Sea Hare Ink Is One of Nature's Most Unusual Bioweapons

Sea hares are known for the colorful, sticky ink they let loose when knocked around by hungry predators (or mean humans). Scientists already knew a few ways this defense helped these squishy creatures escape the dinner plate. But new research reveals another purpose to the defensive ink, and it's unlike anything else seen in the animal kingdom.

At first glance, the sea hare (Aplysia) looks like it would be an easy meal for, well, anything. It doesn't have sharp spikes like sea urchins, nor does it have any appendages or claws to fend off attackers. And it certainly doesn't move very fast. But hungry spiny lobsters beware: When picked on, sea hares release sticky ink that causes "sensory inactivation." Essentially, they give lobsters stuffy noses.

Surprisingly, the sea hare's inkgasm is actually a last line of defense. Many predators are deterred by the marine mollusk's mucous coating, which contains acids and other nasty compounds. The sea hare will only release its noxious ink — which is a combination of actual "ink" and opaline from two different glands — after being manhandled a bit.

"You can go into the field and put them in a dive bag and they don't ink," says lead researcher Charles Derby, a neuroethologist at Georgia State University. "And a lobster can pick them up and wiggle them around a bit before they do it."

When used, this ink seems to be pretty effective, able to fend off predatory sea anemones, fish and crustaceans. For nearly a decade Derby has been working to figure out just how the ink mixture gets the job done. As it turns out, the chemical properties (and color) of the ink varies depending on the type of algae the sea hare is eating, meaning that the ink can affect predators in different ways.

The sea hare distracts a lobster with tasty ink.

In 2005, Derby and his colleagues discovered that the ink is sometimes a "phagomimetic decoy." In this case, the ink is very attractive to spiny lobsters — they drop the sea hare and instead try to manipulate and eat the ink. "I liken this to how some snakes and lizards drop a piece of their tail to escape predators," Derby told io9. Then a few years ago, the scientists found that the compounds in the ink can sometimes be distasteful and act as powerful repellants to some fish and crustaceans.

Through their work the researchers also noticed that some lobsters desperately try to rub off the ink if it gets on their antennules, which function as their nose. So they wondered: Could the ink mixture be messing with the lobsters' ability to smell? And if this does happen, is it because of the stickiness of the opaline, or are the defensive chemicals overpowering the lobsters' senses?

To find out, Derby and his team extracted the water-soluble fraction of the sea hare's opaline. The resulting substance had all of the stickiness and physical properties of normal opaline, but lacked its amino acids and other attractant chemicals. They then painted the substance onto the tips of lobster antennules and presented the crustaceans with "shrimp juice," which is normally palatable to lobsters. They found that the opaline significantly reduced the firing of the lobsters' chemosensory and motor neurons.

The sea hare releases its ink, which gets stuck on the lobster's antennules, clogging its sense of smell.

The team then ran similar experiments with a chemical called carboxymethylcellulose, which mimics the physical properties of opaline (Derby calls this stuff "fake snot"). Again, the lobsters' neurons stopped firing. But when they tested a mixture made of the five most prominent amino acids found in opaline, the lobsters' neurons fired as normal. The results suggest that opaline physically blocks the lobsters' reception and response to food odors, rather than overpowering their senses.

"It's just like getting a stuffy nose, in that the substance prevents the odor molecules from getting to the sensory receptors," Derby says, adding that the substance allows the sea hare to make a clean getaway while the lobster tries to clean the gooey mess off its antennules.

Previous research has shown that other creatures sometimes confuse the senses of their predators, such as moths that produce ultrasound to "jam" the echolocation of bats. But this is the first time an animal has been shown to actually inactivate the senses of another, Derby says. Derby is now interested in seeing if the ink of some cephalopods, such as the octopus, has similar properties to the sea hare's powerful mixture.

The research was detailed today in the Journal of Experimental Biology.

Top image via National Science Foundation.