Phyletic devergence and specialization for pelagic life in the Antarctic nototheniid fish Pleuragramma antarcticum

Living free of the substrate and possessing distinctive morphology, Pleuragramma antarcticum is an atypical nototheniid. Cladistic analyses indicate that Pleuragramma is one of the most phyletically derived species in the family. It is also distinctive in a number of features of its biology. I comment on phylogenetic and developmental aspects of these morphological and physiological features in Pleuragramma: skeleton and notochord, extent of red muscle, lipid storage, blood, retina, lateral line, brain and gastrointestianal system. An entire suite of characters is unique to Pleuragramma, but individual features are convergent within other nototheniid clades living in the water column. Osteology provides the best documented evidence that paedomorphosis has been important in the evolution of Pleuragramma.