Sea Slug Squirts Venomous Boogers at Enemies

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."

PHOTOS: Caribbean Nightlife Under the Waves

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.

NEWS: Snot Affects Sense of Smell

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

August 4, 2011 --

Opening tomorrow at the New York Aquarium and running through the rest of the month, photographs such as the ones here will be part of an exhibit showcasing fluorescent and other underwater images of Caribbean nightlife by Brandi E. Irwin of Liquid Film Photography. Using a Nikon D300s in an Ikelite waterproof housing with two or more strobes emitting near ultraviolet light, invisible to the human eye, she captures the light from the animals as they absorb the blue wavelength from the strobes and fluoresce or glow in different colors. Shown here is a fluorescing Bearded Fireworm.

Fluorescing in red and green the Great Star Coral (Montastraea cavernosa) is formed from colonies of thumb-sized polyps growing together.

Nighttime finds the Caribbean reef octopus (Octopus briareus) at its most active. Usually brown and green, the white coloring shown here indicates this animal was not pleased by the paparazzi.

A queen angelfish (Holacanthus ciliaris) at night shows an array of color. According to the Marine Species Identification Portal, "Queen and Blue angelfish (H. bermudensis) occasionally interbreed, resulting in fish that share some distinctive markings from both species. These hybrids were once incorrectly described as a separate species: Townsend angelfish (Holacanthus townsendi)."

A pair of fluorescing tube anemones (Cerianthus) glow green under the UV lighting.

The orange ball corallimorph (Pseudocorynactis caribbeorum) is an uncommon site in the Caribbean, especially at night. Here Irwin manages to photograph the creature fluorescing without disturbing it.

The corallimorph (Pseudocorynactis caribbeorum) is quick, and retracts its tentacles at the slightest disturbance.

Orange cup coral (Tubastrea coccinea) is often found growing on cave ceilings, dock pilings, and buoys. The species is thought to have originated in the Indo-Pacific region and introduced to the Caribbean in the 1930s or early 1940s.

Puzzled? Several types of brain corals exist in the Caribbean, this images shows what we think might be either a fluorescing boulder brain coral (Colpophyllia natans) or a symmetrical brain coral (Diploria strigosa). Feel free to tell us what you think it is in the comments section below.

As its name implies, the lettuce sea slug (Tridachia crispata), shown here in natural light, has an appetite for salad. The vegetarian nudibranch looks a little leafy too.

A southern stingray (Dasyatis americana) visits the bow of the Hilma Hooker, a ship allowed to sink in 1984 off Bonaire.

A pair of longsnout seahorses (Hippocampus reidi) in natural light. The female is yellow and the male is red.