOLOGY MEETING > SOCIETY FOR INTEGRATIVE AND COMPARATIVE BIOLOGY MEETING, 4–8 JANUARY, ORLANDO, FLORIDA Like squid and octopi, the shallow-water invertebrates known as sea hares eject ink when startled. Quick-moving squid and octopi use their inky clouds as a smokescreen, distracting potential predators while they attempt to escape. Now Charles Derby, a neuroethologist at Georgia State University in Atlanta, is discovering that the slow-moving sea hare's ink contains a host of protective compounds, some tailored to deter or confuse specific enemies. ![Figure][1] Magic potion. The ink released by sea hares is full of chemicals that drive predators away—and sometimes into a feeding frenzy. CREDIT: GENEVIEVE ANDERSON By testing the chemical makeup of inks secreted by two sea hare species facing different predators, Derby and his colleagues have found a complex defense system of deterrents, alarm calls, and feeding stimulants within the milky purplish and pink fluid. “The ink mixture allows the sea slug to persist in the presence of multiple predators through a simultaneous array of surprising mechanisms,” says Erik Sotka, a marine ecologist at the College of Charleston, South Carolina, Grice Marine Laboratory. Although researchers have long studied chemical defenses in the sea hare's skin and digestive glands, they have not looked in depth at the ink, says Derby. Sea hare “ink” actually consists of two substances, each secreted by a different organ into the mantle cavity, where they intermingle before being ejected. The ink is colorful and thin, whereas the other substance, opaline, is viscous. Each contains its own repertoire of compounds, and Derby finds that when ink and opaline meet, innocuous enzymes and amino acids react to form noxious substances. “This animal uses ‘common’ biological chemicals in novel ways,” notes Esther Leise, a marine biologist at the University of North Carolina, Greensboro. At the meeting, Derby described how sea hares ward off giant sea anemones. An anemone's tentacles may snare a sea hare, but the predator immediately spits out the potential meal and vigorously retracts its tentacles once the hare ejects its inky weapon, Derby reported. His postdoc Cynthia Kicklighter has found that opaline alone has no repulsive effect: If anything, it stimulates feeding behavior by the anemone. But the ink contains all sorts of anemone-repulsing chemicals, says Derby. In contrast, when Derby, Kicklighter, and their colleagues tested opaline and ink separately against spiny lobsters, another sea hare predator, the opposite proved true: Opaline was more protective than the ink, they reported last year. The lobster work also suggests that feeding stimulation is actually a defensive strategy. The sea hare's ink-opaline mixture contains substances, such as the amino acid taurine, that set off a feeding frenzy in a lobster—it grabs at the ground and ink cloud as if to ingest it, enabling the sea hare to slink away. As an added bonus, the ink mixture spreads the word among sea hares when danger approaches. Kicklighter and her colleagues have recently presented either ink or opaline to juvenile sea hares. The animals drew in their heads and moved away from either substance. The sea hares did the same when exposed to ink from an octopus and a squid, suggesting a common warning system for all these species. The researchers have identified three of the sea hare's warning compounds, two of which also exist in the squid and octopus inks. Paul Moore, a sensory ecologist at Bowling Green State University in Ohio, calls the sea hare's arsenal “a case of deceptive chemical signaling strategy not seen before [in animals].” Such research “will open up a new avenue of thought for aquatic chemical ecology,” he predicts. [1]: pending:yes