Vision in the rufus snake eel, Ophichthus rufus : adaptive mechanisms for a burrowing life-style

Abstract

A unique retinal topography was found in the rufus snake eel, Ophichthus rufus, a burrowing fish that shows a preference for nocturnal habits. The cells in the ganglion-cell layer formed a high-density semicircular band that covered the dorso-rostral, temporal and ventro-rostral areas of the retina. Three regions of high acuity were identified in the ventro-temporal, ventro-rostral and dorsal regions of the retina. This is the first time that multiple specialisations have been found in a benthic deep-water fish. The ventro-temporal and ventro-rostral visual axes were found at 48° and 54° from the retinal centre, subtending the dorsal field of view, while the dorsal axis was at 66°, subtending the ventral field of view. These characteristics may represent an adaptation to identify the mouth of the muddy burrows in which this species hides or where it looks for food. O. rufus has a duplex retina with numerous slender rod photoreceptors arranged in banks of four layers. This morphology is typical of deep-sea teleosts with a high visual sensitivity, which is useful in a dim-light environment. A few tiny cones were also present, probably enhancing acuity during the eel's daily activity. The retina of O. rufus also possessed a well developed vascularisation, possibly to overcome the hypoxic conditions that may be found in the burrow. The characteristics of the retina of the rufus snake eel indicate singular visual capabilities that are discussed in relation to the burrowing life-style and to what is known of feeding habits of this fish.