Diversification of brain and sense organ morphology in Antarctic dragonfishes (Perciformes: Notothenioidei: Bathydraconidae).

In the subzero shelf waters of Antarctica, fishes of the perciform suborder Notothenioidei dominate the fish fauna and constitute an adaptive radiation and a species flock. The 16 species of dragonfishes of the family Bathydraconidae live from surface waters to nearly 3,000 m and have the greatest overall depth range among notothenioid families. We examined the anatomy and histology of the brain, retina, and cephalic lateral line system of nine bathydraconid species representing 8 of the 11 known genera. We evaluate these data against a cladogram identifying three clades in the family. We provide a detailed drawing of the brain and cranial nerves of Gymnodraco acuticeps and Akarotaxis nudiceps. Bathydraconid brain morphology falls into two categories. Brains of most species are similar to those of generalized perciforms and some basal notothenioids (Class I). However, brains of deep-living bathydraconids (members of the tribe Bathydraconini minus Prionodraco) have a reduced telencephalon and tectum that renders the neural axis visible - the stalked brain morphology (Class II). All bathydraconids have duplex (rod and cone) retinae but there is considerable interspecific variation in the ratio of cones:rods and in the number of cells in the internal nuclear layer. Retinal histology reflects habitat depth but is not tightly coupled to phylogeny. Although the deep-living species of Bathydraconini have rod-dominated retinae, the retinae of some sister species are photopic. An expanded cephalic lateral line system is also characteristic of all members of the Bathydraconini as exemplified by Akarotaxis. This morphology includes large lateral line pores, wide membranous canals, hypertrophied canal neuromasts, and large anterodorsal lateral line nerves, eminentia granulares, and crista cerebellares. The saccular otoliths are also enlarged in members of this tribe. Neural diversification among bathydraconids on the Antarctic shelf has not involved the evolution of sensory specialists. Brain and sense organ morphologies do not approach the specialized condition seen in primary deep-sea fishes or even that of some secondary deep-sea fishes including sympatric non-notothenioids such as liparids (snailfishes) and muraenolepidids (eel cods). The brains and sense organs of bathydraconids, including the deep-living species, reflect their heritage as perciform shorefishes.