Anatomy and histology of the brain and sense organs of the antarctic eel cod Muraenolepis microps (Gadiformes; Muraenolepididae)

Brain regions, cranial nerves, and sense organs in Muraenolepis microps, an Antarctic gadiform fish, were examined to determine which features could b e attributed to a gadiform ancestry and which to habitation of Antarctic wa ters. We found that the central nervous system and sense organs are well de veloped, showing neither substantial regression nor hypertrophy. A detailed drawing of the brain and cranial nerves is provided. The rostral position of the olfactory bulbs and telencephalic size and lobation are common for t he order. The optic tectum and corpus cerebelli are smaller than in most ot her gadiforms. The shape of the corpus cerebelli is not distinctive among g adiforms. The lateral line region is moderately well-developed, but not hyp ertrophied to the extent seen in deep-sea gadiforms. As is the case in gadi ds possessing barbels and elongated pelvic rays, Muraenolepis has well-deve loped facial lobes, although these are smaller and more laterally positione d. The vagal lobes are deeply placed in the rhombencephalon and project int o the fourth ventricle. The brain of Muraenolepis resembles that of a phyle tically derived gadoid, especially a phycid, more than it resembles the bra in of a phyletically basal macrourid. Two histological features of the dien cephalon of Muraeno-lepis appear to be unique among gadiforms: a well-organ ized thalamic central medial nucleus and subependymal expansions. Muraenole pis has a pure rod retina like many deep-sea species but lacks the superimp osed layers of rod outer segments. The histology of the nonvisual sense org ans, especially the olfactory and external taste systems, are well-develope d in Muraenolepis but. not hypertrophied. We relate our findings to what is known about neural morphology in other gadiforms and in phyletically dista nt notothenioids and liparids that are sympatric with Muraenolepis on the A ntarctic shelf. The only feature that reflects an Antarctic existence is th e diencephalic subependymal expansions, which within notothenioids mirror t he habitation of cold waters and have been found in every Antarctic species examined to date. Although the waters of the Antarctic shelf are cold, dar k, and deep, brain and sense organ morphology in Muraenolepis are remarkabl y free of extreme specialization. (C) 2001 Wiley-Liss, Inc.