A new fossil shows how hagfish went back to basics | Science News for Students

A new fossil shows how hagfish went back to basics

The find alters our view of fish evolution
Apr 22, 2019 — 6:45 am EST
a photo of two hagfish in a tank, one is coiled and centered in the image

Hagfish (two shown) are unlike most fish. They don’t have eyes, jaws or bones. Now, a newly discovered fossil helps explain how these fish got their unusual look.

SqueakyMarmot/Flickr (CC BY-SA 2.0)

Hagfish certainly seem prehistoric. Unlike most fish today, these eel-like creatures don’t have jaws, eyes or bones. About the length of a kid’s arm, they make a lot of slime too. Researchers had thought that these strange fish were a throwback to a time before fish evolved jaws and other modern traits. But a stunning new fossil hints that hagfish are not so ancient after all.

Figuring out how hagfish are related to other types of fish has been a challenge. Besides lacking jaws, hagfish do not have a complete spine or true bones. Extinct fish from earlier in life’s history had a similar look. So some scientists thought hagfish were survivors from this branch of the fish family tree. But they weren’t sure because few hagfish fossils had turned up.

Now, a new-found fossil hagfish helps solve the mystery. It came from 100-million-year-old rocks in Lebanon, a nation in the Middle East. Paleontologist Tetsuto Miyashita works at the University of Chicago, in Illinois. He and his colleagues described the fish, February 5, in the Proceedings of the National Academy of Sciences.

The fossil shows the fish’s whole body. Paleontologists named it Tethymyxine tapirostrum. Tethymyxine means “slimy fish from the Tethys.” That’s the name of the ancient ocean the fish swam in. The name tapirostrum combines the words for “tapir” and “snout.” It’s a nod to the fish’s long nose.

a photo of a fossil of an ancient hagfish, Tethymyxine
The fossil of an ancient hagfish, Tethymyxine, reveals what the creatures were like 100 million years ago.
T. Miyashita, Univ. of Chicago

A rare find

Hagfish have left behind few fossils because their bodies are so soft. That means they do not fossilize well. “A hagfish is like a swimming sausage,” notes Miyashita. When scientists come across hagfish fossils, they can look confusing. “If you find a fossil fish that looks a bit like a worm, or a fossil worm that looks a bit like a fish, it has a chance of being a hagfish,” he says.

Paleontologist Lauren Sallan agrees that his team’s discovery is surprising. “It’s difficult to find hagfish fossils because they do not have bones or other hard parts,” says Sallan, who works at the University of Pennsylvania in Philadelphia.  

But delicate details of Tethymyxine suggest it truly is a hagfish. Some of its organs were preserved in stone. And scientists even found slime glands. That means Tethymyxine made slippery goo to ward off predators, just as today’s hagfish do.

If you bite a hagfish, Miyashita says, “within a second your mouth is full of snot-like goo, and the goo goes down the throat and makes you choke.”

Branching off

The fossil’s resemblance to living hagfish is a clue to these slimy seagoers’ origins. Scientists compared Tethymyxine to other types of fish. They found that some traits that make hagfish seem primitive are actually ones that they formed on their own. Hagfish, it seems, are not leftovers from an early stage of fish evolution.

Miyashita notes that hagfish and more familiar fish share an ancestor. This ancestor had bones, eyes and could sense vibrations in the water. Most fish today have these traits; hagfish do not. That means hagfish went down a different evolutionary route than most fish.

Along the way, hagfish shrugged off these traits. They didn’t need backbones — or any bones, Miyashita says. And from their view point, he says, any animal that can't squirt out buckets of slime to fight off enemies is missing out.

The new findings show hagfish in a new light. “This disproves the older story that hagfish evolved before these features appeared among the ancestors of other fishes and us,” Sallan says.

Hagfish are not a living time capsule of how fish used to be. Instead, they’re unique fish. Fish that found their own way to be super gross. “We thought hagfish were simple,” Miyashita says, “but they turned out to be a step ahead in getting rid of things they didn’t need.” 

Power Words

(more about Power Words)

colleague     Someone who works with another; a co-worker or team member.

eel     A fish with a snake-like body and no scales. Many migrate from freshwater to salt water when it’s time to spawn.

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. 

evolutionary     An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

extinct     An adjective that describes a species for which there are no living members.

family     A taxonomic group consisting of at least one genus of organisms.

fossil     Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.

gland     A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.

hagfish     Slimy, eel-shaped fish without hinging jaws, whose skeletons are made of bendy cartilage rather than hard bone, and with eyes far simpler than those of other fish. Like lampreys, they are considered to be “living fossils” similar to the early relatives of vertebrates that lived hundreds of millions of years ago.

journal     (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject.

organ     (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.

paleontologist     A scientist who specializes in studying fossils, the remains of ancient organisms.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

prehistoric     An adjective for something that happened tens of thousands to millions of years ago, periods before people began deliberately recording events.

tapir     A large, nocturnal, plant-eating mammal that is shaped like a pig and closely related to horses and rhinos.

trait     A characteristic feature of something.

unique     Something that is unlike anything else; the only one of its kind.

Citation

Journal:​ T. Miyashita et al. Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological-molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences. Vol. 116, February 5, 2019, p. 2146. doi: 10.1073/pnas.1814794116.