Journal club: Evolution of highly diverse forms of behavior in molluscs

Current Biology 26, R965-71 (2016)
Binyamin Hochner and David L. Glanzman
DOI: http://dx.doi.org/10.1016/j.cub.2016.08.047

This short review paper starts off with the comparative anatomy of the nervous system.  The authors discuss the diversity of the nervous system and its co-evolution with body plan by showing a variety of nervous systems from Solenogastres to cephalopods. Then, cellular mechanisms of synaptic plasticity underlying learning in the gastropod Aplysia and the cephalopod Octopus were discussed.

The first part was fun to read.
Comparative anatomy of the nervous system is a good reminder that the molluscan nervous system, or the medullary cord, is organized in a ladder-like fashion. The loss of collinear pattern of gene expression may explain their simple body plans. The supremacy of Octopus in the motor and cognitive abilities can be due to the high expansion of two developmentally important gene families, extensive transposable element activity, and genome rearrangements.

The second part was somewhat boring.
The title says the diversity of behavior, but this part actually covers just synaptic plasticity in Aplysia (serotonin-mediated long-term facilitation) and Octopus (long-term potentiation). The mechanisms underlying the serotonergic enhancement of synaptic strength has already been described five hundred times elsewhere.  Plus, I don't think this is a valid comparison to discuss about the evolutional process, because the gill-withdrawal reflex and the higher-order learning are completely different brain functions. Such comparison merely shows different types of learning regardless of species, not actually explains the species-dependent differences or the evolution. This is like comparing the spinal reflex and motor learning in two different vertebrate species. No wonder they are different; synaptic plasticity has little to do with the diversity of behavioral expressions.