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Mollusks evolved brains on four different occasions

DWhatley

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#1
VIA Erin on Facebook:

Mollusks evolved brains on four different occasionsBy Alasdair Wilkins, Sep 18, 2011 9:45 AM
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Facebook Twitter StumbleUpon Tumblr Instapaper Close As far as invertebrates go, you don't get much smarter than the members of the mollusk phylum, which includes octopuses, cuttlefish, and squid. But mollusks apparently didn't think through their evolutionary path very well, developing brains four times over.

That's the finding of researchers at Auburn University. Kevin Kocot and his team examined the genetic sequences of the eight main branches of the mollusk phylum. They hoped to determine which branches are most closely related to which others, and in doing so provide a clearer history of the specifics of mollusk evolution. Until now, it was assumed that the two mollusk groups with the most highly organized central nervous systems, the cephalopods (octopus, cuttlefish, squid) and the gastropods (snails and slugs), are the most closely related.

Now it appears that that's actually almost the exact opposite of the truth. According to Kocot's analysis, the gastropods are most closely related to bivalves (clams, mussels, oysters, and scallops), which have far more rudimentary nervous systems and not much of a brain. Even more shockingly, cephalopods - the most intelligent of all the mollusk groups - comes from one of the earliest branches, meaning their evolutionary development predates that of snails, clams, and the rest.

There's no way that cephalopods and gastropods could have evolved together apart from all the other mollusks, which means that their similarly advanced nervous systems must have developed independently. That goes against a lot of longstanding assumptions about the evolution of sophisticated structures, as Kocot's colleague, University of Florida researcher Leonid Moroz, explains:

"Traditionally, most neuroscientists and biologists think complex structures usually evolve only once. We found that the evolution of the complex brain does not happen in a linear progression. Parallel evolution can achieve similar levels of complexity in different groups. I calculated it happened at least four times."

A lot of evolutionary theory has been guided by something akin to Occam's Razor - it's simpler to assume that something as complex as the brain only evolved once in a given group, and that all brainy members of that group come from a single common ancestor. Mollusks appear to be pointing us towards a very different story of evolution, one governed by parallel developments and the repeated emergence of brains in wildly divergent groups. Evolution doesn't have any set goals, but it does appear that it has certain ideas and structures it just keeps coming back to.
 

neurobadger

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#3
Basal cephalopods strongly resemble monoplacophorans. Ellesmereoceras looks like a cone with arms.

This is one of the areas I'm most fascinated by - the commonalities among nervous system development in different clades.

I think I'm going to have to beg Dr. Moroz for a copy of this paper.
 

neurobadger

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#4
I GOT THE PAPER

Here's what the tree looks like:

Mollusks have a common ancestor with annelids. Within the mollusks, it is separated into Conchifera and Aculifera. Aculifera contains the aplacophorans (more specifically the groups called Neomeniomorpha and Chaetodermomorpha ) and polyplacophorans and Conchifera contains the bivalves, gastropods, scaphopods, and cephalopods. Cephalopods split off from the other conchiferans early, then scaphopods, and gastropods and bivalves are more closely related.
 

robyn

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#5
This is an excellent paper - the phylogenetics are really a cut above what has been standard in the field, and probably represent the new standard for total information. Tree building using 300 or so genes is a huge computational task, particularly using different substitution models for each gene. Their discussion of the implications for neural evolution is really interesting. I suspect this will generate a lot of discussion, particularly regarding the deep split of gastropods and cephalopods.
 

SteveM

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Yes this is a far more elegant and organized result than the prior efforts. Elegance is usually a good indicator. The separation of gastropod and cephalopod jumped off the page!

This has bearing on work involving shell microstructure, as we search for a large-mollusk source of foliated aragonite—recently discovered in rare Indo-Pacific pearls of unproven (claimed at source as Nautilus) origin. Heretofore, this material has only been observed in monoplacophora, and in Paleozoic fossils.

A pity that monoplacophora was not included in the analyses and left to triangulation, must assume due to unavailability of sufficient specimen material.

I also recently came across this older open-source paper from the Australian Museum in Sydney that offers complementary results. It also shows Nautilus as having blazed its own trails. (Still pending a close look at all the mollusk species analyzed for taxonomical classification and intra-mollusc interpretation.)
 

gjbarord

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#7
Actually gave an interview to a magazine about this paper.... Not sure I said anything noteworthy. I was nervous as I had ever been. Great paper though!! Not surprising though, really.

Greg
 

SteveM

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gjbarord;181872 said:
Actually gave an interview to a magazine about this paper...
Greg, please point us to that magazine article when it comes out.

The authors write 'Ancestral state reconstruction of most characters of the last common ancestor of Mollusca was unaffected by the placement of monoplacophorans.'

Perhaps I'm being dense but I do not perceive a conclusion or even hypothesis here regarding monoplacophora in the absence of trancriptome data for this study. What is your read? Are you aware if followup work is taking place to fill the gap?

Steve
 

SteveM

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#9
SteveM;181890 said:
The authors write 'Ancestral state reconstruction of most characters of the last common ancestor of Mollusca was unaffected by the placement of monoplacophorans.'

Perhaps I'm being dense but I do not perceive a conclusion or even hypothesis here regarding monoplacophora in the absence of trancriptome data for this study. What is your read? Are you aware if followup work is taking place to fill the gap?
Nature had something up its sleeve the whole time!

The editors appear to have planned this carefully, first Kocot et al introducing the Bivalve/Gastropod sister relationship yet conspicuously missing monoplacophora, in short order followed by Smith et al, reconfirming the Bivalve/Gastropod sisterhood and definitively settling the long-standing argument between paleontologists and neontologists in favor of the paleontologists that Cephalopods descend from Monoplacophorans, and that both predate the branching of Mollusca to the other shelled clades.
 

jhonsmith

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The phylogenetics are really a cut above what has been standard in the field, and probably represent the new standard for total information. Tree building using 300 or so genes is a huge computational task, particularly using different substitution models for each gene....!!! :tongue:
 

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