This week's episode is about two strange animals of the sea: the hagfish, which isn't a fish, and the sea spider, which isn't a spider.

A curled-up hagfish:

The sea spider is actually quite pretty as long as I don't have to touch it:

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

A long long long time ago, and I can’t even remember which episode it was, I mentioned that one day I would do an episode about the hagfish because it’s such a weird animal.

Well, that day is today.

The hagfish isn’t a fish. It looks more like an eel and is sometimes called a slime eel. But it’s not an eel either. In fact, it’s so weird that scientists are still trying to figure out exactly where the hagfish fits in the animal world.

The only living animal that is similar to the hagfish is the lamprey, and current research suggests that they are fairly closely related. We talked about the sea lamprey way back in episode three.

There are a number of hagfish species. The biggest is the goliath hagfish (Eptatretus goliath), which can grow more than four feet long, or 127 cm, but most species are much smaller. As mentioned, it looks sort of like an eel, with a tail that’s flattened like a paddle. It doesn’t have true fins, it doesn’t have a jaw, and it only has a single nostril. It usually breathes by swallowing water, which runs through gill pouches inside the body, but some researchers think it can also absorb oxygen through its skin. It can survive for hours without oxygen.

The hagfish is considered a vertebrate because it has a rudimentary spine, called a notochord. It has eyespots instead of true eyes, which can only detect light, but fossilized ancestors of living hagfish seem to have had more complex eyes. I guess they just didn’t need them.

The hagfish has a lot of blood for its size. Its skin is loose and only attached to the rest of the body along its back and at its slime glands. Since its skin is thick and contains about a third of the body’s blood, the hagfish actually looks kind of like a fluid-filled sock with a tail. If you’ve ever bought an eelskin wallet or other item, it was probably actually made from hagfish skin. Because the hagfish has such low blood pressure, the lowest recorded in any animal, and because its skin is so loose and it only has a few bones, it can squeeze through incredibly small openings. When it does, the blood in its skin is pushed into the rear of its body. This would kill an ordinary animal, but it doesn’t affect the hagfish at all.

There’s so much weirdness about the hagfish that it’s hard to know where to start. Its mouth, for instance. Instead of jaws, its skull has a piece of cartilage that can move forward and backward, with two pairs of comb-shaped teeth attached to the plate. This sounds like it would be an awkward way to bite into food, but it works so well for the hagfish that it hasn’t changed in some 300 million years. It’s more like a toothed tongue or a radula than anything resembling vertebrate jaws. The hagfish also has short tentacles around its mouth.

The hagfish eats anything, but the main part of its diet is probably marine worms that live on the sea floor. It also scavenges carcasses that sink to the bottom of the sea. If you’ve seen that amazing time-lapse video of a blue whale carcass, you’ve seen hagfish. They’re the ones that burrow into the carcass to bite pieces of meat off from the inside, and the ones that will actually tie their body into a knot to help yank food off the carcass. Since the hagfish lives on or near the sea floor, trawlers who drag nets along the sea floor to fish often catch hagfish by accident. Sometimes they catch so many hagfish that by the time they haul the net up, the hagfish have eaten all the fish in the net.

But the hagfish also hunts fish actively, especially the red bandfish that lives off the coast of New Zealand. The red bandfish digs a burrow,