Retinal anatomy in some scopelarchid deep-sea fishes

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Abstract
An account is given of the retinal structure in two species of Scopelarchus , using optical and electron microscopy. The results confirm previous accounts in most respects but several additional features are described and discussed. Scopelarchus has tubular eyes, their visual axes pointing upwards. The floor of the eyecup is lined by main retina, situated at the focal distance of the lens. The medial wall is lined by accessory retina, which extends onto the anterior and posterior walls and is not at the focal distance. The main retina of Scopelarchus güntheri contains anterior and posterior regions of different structure. Anteriorly there is a uniform population of rods related to an unspecialized pigment epithelium, but in the posterior region the rods are grouped in bundles of about 23. These groups are inserted into pits in a reflecting retinal tapetum formed by the pigment epithelium cells, and are optically as well as spatially isolated from each other. The rods within each group are connected at the sclerad end of the inner segments by patches of tight junction. The neuro-retina has the same structure throughout the main retina; the rod synapses are complex in structure, with about six synaptic ribbons each. There is a major and a minor layer of horizontal cells. The major cells are stellate and their lateral processes have wide areas of contact with those of other cells, forming a perforated sheet of horizontal cell tissue. From the vitread surface of these cells long oblique processes arise. These are embedded in swathes of microtubules and the two elements together form a distinct layer. The inner nuclear layer is subdivided by a layer of lamellae of radial fibre origin. These lamellae, present in other deep-sea fishes, are found throughout the neuro-retina. Scopelarchus sagax has an additional region in the main retina. The anterior third of the retina has long outer segments which are not grouped, and which occur in a very regular array. The long-rod region ends in an abrupt step and gives way to short non-grouped rods resembling those of Scopelarchus güntheri . No significant differences are found in the neuro-retina opposite these two regions. The accessory retina in both species has a grouped structure, but the groups are less regular in size and spatial arrangement than those in the main retina. The rod nuclei are sclerad to the outer limiting zone. There are no inter-receptor contacts between the inner segments, and the synapses are of simple type. The neuro-retina is thin and simple, without horizontal cells or the layer of microtubules. The inner nuclear layer is not subdivided. The ganglion cells are few compared to those in main retina. The retinal tapetum is discussed in relation to those in other fishes, particularly those which also have receptor groups, and conclusions are drawn about their functional implications. The long rods of S. sagax are compared with those of certain other deep-sea fishes, and some implications of their size considered. Attention is drawn to the presence of the long-rod region as a possible diagnostic feature.