A moray eel’s second jaws lie in wait (top) before snapping out to help catch prey (bottom). Click here to watch a video.

Researchers studying one species of moray eels have uncovered a deadly secret that helps the snake-like fish to swallow their prey. Like the fearsome extraterrestrial from the sci-fi horror classic Alien, these real-life beasts have a second, extendable pair of jaws — encrusted with sharp teeth — that thrusts forward to ensnare hapless fish and shrimp.

High-speed videos and X-ray photos (above) show how the second jaws, called pharyngeal jaws, lie in wait inside the throat, and then extend forwards into the mouth to grab prey that has been captured by the eel's main teeth. The morsel is then drawn into the eel's oesophagus.

This helps the eels (Muraena retifera) to be deadly hunters, despite the fact that, unlike many other predatory fish, they cannot generate strong suction forces inside the mouth cavity to capture a meal. Zoologists had previously been puzzled as to how moray eels, which live on coral reefs and rocky shorelines all over the world, keep hold of their prey long enough to swallow it.

Unlike Sigourney Weaver's big-screen nemesis, these moray eels cannot extend their second set of jaws out beyond their first. But the ability to deliver not one but two bites is still a potent weapon in helping the eels feed, say Rita Mehta and Peter Wainwright of the University of California, Davis, who made the discovery.

"Eels are well respected and often feared by fishermen for their sharp teeth, but I think their true claim to fame may be their pharyngeal jaws," comments Mark Westneat of the Field Museum of Natural History in Chicago.

Bit between the teeth

Many fish species have extra jaws in their throats, which can function to filter food from water or to grind prey when swallowing. But the eel's extendable jaws are the first throat jaws known to be adapted to help catch prey, rather than simply to help swallow it, the researchers explain in Nature¹ this week.

"It's so exciting because this is another example of the diversity you can see in vertebrates," says Mehta. "This is a really amazing innovation."

The trick also adds to the remarkable similarity between eels and snakes, despite the fact that they are from completely different realms of the animal kingdom. The researchers point out that the eel's double bite, which grabs prey before it gets away, can be compared to the ratcheting process by which snakes gradually swallow huge prey items in distinct stages.

Westneat says the discovery harks back to an age when scientists discovered natural phenomena, rather than developing theories and testing them. He calls it "a classic example of discovery-based science, stemming from a 'wow' moment".

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• References

  1. Mehta, R. S. & Wainwright, P. C. Nature 449, 79-82 (2007).