Mollusc brains: Such a good idea they evolved them four times!


When it comes to intelligence without a backbone, the undoubted brain-boxes of the invertebrate world are found amongst the molluscs, with octopus, squid and cuttlefish well known for their intelligence. But new research is showing that the molluscs' path to a central nervous system was not as straightforward as once thought.

Previously it had been argued that cephalopods (octopus, squid and cuttlefish) and gastropods (slugs and snails) were the most closely related due to their highly centralised nervous systems, but a team led by Kevin Kocot from Auburn University, Alabama has found that this may be far from the truth.

The scientists carried out sophisticated analysis of the genetic sequences common to all molluscs and pinpointed differences that have built up over time, the more variance found in these common genetic sequences when comparing two species showing a more distant relationship.

The results re-map the mollusc family tree showing that gastropods are in fact most closely related to bivalves such as clams, oysters and mussels which only possess a very basic nervous system, while the cephalopods are one of the most ancient branches with their evolutionary development predating snails, slugs, clams and oysters.

All this re-shuffling means that members of the phylum Mollusca appear to have evolved centralised nervous systems independently, showing parallel evolution rather than the expected linear progression. The researchers calculate that this separate brain development has occurred at least four times; in octopuses, Tritonia and Dolabrifera sea slugs and Helisoma, a genus of freshwater snail.

This insight sheds light not only on molluscs but on the whole mechanism of evolution and how apparently related, complex structures can in fact be the product of separate evolutionary paths.

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