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Retinal Structure in Platytroctes Apus, A Deep-Sea Fish with a Pure Rod Fovea

INTRODUCTION The retinae of deep sea fishes have been studied by optical microscopy by several authors. Brauer's (1908) work is the most important early account, and the major recent work is that of Munk (1966a). In this monograph he reviews previous papers including that of Contino (1939), Franz (1907), Hanyu & Ali (1962), Verrier (1931), Vilter (1953, 1954a, b) and Wunder (1958), and also describes the eyes of a number of species himself. Since then Munk (1966b and 1968) has described other species by optical microscopy. Locket (1969, 1970) has used electron microscopic methods of fixation to study two species by optical and electron microscopy. These two fishes, Poromitra nigrofulvus and Sternoptyx diaphana, are not closely related and have different retinal structures. Poromitra has banks of rods in its retina, a condition first described by Vilter (1953) in Bathylagus; Sternoptyx has a single layer of long rods, a condition also found in the present species. The earlier authors worked on material collected during research expeditions and preserved in alcohol or formalin. During the 1966 biological cruise of R.R.S. ‘Discovery’ Locket was able to fix eyes from deep-sea fish with a view to electron microscopy. Though the fixation is not perfect, it does preserve much ultrastructural detail and interspecies variation in retinal structure can be studied in this material.