Sea Hares Shoot Sticky Mucus to Kill Attacking Predators' Sense of Smell

Sea hares seem like that they would be easy prey for a predator. Slow-moving and without any type of shell, the giant, slug-like creatures inch along the bottom of coastal waters, feeding on seaweed. Despite appearances, though, these creatures have a defense that causes most animals to steer clear. They can eject a sticky secretion that seems to fool hungry predators. Now, scientists have found out exactly why these secretions seem to dumbfound a sea hare's attackers.

In order to better study this unique adaptation, researchers examined how the substance affects spiny lobsters, which often try to munch on the giant slugs. During "inking," sea hares produce a mixture of purple ink and a white substance called opaline, a kind of mucus. The researchers used an extract taken from sea hares' glands and then painted it on the tips of the lobsters' atennules, which are used for smelling prey. In a real scenario, it's quite possible that the lobster would be covered with the oppaline. The researchers then presented the lobsters with a pungent shrimp juice. While the lobsters were presented with the juice, the scientists measured the electric activity in the lobsters' chemosensory and motor neurons, which are responsible for detecting odors and sending signals from the brain to the muscles.

It turns out that the sea hare is quite adept at discouraging predators from attacking. The researchers found that the spiny lobsters' responses to tasty smells were significantly reduced when blocked by the mucus.

Yet the ink doesn't only block the smelling receptors; it's also distracting to the predator. Previous studies have shown that high concentrations of amino acids in ink, such as the ones found in the sea hares', can be appetizing to some animals and cause them to focus on the substance rather than their prey.

"A lobster, when it bites a sea hare and gets a whiff of the ink, will drop the sea hare and attend to the ink secretion," said Charles Derby, one of the researchers, in an interview with BBC News.

In the wild, the lobster will often preen and clean itself as the sea hare slithers away. It turns out that the giant slug isn't so defenseless after all.

The findings are published in the Journal of Experimental Biology.