Wednesday, March 16, 2005
Torquarator bullocki
The deuterostomes are a superphylum of animals that includes three phyla: the chordates (us!), the familiar echinoderms (sea urchins and starfish), and a peculiar group called the hemichordates (at times, you will see another phylum, the chaetognaths or arrow worms, grouped in the deuterostomes, but there is now evidence that they don't belong there). All are linked by their pattern of development. During gastrulation, animals form a structure called the blastopore, which is where migrating tissues tuck themselves inward to establish the three germ layers of the embryo. In deuterostomes, the blastopore will eventually become the anus. In the complementary category, the protostomes, which includes annelids and arthropods, the blastopore develops into the mouth.
The hemichordates are probably unfamiliar to most people reading this. They are marine worms that share two characteristics with us chordates: a hollow, dorsal nerve cord and a perforated feeding structure, the pharynx. They lack two others, the notochord and post-anal tail, hence the name hemichordate. There are two classes of hemichordate, the pterobranchs and the enteropneusts, which differ greatly in appearance and lifestyle.
That's an enteropneust worm (also called an acorn worm) to the left, and a pterobranch above. Enteropneusts are burrowing filter feeders—they crawl through the mud and muck sucking up whatever small organisms they find, and are distinguished by the prominent collar and anterior proboscis. Pterobranchs, on the other hand, are more sessile, and on their collar they bear lophophoral arms, tentacles that reach up into the water column and snag microorganisms that drift by. So, both have a collar and dorsal nerve cord and pharynx, but one, the enteropneust, lacks tentacles and crawls wormlike through the mud, while the pterobranch has tentacles and sits in one place to feed.
Years ago, there was a hypothesis to link the two. Their common ancestor was a wormlike crawler with tentacles called the Lophenteropneusta, and there were suspected representatives of this 'missing link' photographed on the deep sea floor. The worms crawl on the surface of the mud, slurping it in one end and excreting it out the other, leaving behind looping trails of tubelike fecal material. These abyssal worms were thought to have tentacles, but the results of a new paper in Nature show that this is not the case, which means that these worms are 'just' enteropneusts, and the morphology of the common ancestor of the two hemichordate classes is once again wide open. That's not so terrible, though; what's interesting are these fascinating new enteropneust species that Holland et al. have found.
Here, for example, are several species that were photographed but not collected. They're beautiful! They're colorful, some with distinctive wide collars. At the bottom right, you can see the spiral trail one has left behind as it forages.
Deep-sea photographs of broad-collared enteropneusts not yet collected and described (depths, longitudes and latitudes are given in Table 1). a, b, Western Pacific (a) and Eastern Pacific (b) forms, with low dome-shaped proboscis and moderately broad collar. c, North Atlantic form with high dome-shaped proboscis and moderately broad collar extended posterodorsally by two conspicuous lobes. d, Mid-Pacific form with high dome-shaped proboscis and very broad collar. e, A second North Atlantic form with small shield-shaped proboscis and moderately broad collar. Scale bar, 1 cm (a, c); 5 cm (b, d, e).
Holland et al. used a suction sampler to collect some of these worms, and here is one species they characterized, called Torquarator bullocki. The top photo shows the critter gliding across the floor, minding its own business, minutes before the hose on the right sucked it up to the ship waiting on the surface; the bottom photo is the dead and fixed animal, so you can better see its organization. It's a classic enteropneust, with a domelike proboscis and a broad collar behind the mouth. And no tentacles; the Lophenteropneusts are going to have to be listed with the canals of Mars as observer errors.
Torquarator bullocki (Phylum Hemichordata, Class Enteropneusta). a, Living specimen (arrowed) crawling on deep-sea floor just before collection by a hose suction sampler (at right). Scale bar, 5 cm. b, Ventral view of the same specimen after collection and fixation showing the proboscis (Pr), collar (Co), gonadal trunk region (Gt) and intestinal trunk region (It); the black arrow indicates the midventral slit in the collar, and the white arrow the artefactual compression of the right side of the collar that occurred during capture and fixation. Scale bar, 1 cm.
The sad specimen above was a bit crumpled and shrunken. Here's a diagram to better show what it looked like in life.
A diagram of Torquarator bullocki in dorsal view. The lateral wings are shown unfolded from the dorsal side and extended laterally to permit a clearer view of the course of the digestive tract (in yellow).
This discovery doesn't entirely invalidate the lophenteropneust hypothesis; it does show that these deep-sea worms aren't an intermediate form, but there may still be evidence uncovered to show that the pterobranchs evolved from the enteropneusts. And of course the hypothesis, even if it were shown to be false, has been productive in driving research to identify these obscure and wonderful animals.
Holland ND, Clague DA, Gordon DP, Gebruk A, Pawson DL, Vecchione M (2005) 'Lophenteropneust' hypothesis refuted by collection and photos of new deep-sea hemichordates. Nature 434:374-376.
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