Eating can be skin deep

Fashion designers love eel-skin: Shoes, purses and wallets are made from the stuff, which looks like leather. Despite its name, though, much of the eel-skin in expensive accessories doesn’t come from eels, electric or other. It comes from long, skinny sea animals called hagfishes. These swimming scavengers delve into dead or dying marine animals and eat from the inside out, leaving behind only skin and bones. (If you invite a hagfish to dinner, be sure to serve something dead.)

Hagfish skin isn’t useful just for fashion designers. Recent tests on the material suggest that when Pacific hagfish dive into dinner, they get some nutrition through their skin ― no need to wait for food to first pass through their guts. Their skin absorbs nutrients.

Other sea animals, like many kinds of worms, also eat this way. The new study is the first to find that hagfish feed this way.

Hagfish are hard to classify, which makes them interesting for scientists to study. They look like eels, but they have different insides. They’re more like fish, but their backs aren’t bony. They have evolved from an ancient group of animals that swam in the seas millions of years before the dinosaurs appeared. The hagfishes you see today look very much like the ones you would see if you rode a time machine back 300 million years into the past. Some scientists who study evolution believe that hagfishes share a common ancient ancestor with modern fish. They might also have evolved from the same animal as modern vertebrates.

Vertebrates are animals, such as fish and mammals, that have hard backbones leading to skulls. Invertebrates, like worms and insects, do not have backbones. Invertebrates are much more common on Earth: Nine of every 10 species have no backbone.

Hagfish may be vertebrates, but scientists continue to debate the question. Each hagfish has a hard skull, but instead of a backbone it has a tissue called a notochord along its back. Chris M. Wood calls the animals “ancient vertebrates.” Wood is a fish physiologist at McMaster University in Hamilton, Canada. That means he studies bodily functions in fish.

He told Science News that the development of skin that could keep an animal’s insides very protected from the outside was important in the evolution of animals that would go on to live on land. With tough, protective skin, land dwellers don’t absorb nutrients directly through the skin.

Wood and his colleagues used skin samples from Pacific hagfish caught near Vancouver Island in Canada. To test how well nutrients were absorbed by the skin, the scientists stretched the skin samples across the tops of small vials, or glass jars. Inside each vial was a solution chemically similar to the inside of a hagfish.

The vials were placed in another solution containing amino acids, which are nutrients. They are the building blocks of proteins, and hagfish devour protein when they eat animals. The vial inside the amino acid solution acted like a laboratory version of a Pacific hagfish inside a carcass.

The researchers tested different solutions containing different concentrations of amino acids. They also tested the skins at different times to measure the amount of nutrients the skins had absorbed. After repeated experiments, the scientists determined that the skin absorbs amino acids, up to a point. Beyond that point, the amount of nutrients leveled off, suggesting that the skin couldn’t absorb any more. That suggests that the tissue was taking in and using nutrients, not just letting those nutrients pass through, Wood told Science News.

In the laboratory, the hagfish skin absorbed amino acids from a solution. More experiments would be needed to show how hagfish use absorption while in their natural habitat, eating carrion — dead animals — on the ocean floor.

“What you can show in a lab isn’t always functionally relevant,” Frederic Martini told Science News. That means, the experiment may not tell scientists just how much the animals end up using the power to eat through the skin. Martini is a biologist at the University of Hawaii at Manoa who also studies hagfish. He says he would like to learn how hagfish get access not only to amino acids, but also to other types of nutrients.

POWER WORDS (adapted from the New Oxford American Dictionary)

vertebrate An animal that has a long backbone or spinal column. Vertebrates include mammals, birds, reptiles, amphibians and fishes.

invertebrate An animal lacking a backbone. About 90 percent of animal species are invertebrates.

amino acids Simple molecules that occur naturally in plant and animal tissues and that are the basic constituents of proteins.

concentration A measurement of how much of one substance has been dissolved into another.