Phil,
Dude, that was brilliant! Thanks for the heads up.
I was thinking about this subject the other day, and I think the discovery of
Acaenoplax lends some credence to the annelid-mollusc link. It looks like a polychaete, sans really noticeable evidence of metamerism in its bauplan (body plan). I don’t think this is accidental though. A little background on why:
In the book
Invertebrates, by Steve and Gary Brusca (1990), four individual theories of molluscan evolution are presented. They are:
The Turbellarian Theory - This theory states that molluscs may have evolved from acoelomate turbellarians, and it based on the homologous (or analogous) mucociliary gliding surface. They don’t like this theory, and I think it’s a long shot at best. I'm sorry, but I'm just not that much of a cladist.
The Modified-Turbellarian Theory – This theory states that molluscs evolved alongside annelids from a turbellarian ancestral group. Hmm… not sure there either. This seems to be dependent on the idea that the aceolmate design "branched" into the annelid coelom and molluscan hemocoel. I think this is also a weak theory, and I go into a bit more detail below.
The Coelomate Theory – This theory states that molluscs evolved from a coelomate ancestor alongside the annelids. Given the idea that molluscs and annelids both show embryonic spiral cleavage, 4d mesentoblast development, and trochophore larvae, I think this is a good theory.
And last, but not least,
The Annelid Theory – This theory states that molluscs are direct descendants of annelids. The Bruscas go on to say that the major problem with this theory is the apparent “de-evolution” of segmentation and coelomic development that would have had to occur in the annelida. Given the aplacophorans and caudofoveatans, and their apparent modifications of the molluscan bauplan, I’m not entirely sold that this isn’t the case. I’ll explain below.
For one thing, “de-evolution” is not an exact term; indeed, body plans are highly modified to suit new patterns in genetic expressions, environmental adaptations, etc. Good examples are arthropod parasites, most notably parasitic crustaceans. Remember the rhizocephalan? At first glance, it appears to be a fungal endo-parasite, assuming hormonal and developmental control of its unlucky crab host. But its larval form betrays its true ancestry. It has a cyprid larva; it is a cirripedian… a barnacle. The modifications on that body plan are insane. I think it was on a neotenic path, and then somehow took a parasitic turn to the dark side. A “simple” parasitic body plan is the result of millions of years of evolutionary specialization, and therefore is not “simple” at all.
The caudofoveatans and aplacophorans are given the “ancestral”, or “primitive” moniker due to their shell-less bodies. Cladists tend to put them low on the molluscan cladogram. Remember H.A.M.? Well, these two groups are enigmatic because, if these cladists are right, the molluscan shell would have vanished from a shelled H.A.M. ancestor, then reappeared. The book says that this is unlikely. HOWEVER, a recent finding in the insecta (Published in Scientific American) seems to indicate that body parts “lost” in the evolutionary process can return (in the given case, it was wings). Darn, I need to find that article… If this is true, then the gene or genes for shell formation would still be found in these two classes, and the lack of shell would be due to extensive modification of their molluscan bauplan.
Imagine evolution reversing its own engineering! Stranger things have happened…
Annelids are segmented, and eucoelomate, but each body part has subtle variations that make it unique. Given their protostome heritage, it is not entirely inconceivable that one or more groups would gradually modify the existing body plan in such a way that the coelom would also be affected. Over time, an ancestral form of annelid (shelled polychaete maybe) could have begun a move toward a less segmented, more molluscan, open-hemocoel form. This is not truly a “de-evolution”, rather it’s a shift in development due to selective pressures and expression of changes in linked genes.
And given the developmental difference, yet structural similarity between coeloms in protostomes and deuterostomes, I think that a lot of genetic analysis will need to be done before we can even consider the coelom as an evolutionary link between groups. The coelom may just be a great idea run through the convergent evolution copy machine.
Hmm... I do play the polyphyly card a lot, don’t I?
Hell, maybe its just my way of copping out, but I can't shake the feeling that similar characteristics do not homologues make.
If the annelid-mollusc links are true, Acaenoplax could be the “missing link” between molluscs and annelids. This is definitely worth a second look.
AUGH!! WHERE IS THE PROOF?!?! @$#%&$*
*sigh* The truth is out there.... oops, wait... that's the X-Files...
Sorry about being so long winded… I think I would have been a lot more coherent if it weren’t for the insomnia.
Sushi and Sake,
John
