The Mystery of the Toothy Eels

by Justine Jackson-Ricketts
Most marine fishes suck food into their mouths like oceanic vacuums. Others use powerful teeth to snag their prey. But only one type of fish began with teeth: the eels.

Scientists suspect that eels were originally biting fishes. All other fishes began as suction feeders, and some evolved biting later. Why and how did eels become biters so much earlier in their evolutionary history?

This mystery intrigues Rita Mehta, a biologist at UC Santa Cruz. She searches the skulls of eels and other biting fishes for clues to this evolutionary puzzle. “I try [to] understand how they’re opening and closing their jaws to grab the prey based on their jaw structure and the muscles in their head,” she says.

There are already many different biting tactics in the ocean. For example, coral-munching bumphead parrotfish have beak-like jaws with teeth resembling cement blocks. Moray eels and barracudas have long, sharp-toothed jaws. Many cleaner fish bite quickly with their small jaws.

Mehta, however, has always been interested in how snake-like animals feed. “All of my graduate work was on snake feeding behavior,” she says. “Imagine eating without any limbs. That’s really challenging.” After looking at a moray skull, she realized, “I don’t think it’s using suction to capture prey.” So she began wondering how moray eels feed.

A few years ago, using high-speed video, she got her first glimpse. Reticulated moray eels use a second set of jaws in their throats in a frightening way: these inner jaws dart forward to grasp a victim held within the outer jaws and draw it back into the throat.

Prior to this discovery, the mystery of moray feeding gripped her. “I knew, Hell or high water, I was going to study this,” Mehta says. It paid off. “When we got the high-speed video, sure enough, they have this novelty in the way they not only capture but swallow their prey.” She published her findings in Nature in September 2007, sparking widespread interest in moray eel jaws. The New York Times published an article about her work, and Joe Palca interviewed her on NPR’s Morning Edition.

Now that she holds a faculty position, Mehta is putting a lab together. She has dubbed her lab “ELVer,” for Elongate Limbless Vertebrates – and, slyly, the term for a baby eel. Upon entering ELVer, visitors see a clean, well-ordered room. A set of shelves contains jars of fish specimens and empty tanks awaiting live eels. A counter along one wall holds stacks of small clear plastic boxes, each neatly labeled and containing at least one skinned and colorfully stained eel. Peering into a box is like looking at a colorful X-ray printout. You can clearly see the inner jaws with their sharp, curved teeth.

She has about 250 specimens spanning nearly 180 eel species. Mehta builds her eel collection in a few different ways. She relies heavily on museums such as the California Academy of Sciences to achieve the diversity she needs for comparative studies. She also goes diving in the Caribbean to bag them herself. To understand the more common species, Mehta orders eels from aquarium suppliers. When live individuals arrive in her lab, the investigation can begin.

For her feeding studies, Mehta must first take high-speed video of eels feeding. Her camera captures 100 images every second. When viewed in real time, the food seems to disappear. However, when Mehta slows down the video, you can see the tips of the eel's inner jaws reach forward, snatch the food, and pull it back.

To get her high-speed video, Mehta puts eels through their paces. She must capture the precise moment that an eel closes its jaws on its victim. For this, she must film live eels in profile when they attack, meaning that Mehta must train them to feed facing perpendicular to her camera. “It’s pretty hard,” she says, “only because they’re elongate fishes without any limbs. That gives you extra maneuverability. You basically are able to twist and pitch and roll your body.”

So, Mehta trains them to come straight at the offered food item rather than twist around. Sometimes this means “putting the food far away,” she says. “And so if it’s really hungry, it’ll rush in, and the faster it rushes in, it doesn’t have time to twirl or do these crazy things.” Another way she can ensure a straight attack is to place a board in the eel’s tank to narrow its field of view. However, this tactic doesn’t work with skittish eels.

