I started this series because I really wanted to know more about Chaetognaths. I've never run into them in any samples I've collected nor have any real experience with them at all. This may be the reason for my fascination of them. After reading the 2 articles from Current Biology that I discussed in Part 1 and Part 2, I have certainly developed an appreciation of their evolution and Chaetognaths as organisms in the general sense. That is where this post comes in. In that same issue of Current Biology, there is - believe it or not - a third article on Chaetognaths by Ball & Miller. This paper discusses more of the biology of chaetognaths and why they just plain cool creatures.
They are a relic of the Cambrian and fossils show that not much has changed in their body plan (above).
"The chaetognaths are an ancient lineage of invertebrates that shares some characteristics with just about every other major invertebrate phylum and has consequently puzzled taxonomists ever since its original description in 1769. Darwin described chaetognaths as ‘‘remarkable for the obscurity of their affinities’’ and they have puzzled a succession of eminent zoologists ever since. Though unfamiliar to most biologists, chaetognaths are typically the most abundant planktonic predators, sometimes accounting for more than 10% of zooplankton biomass and being outnumbered only by their major prey, the copepods."-Ball & Miller 2006Chaetognaths feed on copepods by sensing their vibrations and are able hunters in darkness (click here for a movie of a chaetognath hunting!). Some even have a neurotoxic venom. They are also simultaneous hermaphrodites. One species, Paraspadella gotoi, exhibits interesting mating behavior with ritualistic dance before exchanging sperm packet(below).
I found it interesting to that the authors note the nervous system bears a resemblance to nematodes, kinorhynchs and priapulids. In the phylogenetic papers of Matus et al. 2006 (see Part 1) and Marlétaz et al. 2006 (see Part 2), the authors take out priapulids from the analyses because they didn't like that Chaetognaths grouped with them within the Ecdysozoa. The Ecdysozoa clade is supposed to encompass the moulting animals. So Chaetognath's nervous system resembles members of the Ecdysozoa AND Chaetognaths have a cuticle. To my knowledge, animal cuticles get shed periodically? I'm not entirely certain if this is true or not, but something to think about. Another thing to about are a priori assumptions, such as we would like Chaetognaths to be closely allied to the Deuterostomes yet have some affinity with Protostomes too. You can't have your cake and eat it to. I think if you are going to use a consensus tree from dozens of genes, you should use the taxa possible/applicable to the analysis. In this case, all animal phyla because the question related to a phylum's position in the phylogeny of the animals.
I have no doubt from the molecular work shown to date that chaetognaths align with protostomes. But the protostomes are a HUGE group encompassing some of the most diverse animal taxa such as the arthropods, molluscs and annelids. Where they fit in that tree means alot evolutionarily. There are alot of biologists that outright reject the Ecdysozoa hypothesis for a variety of reasons. Libbie Hyman, who wrote book(s) on invertebrate diversity quite literally, recognized the affinities of the Chaetognaths to the protostomes:
‘‘It seems probable that the chaetognaths should be regarded as having diverged at an early stage from the primitive ancestor of the Bilateria.’’-Quoted in Ball & Miller 2006But Chaetognaths aren't the only unresolved taxon. There are many minor taxa which have few people working on them. Some taxa don't even have anyone in the U.S. as a specialist working on that group. Many of these taxa are in desperate need of study and revision and can potentially fill in alot of holes in animal phylogeny.
More Chaeognath Stuff:
Erik Thuesen's Chaetognath Webpage
PZ Myers Chaetognath Friday Blogging (2004)