Monday, January 7, 2008

More Anguilliform than Anguillidae

As I realized with the pig-nosed turtles, writing on just one species can be very difficult. I'm still not entirely sure how some people can write far-reaching articles summarizing entire groups, my hat goes off to them. Despite spending an entire semester coping with writing papers on the death penalty, juggling 20 articles or more at once still ain't easy, even on vacation. I don't want to spend another week on a post like I did the last one, yeesh.

Well, enough complaining. As you could tell from my previous post I clearly had "primitive fishes" on the mind, in addition to turtles, particularly lampreys and hagfish. Discovering that I do indeed have access to the book Primitive Fishes outside of Maine made writing a post on them inevitable. Note the title for a hint at where this is leading.

While the pig-nosed turtle was a relict that could shed light on early cryptodire or possibly chelonian evolution, hagfish (Myxiniformes) and lampreys (Petromyzontiformes) branched off somewhere close to the origin of vertebrates themselves. While they share features such as pouch-like gills, a lack of jaws, horny "teeth", and a protractible/retractable "tongue"; hagfish lack vertebrae and display many "invertebrate" characteristics. Genetic and morphological evidence still hasn't determined if lampreys and hagfish form a monophyletic group ("cyclostomes") or not; if they do then either hagfish have many reverted characteristics or lampreys have converged significantly upon jawed fishes (Janvier p. 13). The concept of lampreys being the sister grouped to jawed vertebrates and hagfish being non-vertebrates seems to be the most common, and in my opinion the best supported. Hagfish and lampreys have apparently retained highly derived characteristics for at least 305 and 360 million years respectively (Janvier p. 6-7) and presumably these adaptations allowed them to survived for as long as they have. I don't really prefer the term "primitive" because it conveys that there is something inadequate about the creature, but evolution works by the concept of "if it ain't broken, don't fix it?" Hagfish are capable of incredibly slime production, which suffocates fish gills (indicating that they can tolerate no oxygen for extended periods), and their flexible spine allows them to tie themselves in knots to clean off mucous or make it hard for predators to handle (Ilves & Randall p. 523). At least one species of lamprey is capable of surviving out of water in humidified are for at least four days while maintaining the same metabolic rate, and they are very capable swimmers (Brauner & Berenbrink p. 229).

This brings us, eventually, to the what inspired this post. From the "Locomotion in Primitive Fishes" chapter it is noted that eels (Anguillidae) move with anguilliform locomotion which involves bending a large portion of a uniform body; but lampreys and hagfish exhibit a lot of rostral movement which eels lack, and and arguably have a more characteristic anguilliform locomotion than the eels themselves (McKenzie et al., p. 322-323). Despite lots of research on the neurology of lampreys, the swimming method has not been very well studied. At low speeds in hagfish, tail beat frequency decreases with increased speed (possibly due to the notochord stiffness) although at a point speed increases with beat frequency. The authors suggest that "The recent work on hagfish suggests that there are likely to be interesting and fundamental principles of swimming that have yet to be described in these unusual fishes". That's an absolutely remarkable statement, although given the fishes in question perhaps it is not that surprising. Lamprey and Hagfish have encountered numerous radiations of animals throughout hundreds of millions of years with no major structural changes, so it is likely that there are many fundamental differences between them and conventional fish.

Exactly what advantages this form of locomotion has cannot be answered, but perhaps this locomotion is the ancestral condition for craniates/vertebrates which these two groups subsequently modified.

While not the exact same form of locomotion, tadpoles are also noted as wobbling their heads as they swim, which has been noted as poor swimming, and even used as a model for an inefficiently swimming vertebrate (Azizi et al., 2007). One early authority even declared that ‘‘the locomotion of the oldest vertebrates must have been of the relatively ineffective and uncontrolled type seen in a frog tadpole’’. However, oscillations of the head actually do appear to contribute to thrust, and a model of a tadpole without the "wobbliness" was less efficient. The vertebral column of tadpoles is very short (Azizi et. al, 2007), so it isn't too clear how comparable this could be to primitive fishes. Still, this and the remarkable swimming performance of lampreys suggests that "primitive" or "inefficient" locomotion can have some hidden surprises.

Since this article also somewhat functioned as an introduction to "cyclostomes", it looks like I'll have to make a few more posts...

Oh, and with the tadpoles, I should mention that this is the Year of the Frog.


References:

Azizi, Emanuel et al. 2007. Vertebral function during tadpole locomotion. Zoology 110, 290–297.

Brauner, C. J. & Berenbrink, M. "Gas Transport and Exchange" from Fish Physiology pages 213-283.

Ilves, K. L. & Randall, D. J.. "Why Have Primitive Fishes Survived" from Fish Physiology pages 515-537.

Janvier, Philippe. "Living Primitive Fishes and Fishes from Deep Time" from Fish Physiology pages 1-51.

McKenzie, D. J. et al. "Locomotion in Primitive Fishes" from Fish Physiology pages 319-380.

2 comments:

Mickey Mortimer said...

Man, and here I thought the title meant you'd be discussing the non-anguillid anguilliforms- serrivomerids, derichthyids, myrocongrids and the like. :(

Caitling said...

Slime, you should talk more about slime.

Slightly more appealing than slime is my amazement at the fact that there are still ways of swimming yet unknown to scientists. There really shouldn't be that many different variations on swimming.I guess it would work as a good distraction. People become too fascinated by watching you swim that they don't realize that you're swimming away from them, thus avoiding being eaten. Good plan old chum.