Ah, Aplysia. Also known as the sea hare, Aplysia is a common preparation used in neurobiology labs; it's a good sized beastie with the interesting defense mechanism of spewing out clouds of mucusy slime and purple ink when agitated. I well remember coming into the physiology lab in the morning to find a big bucket full of squirming muscular slugs in a pool of vivid purple goo. And then I'd reach in to grab one, and they were all velvety soft and undulating and engulfing my whole arm in this thick, slick, wet, slippery knot of rippling smooth muscle…

Ahem. Well. Let me compose myself for a moment. I will say that I always thought handling Aplysia is an amazingly sensuous experience.

It turns out that I'm not the only animal that feels that way. A reader mentioned this fine article in Current Biology on Aplysia secretions, and it turns out that what it is spewing induces complex responses in predators—it's more than just an "ooo, ick, yucky" reaction to drive them away. Aplysia produces two kinds of secretions, a white, thick goo called opaline, and the intensely purple ink. Investigators paired hapless sea hares with a predator, the spiny lobster, in a tank, and observed what happened. Surprisingly, the soft little slug escaped from the spiky hungry lobster two thirds of the time, as can be seen in this video:

(3.4MB Quicktime movie)

Toss in a sea hare with neither opaline nor ink, though, and it was lunch more than 80% of the time. And it wasn't the purple ink that was critical, either: sea hares with ink but no opaline got turned into lobster chow 83% of the time.

It turns out that opaline is loaded with stuff that stimulates complex responses in the lobster. Some trigger escape responses, so the lobster reflexively flicks its tail and darts away. Some stimulate feeding behaviors, such as digging their legs into the substrate or clutching the frontmost pair to the mouthparts. And others cause the lobster to start grooming its antennae and mouthparts. The poor crustacean gets slammed with mixed signals.

Here's an analysis of just what is in that slime:

Composition of Opaline, Ink, and Hemolymph of Sea Hares. The three pie charts in the top row represent absolute amounts on the same scale; pie charts in the bottom row show the same results but on a relative scale.

Note the large quantities of taurine (one of the major components of familiar energy drinks) and lysine. One of the signals the sea hare is sending is that of food—it's called phagomimicry. It squirts out this viscous textured goo colored with inks and flavored with amino acids, so it's as if the lobster has been hit in the face with a Red Bull flavored cream pie. Then, while it is licking its lips and cleaning the glop off its face, the sea hare wriggles away.

Kicklighter CE, Shabani S, Johnson PM, Derby CD (2005) Sea Hares Use Novel Antipredatory Chemical Defenses. Current Biology 15(6):549-554.