The Ancient Back Story of the Slimiest Animal in the Sea

The hagfish, a deep-sea scavenger about the size and shape of a tube sock, has the curious ability to smother itself in its own snot. The mucus is a defense mechanism, released into the water (or in one unfortunate incident, all over an Oregon highway) when the fish feels threatened. Once it hits seawater, a tiny amount of the ooze expands to 10,000 times its original size in a fraction of a second, forming a tenacious web of goo. A shark trying to take a bite of a hagfish will find itself suddenly unable to breathe, its gills clogged with the slime.

But the same is true of the hagfish itself, which finds the process of being captured by scientists late at night on boats in the black ocean a bit stressful. Juan Pascual-Anaya, a biologist at the University of Málaga in Spain who has spent summers collecting hagfish off the coast of Japan, recalls having to strip the elastic gel off the animals with his hands.

“We have to be removing the mucus all the time on the ship, or they will die,” he said.

All this mucus-handling was in pursuit of a rather deep evolutionary unknown: what the hagfish’s genome can tell us about the earliest vertebrates. The hagfish has no jaw, making it part of a group that diverged long ago from the ancestors of jawed vertebrates like ourselves.

Last week, Dr. Pascual-Anaya and other scientists reported in the journal Nature Ecology & Evolution that they have sequenced the genome of the hagfish, discovering evidence that jawless fishes probably diverged from those with jaws more than a half-billion years ago. The ancestors of hagfish tripled the size of their genome more recently than that, after they’d already split from the marine life that were our ancestors.