To study what happens under the skin, Mehta euthanizes her research subjects and preserves them through pickling. She then takes measurements of each eel’s head and weighs it. Next, she skins it, which turns out to be remarkably easy. “Because they’re elongate, the skin comes off like a sock, pretty much,” she says. After that, Mehta stains the bones red and cartilage blue so that she can easily differentiate between them when visualizing how the jaw moves during feeding. Finally, she takes many more different skull measurements.

This careful set of procedures led to her pivotal discovery in 2007. She was working with Peter Wainwright at UC Davis, studying the feeding methods of the reticulated moray eel. Like most bony fishes, reticulated morays have a second set of jaws behind their heads, called “pharyngeal jaws.” These jaws have sharp, curving teeth. Mehta discovered that this dramatic skull structure does not exist simply to help the eel swallow, as scientists previously believed. Rather, after the outer “oral” jaws snag an unsuspecting victim, the pharyngeal jaws shoot forward, grab the struggling creature, and drag it back into the esophagus.

It sounds like something out of a science fiction movie, and it’s pretty close. The title creature in Ridley Scott’s Alien had a second jaw to snatch its prey. The alien jaw, however, wasn’t nearly as fast as a real moray jaw — and it was the only jaw that grabbed the prey. In morays, “both jaws grab a hold of the prey before the oral jaws let go,” says Mehta.

Recently, she discovered something else unexpected. Mehta and other scientists had thought that biting didn’t require as much skull specialization as suction feeding, because biting actually is easier. Suction feeders must expand their mouths quickly to create a force that wraps their prey in a vortex of water and pulls it into the mouth. Many skull elements must work together to make this happen. In biting, however, the fish simply snaps its jaws closed around its victim. But Mehta found that the skulls of biters show greater diversity than those of suction feeders. This surprise hints that there’s still much to learn about how eels and other biting fishes developed—and how they persevere.

It also means there’s a lot more to uncover about evolution in general. Although scientists have worked out the basics, every once in a while an exception arises. Moray eels, as the only fish species that began as biters, are one such exception. Perhaps morays are a stepping stone to a more complete understanding of the complex process of evolution and how a marine lifestyle shapes how bodies develop.

Although Mehta has her own questions about eel evolution, she will take requests. “People always ask me, ‘How big of a prey can morays eat?’ And I don’t exactly know,” she says. A six-foot moray can wrestle a three-foot octopus into submission. “And an octopus isn’t that challenging because it’s really squishy.”

People also ask her if all morays eat large fish. “But morays don’t always eat fish,” Mehta says. About 10-15% of the 202 species of moray are durophagous: they only eat hard-shelled animals. While fish-eating, or piscivorous, eels have sharp, curved teeth, the teeth of durophagous eels resemble pebbles. One of Mehta’s postdoctoral researchers is studying the evolutionary paths morays have taken to become durophagous. They have found that while all durophagous morays have evolved shorter jaws, some also have smaller teeth, a wider skull, or a thicker lower jaw. A number of divergent routes over time may have converged on durophagy.

Mehta believes morays are biters because they live in crevices and must eat large prey to sustain their size. Morays don’t have the space in their crevices to expand their mouths like suction feeders, so they must quickly lunge out of their crevices to grab prey. But Mehta needs more evidence for this hypothesis. She and collaborators at UCLA are fitting together the puzzle pieces they have so far by assembling a network of eel lineages. However, it’s an 800-piece puzzle, and about 700 pieces are still missing.

The biggest challenge is that “they’re really hard to study, because they’re elusive, they’re hard to obtain,” says Mehta. “How do we get the general public excited [about] helping to discover things unless we show them discoveries [and] cool insights?” That very challenge, however, drives Mehta to continue learning as much as she can about morays. As she says, “Eels are just one big shocker after another.”

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Justine Jackson-Ricketts, a second-year graduate student in Ecology and Evolutionary Biology, wrote this article in spring 2011 for SCIC 160: Introduction to Science Writing.