Sea Slug Squirts Venomous Boogers at Enemies

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Sea hares possess a weapon that even the best comic book writers couldn’t have dreamed up: an inky mucus-like substance that, when squirted at enemies, prevents them from smelling.

The discovery, reported in The Journal of Experimental Biology, reveals how complex and effective some natural chemical defenses can be.

Sea hares are a type of sea slug that has head appendages resembling rabbit ears (or at least someone thought that), hence the name. In Australia, they’re called “beach blobbies.”

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Many marine inhabitants steer clear of them, and it’s easy now to understand why.

Charles Derby from Georgia State University and colleagues Tiffany Love-Chezem and Juan Aggio analyzed what’s in a substance known as “opaline,” which sea hares squirt at enemies, along with ink. Opaline essentially is a type of shimmery white, sticky mucus.

The researchers studied how this substance affects lobsters, which attempt to hunt sea hares. Spiny lobsters, in particular, occasionally try to snack on the mushy, big slugs.

When the substance was applied to the tips of the lobster antennules, used for smelling, the lobsters were unable to detect an enticing, pungently-scented shrimp juice presented to them.

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To figure out why, the scientists measured electrical activity in the lobsters’ chemosensory and motor neurons. These neurons stopped firing in the presence of the snotty gunk.

The researchers next isolated amino acids from the substance, but found that they alone had no affect on the lobsters. In fact, the lobsters’ neurons “fired robustly” in reaction to the “delicious shrimpy aroma.”

When the scientists mixed the amino acids with the sticky substance carboxymethylcellulose, aka cellulose gum, the lobsters again were fooled. Like sticking a wad of chewing gum on a human nose, it blocks odors from reaching aroma receptors.

The lobsters are usually left in dismay, preening and cleaning themselves while the sea hare slithers away.

Image: A sea hare that just released the inky, mucus-like defense substance; Credit: Colin Brown/Flickr