Hagfish, the eel-like, slime-emitting wrigglers of the sea floor, just can't get enough of decaying corpses. When they come across a dead fish, they snuggle their sinewy bodies down into its cavities and stay there, writhing blissfully. But what are they doing?

Probably eating their way out of the carcass, according to a new study—and not with any sort of politesse, either. When they can't pack enough flesh into their tentacle-lined mouths, hagfish absorb the carcass's nutrients right through their skin.

The salt concentration of a hagfish's tissue is the same as that of the seawater it normally swims in, suggesting that dissolved substances can cross the skin. To determine whether the animals can soak up nutrients as well as salt, physiologist Chris Glover of the University of Canterbury in Christchurch, New Zealand, and colleagues took a piece of hagfish skin and stretched it out in a flask containing a seawater-like solution on one side and a solution resembling a hagfish's own bodily fluids on the other. In the seawater, they dissolved radioactive amino acids and sugars, along with food coloring to test how permeable the skin was. After a few hours, the skin started to become radioactive as it absorbed the amino acids, the researchers reveal online this week in the Proceedings of the Royal Society B. The skin didn't allow food coloring through, however, suggesting that it selects nutritious detritus.

Credit: J. Drazen and M. Kay

This is the first time such feeding habits have been observed in a non-invertebrate animal, suggesting that hagfish may be a transitional organism between modern fish and invertebrates such as aquatic worms, which can absorb food through their skin.

Although they don't have true vertebrae, hagfish are considered "proto-vertebrates" and have many similarities to vertebrates. The researchers say that their physiology holds hints to the origin of some vertebrate traits. Their absorbent skin appears to act like the gut in more complex animals, for example, and may help scientists understand how this system evolved.

"It's an interesting and novel result," says fish ecologist Jeffrey Drazen of the University of Hawaii, Manoa. When it's not busy consuming cadavers, the hagfish's larger role in the food web remains a mystery, he says, as it's difficult to study fish that live this deep in the ocean. The new finding is the latest in a list of hagfish feeding habits that would make Miss Manners weep. For instance, hagfish coat a corpse they're interested in with slime exuded from pores in their bodies, keeping other scavengers at bay. "Nobody else wants it after that," Drazen says.