A semi-real life "Alien."

[selke]

The moray eel has two sets of jaws.  The main set and a smaller set inside the mouth/throat, to grip the food.



The vertebrate jaw is a product of evolution - we have a serially repeated array of pharyngeal structures as embryos (and fish retain them in all their bony glory as gill arches), and the anterior most arch is modified during our development to form the jaws. The fact that they're serially repeated raises an interesting possibility: what if, instead of just the one developing into a jaw, others were transformed as well? You could have a whole series of jaws!

One animal has done exactly that. The moray eel has modified one of the more posterior pharyngeal arches into a second pair of jaws, with a set of muscles that can slide it forward to bite prey already held in the mouth.



a, Posterior placement of the pharyngeal jaws in relation to the skull. The arrow points to the pharyngeal jaws. b, Pharyngeal jaws in their protracted position. The arrow points to the upper pharyngobranchial. Scale bar for a and b, 1 cm.
I know, everyone is thinking HR Giger and the Alien movies … and it is similar to that. These animals have toothed jaws in their throat, and a set of protrusive and retracting muscles to move them forward and back.

Why? In teleost fish, the jaws are wonderfully complex and malleable, and are impressive examples of morphological diversity. In most predatory fish, the common technique for capturing prey is suction feeding: opening the jaws, depressing the floor of the oral cavity, and flaring the opercula (gill flaps) creates a partial vacuum that sucks the prey into the mouth and holds it there while the teeth engage in their bloody work. This is a problem for eels, since they're adapted for living in confined spaces and narrow crevices - a morphological feature that requires expanding the mouth and pharynx may not work well. And if they can't use suction to draw food deeper into the throat, it might drift away each time they open their mouth to take another bite.

The moray has evolved a new eating strategy. Bite into the prey with the usual jaws, and then reach forward with the pharyngeal jaws and bite again. Open the main jaws and release, and the pharyngeal jaws pull the prey deeper into the mouth. The main jaws bite and hold again, and the pharyngeal jaws can reposition and bite more. The food is ratcheted deeper and deeper into the gullet by pairs of jaws taking turns to seize and tug.

The pharyngeal jaws can be seen in this quicktime movie. The eel bites on a bit of squid, and then about half way through the clip, you'll see a disturbing, creepy something between its gaping jaws reach forward and bite into the meal. It is so cool, and so unexpected.

It's a wonderfully, beautifully wicked mechanism.



(click for larger image)
The left dentary has been removed in a-c, and the left maxilla has been removed in b and c. a, Pharyngeal jaw apparatus at rest. b, Pharyngeal jaw protracted: the levator internus (LI) and levator externus (LE) protract the upper jaw into the oral cavity, whereas the rectus communis (RC) protracts the lower jaw. During protraction, the upper pharyngobranchial is dorsally rotated by contraction of the LI and the obliqus dorsalis (OD). c, After prey contact, the adductor (AD) contracts to bring the upper and lower jaws together to deliver a second bite. The dorsal retractor (DR) and pharyngocleitheralis (PHC) retract the pharyngeal jaws back to their resting position behind the skull. Scale bar, 1 cm.
The closest resemblance to this mechanism elsewhere in vertebrates is snakes, which use gnathic transport in a similar way. They don't have quite as elaborate a set of pharyngeal jaws, but they do use a pharyngeal ratchet to pull prey down into their throat.


http://scienceblogs.com/pharyngula/2007/09/jaws_of_the_moray.php

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