Amphioxus and the evolution of the chordate genome

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This is an amphioxus, a cephalochordate or lancelet. It’s been stained to increase contrast; in life, they are pale, almost transparent.

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It looks rather fish-like, or rather, much like a larval fish, with it’s repeated blocks of muscle arranged along a stream-lined form, and a notochord, or elastic rod that forms a central axis for efficient lateral motion of the tail…and it has a true tail that extends beyond the anus. Look closely at the front end, though: this is no vertebrate.

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It’s not much of a head. The notochord extends all the way to the front of the animal (in us vertebrates, it only reaches up as far as the base of the hindbrain); there’s no obvious brain, only the continuation of the spinal cord; there isn’t even a face, just an open hole fringed with tentacles. This animal collects small microorganisms in coastal waters, gulping them down and passing them back to the gill slits, which aren’t actually part of gills, but are components of a branchial net that allows water to filter through while trapping food particles. It’s a good living — they lounge about in large numbers on tropical beaches, sucking down liquids and any passing food, much like American tourists.

These animals have fascinated biologists for well over a century. They seem so primitive, with a mixture of features that are clearly similar to those of modern vertebrates, yet at the same time lacking significant elements. Could they be relics of the ancestral chordate condition? A new paper is out that discusses in detail the structure of the amphioxus genome, which reveals unifying elements that tell us much about the last common ancestor of all chordates.

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The wisdom of the cephalopod

That smart guy, Carl Zimmer, has written an article on those smart molluscs, the octopus. I like that his conclusion is that we can’t really judge their intelligence, because it is different than our own.

That’s the same answer I give to questions about the existence of intelligent life in the universe. I suspect that it’s there (but rarer than most astronomers seem to think — intelligence is an extremely uncommon adaptive strategy here on Earth, as is probably likewise elsewhere), but that it will be radically different in intent and action than our own, as different as we are from a squid, or a dolphin, or an elephant, to name a few forms that have evolved large brains. Often, the question of alien intelligence is more like, “Are there people like us out there?”, and I think the answer to that one is clearly no, almost certainly not. There are too many alternative pathways.

How do you make a cephalopod drool?

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We’re all familiar with Pavlov’s conditioning experiments with dogs. Dogs were treated to an unconditioned stimulus — something to which they would normally respond with a specific behavior, in this case, meat juice which would cause them to drool. Then they were simultaneously exposed to the unconditioned stimulus and a new stimulus, the conditioned stimulus, that they would learn to associate with the tasty, drool-worthy stimulus — a bell. Afterwards, ringing a bell alone would cause the dogs to make the drooling response. The ability to make such an association is a measure of the learning ability of the animal.

Now…how do we carry out such an experiment on a cephalopod? And can it be done on a cephalopod with a reputation (perhaps undeserved, as we shall see) as a more primitive, less intelligent member of the clade?

The nautilus, Nautilus pompilius despite being a beautiful animal in its own right, is generally regarded as the simplest of the cephalopods, with a small brain lacking the more specialized areas associated with learning and memory. It’s a relatively slow moving beast, drifting up and down through the water column to forage for food. It has primitive eyes, which to visual animals like ourselves seems to be a mark of less sophisticated sensory processing, but it has an elaborate array of tentacles and rhinophores which it uses to probe for food by touch and smell/taste. Compared to big-eyed, swift squid, a nautilus just seems a little sluggish and slow.

So let’s look and see how good a nautilus’s memory might be. First, we need a response to stimuli that we can recognize and measure, equivalent to the drooling of Pavlov’s dogs. While they don’t measurably salivate, the nautilus does have a reaction to the hint of something tasty in the water — it will extend its tentacles and rhinophores, as seen below, in a quantifiable metric called the tentacle extension response, or TER.

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The scoring system for tentacle extension response (TER) in chambered nautilus. TER was graded every 5 s from a minimum score of 0 to a
maximum score of 3. Each level corresponds to a range of percentage extension relative to the length of the animalʼs hood. Zero is recorded when all
tentacles are retracted into their sheaths. A score of 1 corresponds to an extension of <33% of the hood length. A score of 2 corresponds to extension
between 34% and 66%, and 3 is given when tentacles are extended beyond 67% of hood length.

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Gerobatrachus hottoni

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It’s another transitional form, this time an amphibian from the Permian that shares characteristics of both frogs and salamanders — in life, it would have looked like a short-tailed, wide-headed salamander with frog-like ears, which is why it’s being called a “frogamander”.

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Complete specimen in ventral view, photograph (left) and interpretive outline drawing (right). Abbreviations: bc, basale commune; cl, cleithrum; cv, clavicle; dm, digital elements of the manus; dt3, distal tarsal 3; fe, femur; h, humerus; ic, intercentrum; il, ilium; is, ischium; op, olecranon process of ulna; pc, pleurocentrum; r, radius; sr, sacral rib.

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