I, BM

299 792 458 m·s^−1 6.67384(80)×10−11 m³·kg^−1·s−2 6.626 069 57(29) × 10^−34 J·s 1.054 571 726(47) × 10^−34 J·s 4π × 10^−7 N·A^−2 = 1.256 637 061... × 10^−6 N·A^−2 8.854 187 817... × 10^−12 F·m−1 376.730 313 461... Ω 8.987 551 787... × 109 N·m²·C^−2 1.602 176 565(35) × 10^−19 C 9.274 009 68(20) × 10^−24 J·T^−1 7.748 091 7346(25) × 10^−5 S 12 906.403 7217(42) Ω 4.835 978 70(11) × 10^14 Hz·V−1 2.067 833 758(46) × 10^−15 Wb 5.050 783 53(11) × 10^−27 J·T^−1 25 812.807 4434(84) Ω 5.291 772 1092(17) × 10^−11 m 2.817 940 3267(27) × 10^−15 m 9.109 382 91(40) × 10^−31 kg 1.166 364(5) × 10^−5 GeV^−2 7.297 352 5698(24) × 10^−3 4.359 744 34(19) × 10^−18 J 1.672 621 777(74) × 10^−27 kg 3.636 947 5520(24) × 10^−4 m² s^−1 10 973 731.568 539(55) m^−1 6.652 458 734(13) × 10^−29 m² 0.2223(21) 1.660 538 921(73) × 10^−27 kg 6.022 141 29(27) × 10^23 mol^−1

« I was wrong. THIS is the best thing Experimental Vibrational Analysis for Hire. | Main | My kind of Dr Seuss. »

Rate this Entry

What ails thee, eels? Say ahhhhOH MY GOD

Posted 01-03-2009 at 05:34 PM by Bullet Magnet

No. 11 of the 15 Evolutionary Gems: recent important developments in evolutionary biology that support the theory of evolution: contingency in the evolution of pharyngeal jaws in the moray.

Before I get to the awesome cool stuff, I'll have to run you down on Biology 101. If you don't want the background lesson, skip down to the part labelled "SMEG FOR BRAINS".

The vertebrate jaw: that of humans, other mammals, birds, reptiles, amphibians and jawed fishes extant and extinct (collectively the clade known as the Gnathostomes); is evolved from the gill arches of the ancestral jawless fish. As embryos, all chordates develop "pharyngeal structures" (part of the pharynx) in the pharyngula stage of development. You may have heard of these so erroneously referenced in the media and biology classes as the "gill slits" we supposedly develop in the womb.

Actually, they only develop into gills in fish, where they are expressed in all their bony glory as skeletal gill arches. In other vertebrates, they developed into the structures that gills evolved into (or rather, that the pharynx was repurposed for). In us, the pharynx ultimately becomes this general area:

However, the development of the most anterior (frontal) gill arch is redirected during embryonic development to become jaws in jawed vertebrates. But the fact that gill arches, and the pharyngeal structures they develop from, are serially repeated raises an interesting possibility. Suppose that instead of just one developing into the jaw, the others followed suit? It would be a fairly simple mutation, and you could have a whole sequence of jaws!

SMEG FOR BRAINS

As you can no doubt tell from the x-ray, this is a moray eel, and it has done just that: evolved a second pair of jaws inside its throat, and a set of protrusive and retractive muscles to move them forward and back. Yes, yes, I know you're all thinking "HR Giger" and the "Alien" movies...

Why would a moray eel have a second set of jaws? This requires more explanation, so to avoid it, skip to the ALPHABET HEAD section.

The jaws of teleost fish (bony fish, as opposed to cartilaginous sharks or jawless fish) are impressive examples of morphological diversity. In most such predatory fish (and a great many fish are, one way or another, predatory), prey are capture mainly by suction. Food is literally inhaled, and anyone who as ever watched pet, pond or river fish feed will know what I mean. The jaws are opened, the floor of the oral cavity is depressed and the opercula (gill flaps) are flared, creating a partial vacuum that sucks the prey into the mouth, holding it there for the teeth to get to work in the usual way.

If you are familiar with the lifestyle of moray eels, you will have noticed a problem with their using of this technique. They live in confined spaces (like so

so any feature that requires expanding the mouth and pharynx is such a way isn't going to fly. And if they can't use suction to hold prey items in place and pull into into the throat, it would drift away each time it opened its mouth to take another bite or shift it further down its throat.

Enter pharyngeal jaws. With two jaws, one can hold onto the food while the other lets go to bite onto a second position further up, a sort of "ratcheting" action.