Why Do Moray Eels Have Two Sets of Jaws? Pharyngeal Jaws

What are pharyngeal jaws?

Pharyngeal jaws are a “second set” of jaws in the throat, or pharynx, separate from the jaws at the front of the mouth. They evolved from gill arches, the bony supports of the gills, in much the same way that the main jaws of vertebrates did long ago.

Pharyngeal jaws are not unique to morays. Most bony fish have them, often with teeth, and use them to process food after it has been swallowed. Cichlids use theirs to crush snails or tear flesh, and many fish rely on them to grind or position food before it goes down. In nearly all of these fish, though, the throat jaws stay in place in the throat. The moray’s are different because they move.

How the moray’s second jaw works

A moray feeds in two stages:

1. The bite. The eel lunges and seizes the prey with its main (oral) jaws, which are lined with long, sharp teeth in most species.
2. The throat jaws strike. The pharyngeal jaws, normally tucked behind the skull, are pulled forward into the mouth. They bite down on the prey, then retract, hauling it backward into the throat and esophagus.

The oral and throat jaws take turns gripping, so the prey is “walked” down the throat a bite at a time. This ratchet-like action is mechanically similar to the way snakes swallow large prey, a striking case of convergence between two long, narrow predators. The extreme reach of the moray’s throat jaws comes from greatly lengthened muscles that pull them forward and back, together with reduced gill-arch structures around them that leave room for the movement.

Other features help keep a grip. Moray mouths extend far back on the head, and in at least one species, the California moray, teeth on the roof of the mouth fold down as prey slides backward, so they do not snap and the prey cannot slip out.

Why morays need a second set of jaws

Most bony fish are suction feeders. They expand the mouth and gill chambers very quickly, so water rushes in and carries the prey with it, then the throat jaws finish the job. Moray eels are poor at suction feeding. Their heads are narrow, their gill openings are small and far back, and they hunt inside reef holes where there is little room to expand the head.

Instead of sucking prey in, morays grab it and pull it in. This works well for their prey of fish, octopuses and crabs, which are often large compared to the eel’s head and fight hard. The researchers who described the system suggested it may have helped make morays such successful predators in the tight spaces of coral reefs.

The teeth on both sets of jaws match the diet. Fish-eating morays such as the green moray have long, pointed teeth, while crab-eating species such as the zebra moray and the snowflake moray have short, rounded jaws with blunt, molar-like teeth. The same patterns show up on their pharyngeal jaws.

How the discovery was made

The moray’s secret came to light in 2007, when Rita Mehta and Peter Wainwright of the University of California, Davis, published their findings in the journal Nature. Working with the reticulate moray (Muraena retifera), they filmed feeding eels and studied their anatomy, and showed that the pharyngeal jaws travel out of the throat into the mouth cavity to grab prey.

It was the first described case of a vertebrate using a second set of jaws to both restrain prey and carry it down the throat. Before that, scientists had assumed morays swallowed like other fish, even though their weak suction made it unclear how they managed large prey.

Can morays eat out of water?

Some crab-eating morays have been seen wriggling out of the sea onto rocks to grab crabs in the intertidal zone. A 2021 study led by Rita Mehta trained snowflake morays to leave the water and feed on land. Out of water, where suction is impossible, the eels still swallowed their prey using their pharyngeal jaws, and the swallowing movements and feeding times were similar on land and in water.

This makes sense: because morays never relied on suction in the first place, being out of the water does not stop them from swallowing. The researchers suggested that the long body, a flexible head and the extreme movement of the throat jaws all help these eels feed on land.

The Alien comparison

When the discovery was announced, it was widely compared to the alien creature in the Alien films, which shoots a second, inner set of jaws out of its mouth. The likeness is real but coincidental. The creature was designed by artist H. R. Giger for the 1979 film, long before biologists knew that moray throat jaws could move so far, and Giger said he had not studied any animal when designing the double jaw.

There is one important difference from the movie monster. A moray’s throat jaws do not shoot out of the mouth to attack. They only reach forward into the mouth cavity to grab prey that the front jaws have already caught.

Moray eels are not the only reef predators with surprising tricks. Some even team up with other fish to hunt, as described in Do moray eels hunt with groupers?, and the real differences between these fish and snakes are covered in eel vs snake.