I ran across this word in the form “autotomized” the other day. It’s one of those words that not many people may know but that describes something most everyone knows an example of. It’s the shedding of body parts, usually as a defensive measure. Many lizards do it, a lot of arthropods are capable of it, and sea cucumbers throw up their internal organs when threatened, later regrowing them. It’s an interesting process, since usually (honeybee stings are an exception) there are anatomical modifications that act to prevent trauma and circulatory fluid loss following the shedding, and to regrow the lost body parts.

We usually think of Annelida as being the phylum that contains earthworms and leeches, but the largest group among the annelids is marine polychaetes, or bristleworms. They’re very diverse, occupying many ocean habitats, ranging from freeswimming to bottom dwelling. You’d recognize some of them: Christmas tree worms, tubeworms, sandworms, and clamworms (a few photos here), although you might not have thought of some of these as worms. Do a google images search on “polychaetes.”

So that brings us to the recent discovery of seven new species of polychaetes. I found the report in the most recent issue of Science (Osborn et al., Science 325, 964 (2009); subscription wall). Here’s an individual of one of the new species, Swima bombiviridis:



All of these species are very deep ocean dwellers. They were discovered by remote operated vehicle one to two miles deep. While most polychaetes are demersel (bottom dwellers), three of the new species are strong swimmers, found as far as 1200 feet above the ocean floor. This individual has elaborate bristles extending from the parapodia ("legs") that act as swimming paddles.

Five of the species have four pairs of fluid-filled organs, and here’s where the word autotomy comes in. When disturbed, the worms drop these “bombs,” and the globules then produce brilliant green bioluminescence, presumably distracting attention from the real worm. Is that cool, or what?

Here is a diagram of a cross section of a typical polychaete, taken from this interesting discussion of polychaete anatomy. Unlike earthworms and leeches, polychaetes have a number of head appendages and tentacles, including structures called “branchiae.” In the interests of making them especially creepy-looking, these provide the foundation for the elaborations that can give polychaetes a spectacular appearance. In the case of the bomb-carrying species, the bombs are modifications of, and therefore homologous to, the branchiae.