Re-Discovering the Squid Brain

Abstract

Cephalopods have the most complicated central nervous system (CNS) of all invertebrates at both anatomical and functional levels as demonstrated by the pioneering neuroanatomical work of Cajal and Young decades or indeed a century ago. An octopus brain with over 200 million neurons is equivalent, numerically at least, to that of a rat. Brain complexity, their rich behavioral repertoire and most famously the eye, highlight cephalopods and vertebrates as an example of convergent evolution. Perhaps due to its size and complexity, our knowledge of the cephalopod brain still relies mostly on old histological approaches and a few attempts at dye-labelled projections. Here using the new technique of diffusion magnetic resonance imagery (dMRI) tempered with highly conservative tract-acceptance thresholds and cross-validated with traditional histology and neural tracers, we reveal 153 previously undescribed lobe interconnections and novel within-lobe morphologies in the brain of the reef squid, Sepioteuthis lessoniana. Almost all established connections are re-confirmed and the beginnings of a new brain-wide neural connection “atlas” is erected. As with vertebrate examples, using the complementary techniques of MRI, traditional histology and dye-based tractography, such a ‘circuit diagram’ for cephalopod neural architecture leads to a better understanding of their complex behaviors. It also supports emerging hypotheses around anatomical and functional convergence with parts of the vertebrate central nervous system, including projections of overall body axes. Notably it provides a firm base upon which to place the currently fashionable but contentious cephalopod cognitive capabilities.