Brain organization and specialization in deep-sea chondrichthyans

Chondrichthyans occupy a basal place in vertebrate evolution and offer a relatively unexplored opportunity to study the evolution of vertebrate brains. This study examines the brain morphology of 22 species of deep-sea sharks and holocephalans, in relation to both phylogeny and ecology. Both relative brain size (expressed as residuals) and the relative development of the five major brain areas (telencephalon, diencephalon, mesencephalon, cerebellum, and medulla) were assessed. The cerebellar-like structures, which receive projections from the electroreceptive and lateral line organs, were also examined as a discrete part of the medulla. Although the species examined spanned three major chondrichthyan groupings (Squalomorphii, Galeomorphii, Holocephali), brain size and the relative development of the major brain areas did not track phylogenetic groupings. Rather, a hierarchical cluster analysis performed on the deep-sea sharks and holocephalans shows that these species all share the common characteristics of a relatively reduced telencephalon and smooth cerebellar corpus, as well as extreme relative enlargement of the medulla, specifically the cerebellar-like lobes. Although this study was not a functional analysis, it provides evidence that brain variation in deep-sea chondichthyans shows adaptive patterns in addition to underlying phylogenetic patterns, and that particular brain patterns might be interpreted as ‘cerebrotypes’.