Catfish eating its dinner

John Lynch beat me to this story about catfish feeding on land, so I’ll be brief. It shows how the eel catfish, Channallabes apus, can manage to take an aquatic feeding structure and use it to capture terrestrial meals. Many fish rely on suction feeding: gape the mouth widely and drop the pharyngeal floor, and the resulting increase in volume of the oral cavity just sucks in whatever is in front of the animal. That doesn’t work well at all in the air, of course—try putting your face a few inches in front of a hamburger, inhale abruptly, and see how close you come to sucking in your meal. So how does an aquatically adapted feeder make the transition to eating on land?

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Malacology morning

My wife thought this story about left-handed snails having a competitive advantage, in that they seem to be better able to escape predation by right-handed crabs, was pretty cool. She also recalled that I’d scribbled up something about snail handedness before, so to jump on the bandwagon, I’ve brought those stories over from the old site.

The handedness of snail shells is a consequence of early spiral cleavages in the blastula. It’s a classic old story in developmental biology—everyone ought to know it!

There was also a story last year about shell chirality in Euhadra. There, it wasn’t a matter of predation, but a potential isolating mechanism, and one where mating compatibility and character displacement could play a role.

Everyone can read up on snails while I’m off at class this morning.

Chirality in Euhadra

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Since Coturnix turned me on to this paper on snail chirality in PLoS (pdf), I had to sit down and learn something new this afternoon.

Chirality is a fascinating aspect of bilaterian morphology. We have characteristic asymmetries—differences between the left and right sides of our bodies—that are prescribed by genetic factors. Snails are particularly interesting examples because snail shells have an obvious handedness, with either a left-(sinistral) or right-handed (dextral) twist, and that handedness derives from the arrangement of cell divisions very early in development.

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Tiktaalik makes another gap

Paleontologists have uncovered yet another specimen in the lineage leading to modern tetrapods, creating more gaps that will need to be filled. It’s a Sisyphean job, working as an evolutionist.

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This creature is called Tiktaalik roseae, and it was discovered in a project that was specifically launched to find a predicted intermediate form between a distinctly fish-like organism, Panderichthys, and the distinctly tetrapod-like organisms, Acanthostega and Ichthyostega. From the review article by Ahlberg and Clack, we get this summary of Tiktaalik‘s importance:

First, it demonstrates the predictive capacity of palaeontology. The Nunavut field project had the express aim of finding an intermediate between Panderichthys and tetrapods, by searching in sediments from the most probable environment (rivers) and time (early Late Devonian). Second, Tiktaalik adds enormously to our understanding of the fish-tetrapod transition because of its position on the tree and the combination of characters it displays.

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Taphonomy of fossilized embryos

There are these fossilized embryos from the Ediacaran, approximately 570 million years ago, that have been uncovered in the Doushantuo formation in China. I’ve mentioned them before, and as you can see below, they are genuinely spectacular.

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Parapandorina raphospissa

But, you know, I work with comparable fresh embryos all the time, and I can tell you that they are incredibly fragile—it’s easy to damage them and watch them pop (that’s a 2.3MB Quicktime movie), and dead embryos die and decay with amazing speed, minutes to hours. Dead cells release enzymes that trigger a process called autolysis that digests the embryo from within, and any bacteria in the neighborhood—and there are always bacteria around—descend on the tasty corpse and can turn it into a puddle of goo in almost no time at all. It makes a fellow wonder how these fossils could have formed, and what kind of conditions protect the cells from complete destruction before they were mineralized. Another concern is what kinds of embryos are favored by whatever the process is—is there a bias in the preservation?

Now Raff et al. have done a study in experimental taphonomy, the study of the conditions and processes by which organisms are fossilized, and have come up with a couple of answers for me. Short version: the conditions for rapid preservation are fairly easy to generate, but there is a bias in which stages can be reliably preserved.

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gen•e•sis

Some fields of science are so wide open, such virgin swamps of unexplored territory, that it takes some radically divergent approaches to make any headway. There will always be opinionated, strong-minded investigators who charge in deeply and narrowly, committed to their pet theories, and there will also be others who consolidate information and try to synthesize the variety of approaches taken. There are dead ends and areas of solid progress, and there is much flailing about until the promising leads are discovered.

Origins of life research is such an unsettled frontier. I wouldn’t want to work there, but the uncertainty and the confusion and the various small victories and the romance of the work do make for a very good story. And now you can read that story in Robert Hazen’s Gen•e•sis: The Scientific Quest for Life’s Origins (amzn/b&n/abe/pwll).

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Modeling metazoan cell lineages

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A while back, I criticized this poorly implemented idea from Paul Nelson of the Discovery Institute, a thing that he claimed was a measure of organismal complexity called Ontogenetic Depth. I was not impressed. The short summary of my complaints:

  • Unworkable idea: There was no explanation about how we could implement and test the idea, and despite promises at the time, Nelson still hasn’t produced his methods.
  • False assertions and confusing examples: He claims that all changes in early lineages are destructive, for instance, which is false.
  • Bad metaphors: He uses a terribly flawed metaphor of a marching band to explain how development works; I’d say that it’s a better example of how development doesn’t occur.
  • No research: Which is really a major shortcoming for a research program, that no research is being done.

Recently, Nature published a paper by Azevedo et al. that superficially might resemble Nelson’s proposal, in that it attempts to quantify the complexity of developing organisms by looking at the pattern within their early lineages. The differences are instructive, though: this paper clearly explains their methodology, presents many of the limitations, and draws mostly reasonable conclusions from the work. It is an interesting paper and contains some good ideas, but has a few flaws of its own, I think. My main objections are that its limitations are even greater than the authors mention, and there are some conclusions that are driven by an adaptationist bias.

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