Monday, April 12, 2010

Moray eels prove to be an exception to the rules

Eels have many traits that suggest intelligence. They are known to hunt cooperatively with groupers in the wild, and go on hunger strikes when in captivity. They are also famous for being spontaneously aggressive. Moray eels have two sets of jaws; oral jaws and pharyngeal jaws (in their throat). The pharyngeal jaws serve to aid the eel in sending their prey down their throats. Each set of jaws has a set of rear facing fangs. When eels feed, they actually bite their prey twice. Joshua Reece, a graduate student at Washington University, decided to research eels for his dissertation when he saw seven species of eel occupying the same niche and eating the same diet. Upon discovering this he said "species don't do that; if they exploit the same niche they don't diversify, and if they diversify they don't exploit the same niche." This study involves two species of eel: the undulated moray (Gymnothorax undulatus) and the yellow-edged moray (G. flavimarginatus), sampled at a dozen different locations across the Indo-Pacific ocean. The are searching for genetic variations that might indicate a past or recent interruption among the gene flow in the eel populations. Reece and colleagues have found that both species of eel are genetically homogeneous across the entire ocean, despite the vastness of the Indo-Pacific. This proves that eels are the most "cosmopolitan of reef fish" and raises many questions as to how and why the 150 species of moray eel formed separate species at all. One hypothesis stems from a proposed "rule of thumb" that the longer the larvae live pelagically, the more genetically homogeneous the species is likely to be. Moray eels will be great to test this idea. As juveniles, they are poor swimmers, and adults only stick to a few square meters of reef. Eel have extremely long-lived pelagic larvae (several months or even years). These larvae (leptocephalus) is the simplest of the self-sustaining vertebrate forms. It has a slender body that is only one cell thick, and it has relatively no digestive tract. Their simplicity means that they cannot digest plankton, so they feed on the shed exoskeleton or the waste products of plankton. A second hypothesis is that their may be barriers in the ocean that the larvae cannot cross such as the Eastern Pacific Barrier and the Sunda Shelf. This hypothesis didn't seem to hold up when tested. Genetic analysis showed that where the eels were living didn't seem to have anything to do with their genetic makeup. These questions are still under investigation.
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  1. Neat post! I never knew eels have two pairs of jaws. Nor was I aware of how complex their behavior may be.

  2. I enjoyed this post! I actually saw something on television where a scuba diver was feeding an eel sausages and the eel got a little frustrated that the diver was taking to long with the sausages so it bit his finger off because the eel thought it was a sausage!

  3. Interesting how two different species were found to exploit the same niche. I've come to realize that in Biology, you have to be careful when you make up a "rule", because that rule will most likely be broken by some "oddball" organism eventually.