You would have thought that having been around for over 500 million years, being so valuable to humans in the form of clam chowder and oysters and being so prevalent on land and in water, that scientists would have learned all about mollusks long ago. But, it would seem not.
"Here's this big, diverse group of animals, and we don't know how they were related to each other," said Casey Dunn, an evolutionary biologist at Brown University who specialises in building evolutionary trees. Some branches were well known, Dunn said, "but what we really lacked was a breadth of sampling."
Now, in a paper in Nature, researchers from Brown University and collaborating institutions have pieced together an evolutionary tree (phylogeny) for mollusks.
By collecting hard-to-find specimens, including samples of an organism previously thought to have been extinct for millions of years, the team was able to sequence hundreds of thousands of genes from the specimens.
Using Brown’s supercomputer, computational analyses were used to match the genes.
The result of the collaborative effort is that mollusk phylogeny is now "resolved at a broad scale," said Dunn, assistant professor of biology in the Department of Ecology and Evolutionary Biology and the paper's corresponding author.
The study has, for the first time, placed a mysterious group of deep-ocean animals known as Monoplacophora, a group that was thought to have been extinct until 1952.
This means that monoplacophorans become a sister clade to the cephalopods (octopuses, squid and nautiluses). "Cephalopods are so different from all other mollusks, it was very difficult to understand what they are related to. They don't fit in with the rest," Dunn said. "Now, we have a situation where two of the most enigmatic groups within the mollusks turn out to be sister groups."
Now that a close evolutionary relationship between monoplacophorans and cephalopods has been made, the researchers say the question of a single origin for shelled mollusks has been answered. The ancestor species is still not known, but the group is sure that on phylogenetic basis, monoplacophorans and cephalopods share more in common with shelled mollusks than with the non-shelled.
By establishing the close evolutionary relationship between monoplacophorans and cephalopods, the researchers say they have squarely answered the question of a single origin for shelled mollusks. That ancestor species is not known, but the group is confident that monoplacophorans and cephalopods share more in common, evolutionary speaking, with shelled mollusks than with the non-shelled groups.
"What we found is these worm-like mollusks (aplacophora) and chitons (polyplacophora) are more closely related to each other, and they diverged prior to the origin of the shell," Dunn said. "They are mollusks, but they formed this group that split off before shells came along."
"We are trying to understand how these species are related, their evolutionary relationships. We do this by analysing the conserved parts of their genomes and constructing an evolutionary tree," said Stephen Smith, a postdoctoral researcher at Brown and the paper's first author, who designed the genetic computational analysis.
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