Nocturnal Solitary Sneakers Versus Diurnal Social Explorers – Brain Evolution in Octopods

Octopods are masters of camouflage, solving complex tasks, expressing emotion and their cognitive ability is said to approach that of some small mammals. Despite intense interest and some research progress, much of our knowledge of octopus neuroanatomy and its links to behaviour and ecology comes from one coastal species, the European common octopus, Octopus vulgaris a largely nocturnal coastal dweller. Octopod species are found in habitats including complex coral reefs to the relatively featureless deep mid-water. There they encounter different selection pressures, may be nocturnal or diurnal and mostly solitary or partially social. How these different ecologies and behavioural differences influence the octopus central nervous system (CNS) or brain remains largely unknown. Here we present a phylogenetically-informed comparison between diurnal and nocturnal coastal and a deep-sea species using brain imaging techniques. This study shows that characteristic neuroanatomical changes are linked to their life modes and ecological niches. Octopods are voracious visual predators and we demonstrate enlargement and division of the optic lobe as well as structural foldings and complexity in the underlying CNS. It appears these modifications are linked to behavioural adaptation (diurnal versus nocturnal; social versus solitary) and ecological niche (reef versus deep sea), but phylogeny may play a part also. The difference between solitary and social life is mirrored within the brain including the formation of multiple compartments, or gyri, in the vertical lobe. This brain region has previously been linked to cognitive capability and is likened to the vertebrate cortex. Some of the life-mode related modifications described here are convergent with similar progressions of complexity found within the vertebrates. These findings continue the case for convergence between cephalopod and vertebrate brain structure and function. Notably, within the current push towards comparisons of cognitive abilities, often with unashamed anthropomorphism at their root, these findings provide a firm grounding from which to work.