Sea Slug Chemical Blast Deters Lobster Predators

Matt Kaplan
for National Geographic News
August 13, 2008
Sea hares, a type of sea slug with tentacles that resemble rabbit ears, have developed a crafty concoction of bodily fluids that deters predatory spiny lobsters, a new study says.

When threatened, the marine invertebrates produce two chemical secretions, ink and a milky mix of substances called opaline. The chemicals are mixed together moments before the animals squirt out a dark cloud.

A reaction between ink and opaline also makes a byproduct of hydrogen peroxide, commonly used by humans as an antiseptic.

At first glance, the combined secretions appear to create a cover by which the soft slugs can escape lobsters.

But the new study shows that the chemical blast causes lobsters to display anxious behaviors, as well as prevents them from eating the sea hares.

"We knew these secretions were incredibly complex and used for defensive purposes, but we didn't know what role the individual secretions were playing or what specifically they were doing," said study lead author Juan Aggio of Georgia State University in Atlanta.

(See colorful photos of toxic sea slugs.)

Toxic Cocktail

Aggio and co-author Charles Derby, also at Georgia State University, placed lobsters in aquariums and exposed them to small doses of shrimp juice to whet their appetites.

The scientists exposed the lobsters to one of three chemical compounds: ink, opaline, or hydrogen peroxide.

Ink on its own had little effect on lobster behavior. But opaline and hydrogen peroxide caused lobsters to rub their mouthparts and flip their tails—actions associated with anxiety and escaping danger.

Next the team gave lobsters shrimp laced with one of the three compounds. The crustaceans took considerably longer to eat shrimp containing ink and hydrogen peroxide than they did normal shrimp.

"Hydrogen peroxide is an antiseptic. I really did not expect to see it have any effect on its own, I figured it would need to be combined with other compounds to create such a dramatic reaction," Aggio said.

The lobsters also quickly spit out opaline-laced shrimp.

The research appeared online recently in the Journal of Experimental Marine Biology and Ecology.

No Silver Bullet

"It is surprising how many compounds the sea hares use," said chemical biologist Richard Zimmer at the University of California, Los Angeles.

"We kind of expected to see a silver bullet, a single compound that has a powerful defensive effect, but it is quickly becoming clear that there are a number of bullets being used in sea hare ink," he said.

Mark Hay is a marine ecologist at the Georgia Institute of Technology in Atlanta.

"People have assumed for a long while that [sea hares] have been eating things with toxins in them, collecting the toxins in their bodies, and then spewing them out," Hay said.

(Read: "Toxic Frogs Get Their Poison From Mites" [May 14, 2007].)

"Obviously, this research is showing that there is a lot more than that going on."

The next step will be to evaluate if other predators respond to these compounds in the same way, Hay added.

"It is quite possible that sea hares evolved different chemical defenses to deal with different predators like fish and crabs, and that combining these defenses provides even greater protection for the sea hares."

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