Nervous and sensory system correlates of an epibenthic evolutionary radiation in antarctic notothenioid fishes, genus Trematomus (Perciformes; Nototheniidae).

The perciform suborder Notothenioidei consists of 120 species, with 94 confined to the Antarctic Region of the Southern Ocean. On the Antarctic shelf, this phyletic radiation has been accompanied by a substantial morphological and ecological diversification towards a pelagic existence. For example, the primarily benthic genus Trematomus contains an epibenthic radiation that includes T. loennbergii, T. lepidorhinus, and T. eulepidotus. By comparing these epibenthic species with three congeneric benthic species (T. scotti, T. pennellii, and T. bernacchii) we tested three null hypotheses regarding brain variation in Antarctic trematomids: 1) that there is no difference in brain morphology among the six species; 2) that phylogenetic and ecological factors do not influence brain morphology; and 3) that peripheral sensory structures do not influence brain morphology. We rejected each of these hypotheses, leading us to conclude that Trematomus brains vary interspecifically, between benthic and epibenthic species, and with a species' depth distribution. Further, we conclude that brain variation is correlated with differences in peripheral sensory systems and motor activity. Specifically, epibenthic Trematomus have larger percentages of their brain volume devoted to lateral line mechanoreceptive and motor (cerebellar) structures. Species living at greater depths have low ratios of cones:rods in the retina and larger olfactory structures.