The hidden protractable jaw of the moray eel

Scanning electron micrograph of the moray eel's secondary jaw, with highly recurved teeth. Scale bar= 500 micrometres. (Rita Mehta/ Nature) 

In today's issue of Nature, evolutionary biologists from the University of California, Davis report that the moray eel (Muraena retifera) has a protractable jaw that it uses to grasp and swallow prey, in a manner that is reminiscent of the creatures in the Alien films. 

Most bony fish feed by a suction mechanism. The suction-feeding abilities of morays are limited, so until now it was not known how they could swallow the large fish and cephalopods on which they prey.

At rest, the moray's second set of jaws remains hidden behind the skull in the back of the throat. When the moray captures something in its mouth, the jaws are launched out of the throat to grasp the struggling prey and pull it back through the throat and into the oesophagus.

Rita Mehta and Peter Wainwright had previously found that some morays do not use suction at all when feeding. This therefore led them to search for alternative methods that the morays might use to transport large prey into the gullet.

First, they used high-speed video to record morays feeding in laboratory aquaria. This showed that prey is first captured with the powerful outer set of jaws. The secondary jaws are then protracted, moving forward almost the entire length of the moray's skull in the process. The recurved teeth on the secondary jaws ensnare the prey as the jaws are retracted.

Once the prey has reached the back of the throat, the oesophageal sphincter muscle contracts, and a compression of both sides of the body produces a backwards-moving wave. These actions pull the prey into the oesophagus. All this happens within a fraction of a second:

Snakes feed in a similar way, using a mechanism called ratcheting, whereby the left and right jaw arches advance alternately over the prey. Most bony fish have secondary jaws, but they are nothing like as mobile as those of the moray, because their movements are restricted by large bones near the front of the jaw. Other bony fish species therefore grasp prey with their outer jaws , then suck it back towards the secondary jaws.

The findings of Mehta and Wainwright are therefore the first example of a protractable secondary jaw in an aquatic vertebrate. The movements of the jaws are made possible by elongated throat muscles and a reduction in the size of the bones that restrict the movements of the secondary jaws in other fish.

The authors suggest that morays evolved such a feeding mechanism because rocky crevices in which they hunt for prey may have set limits on cranial expansion required to generate the water movements that occur during suction-feeding. They also show that slight modifications to existing structures can lead to striking novelties in function.

Reference:

Mehta, R. S. & Wainwright, P. C. (2007). Raptorial jaws in the throat help moray eels swallow large prey. Nature 449: 79-82. [Abstract]