123 — Evolution of highly diverse forms of behavior in molluscs

This primer explores the rise of complex behaviors and biophysiologies in molluscs — the phylum of animals that includes octopus, clams, and cuttlefish. These nervous systems developed entirely separate from vertebrates’, and evolution made sometimes very similar and sometimes very different design decisions.

Cephalopods — the grouping containing squid, cuttlefish, and octopus — are particularly distinct in that they lack any skeletal joints or bones at all. Skeletons are useful for a whole bunch of things, but in particular they enable an animal to use levers for mechanical advantage. Cephalopods lack that advantage.*

Cephalopods are also distinct among their mollusc brothers and sisters in that they are really good at moving. Remember, those brothers are sisters are clams et al; that anyone in this family could move like an octopus moves is pretty outrageous. This movement is enabled by the explosion in the number of neurons found in a cephalopod versus other molluscs. This is possibly a result of an early evolutionary split that changed the genetic underpinnings of body-layout (possibly to a Hox-like gene). Additional mutations to likely led to high genomic reconfigurability, allowing for much more coding to take place in these animals than in other molluscs.

A bit of a buried lede here: Octopus arms’ nervous systems resemble the full body nervous systems of other invertebrates. And the cephalopod nervous systems vary from molluscs in other fundamental ways as well; but in general, individual ganglia exist close to their sensorimotor targets; ganglia exist near the octopus suckers, near the Aplysia mouth, et cetera.

One of my favorite invertebrates is the giant sea slug, Aplysia. Aplysia exhibits the Defensive Withdrawal Reflex (DWR), which acts as a great model for its synaptic plasticity and learning-mechanisms. That is, you can condition an Aplysia to ignore touch after enough interaction; this mechanism of long-term synaptic memory that mediates this response is based upon synapse-local protein synthesis. But similar learning in Octopus is not based upon protein synthesis. Instead, shorter-term learning keeps the new memory in a “cached” state until it can be digested by central circuitry that the octopus brain controls.

This means that even within the cephalopod world, learning and neural plasticity are widely variegated. In some ways, we can use these differences to learn about the progression of neurological development in invertebrates as it parallels that of vertebrates.

* I argue that they make up for it by being extremely awesome in every other way.