November 28, 2007

Strange BiologyAn Alien Among Us!

A moray eel emerges from a crevice in a reef.

Truth is often at least as strange as fiction. Rita Mehta, an evolutionary biologist at the University of California, Davis, found that the moray eel uses a secret weapon to eat its prey, one that seems straight out of the movie Alien.

Moray eels are ray-finned fish, but they don’t hunt or eat like other ray-finned fish do. Most ray-finned fish, including other types of eels, feed by suction. A typical fish will get close to its prey, then swell its mouth cavity, sucking water and the hapless morsel into its mouth. Next, the fish uses a second set of jaws in the throat, called pharyngeal jaws, to grind the food and push it to the stomach.

This feeding mechanism works very well for many fish. However, moray eels live in and hunt from narrow crevices in coral — a cramped environment for suction feeding. Instead, moray eels eat by biting. This, however, poses a problem: How does a moray eel get a large, slippery animal down its throat? If the moray lets go of its prey to swallow it, the meal might just escape.

The monster from the movie “Alien.”

Moray eels are very efficient predators that evolved an effective feeding strategy. Once the moray gets a bite on its prey, it moves a set of ferocious pharyngeal jaws out of its throat that clamp down on the prey and drag it down the eel’s esophagus. Much like the monster in Alien, the first set of jaws is dangerous, but the second set does the dirty work.

Oral Gymnastics

While researching moray eel feeding habits, Mehta and her colleague Peter Wainwright filmed morays eating. When Mehta and Wainwright were reviewing the film, they discovered the double bite. “When we got the movies, we sat and stared in disbelief,” Wainwright said.

”The pharyngeal jaws in their throat exhibit a very different architecture from the jaws of other bony fishes,” Mehta said. "[They] look like a fancy pair of forceps with large, sharp recurved teeth.” They further investigated the way these pharyngeal jaws work and how they evolved.

X-rays that show the moray eel’s pharyngeal jaws both at rest (top) and while in use (bottom).

Mehta and Wainwright recorded the moray eels while they were eating using an X-ray imaging technique and high-speed video; Mehta also carried out anatomical dissections. Through their research, they found unusually long muscles running from the second set of jaws to the moray’s skull. These muscles pull the pharyngeal jaws up the entire length of the skull to grab the prey.

The pharyngeal jaws rest behind the skull when they are not in use. When needed, they quickly move up and forward into the mouth to grasp the prey and yank it back into the esophagus with a gulping motion. The two sets of jaws work together until the whole animal has been swallowed.

How This Fierce Double Bite Evolved

Mehta found that moray eels evolved forceps-like pharyngeal jaws in response to their environment. There simply is no room to expand the mouth cavity with the required suction within the coral crevices in which they hunt. In fact, Mehta found that morays do not have the strong muscles and bones needed to expand the mouth cavity to suck in prey. Suction feeding also restricts the size of prey that can be drawn into the mouth. Biting allows morays to grab larger, more slippery prey such as squid and octopuses.

A close-up view of a moray eel’s pharyngeal jaw.

It is this major adaptation that may be responsible, in part, for the success of moray eels as chief predators hunting within the complex crevices of coral reefs.

According to Mark Westneat, curator of zoology at the Chicago Field Museum of Natural History who studies the feeding behavior of fish, having pharyngeal jaws is common in fish that lack teeth for grinding food. However, only morays have been found to have ones that are so highly evolved and use them so uniquely. “Having a jaw in your throat that has long, recurved, canine teeth — and can actually shoot out of your throat into your mouth, grab a fish or something, and pull it down the throat — is highly unusual,” he said.

Sometimes real-life can be as frightening as in the movies. When asked which set of teeth should be feared most, Mehta replied, “Oh, man, that’s tough, the front ones are really long and sharp, too. I’d be afraid of both!”

More to Explore

• If the First Bite Doesn’t Do It, the Second One Will
• Moray Eels Grab Prey with “Alien” Jaws
• Raptorial Jaws in The Throat Help Moray Eels Swallow Large Prey
• Moray Eels Are Uniquely Equipped to Pack Big Prey Into Their Narrow Bodies (includes video of eel feeding)
• Concealed Weapon: Eels’ Second Set of Teeth

Last updated November 28, 2007 | Comments (17) | Permalink | Strange Biology