The ocular morphology of the southern hemisphere lamprey Mordacia mordax Richardson with special reference to a single class of photoreceptor and a retinal tapetum

A description is given of the morphological and ultrastructural characteris tics of the: eye of Mordacia mordax, one of the two parasitic species of th e southern hemisphere lamprey family Mordaciidae. The avascular retina is w ell-differentiated, containing two ganglion cell sublaminae, two layers of horizontal cells and a large population of displaced bipolar cells. Interme diate filaments, which stain positively for glial fibrillary acidic protein (GFAP), lie in perpendicularly-arranged bundles between the two layers of horizontal cells. M. mordax contains only a single type of rod-like photore ceptor, in contrast to representatives in the other two lamprey families Ge otriidae and Petromyzontidae, which possess three (two cone-like and one ro d-like) and two (one cone and one rod) types of photoreceptor, respectively . The large photoreceptors of M. mordax possess a large mitochondrial ellip sosome, a cylindrical outer segment with incisures, and a nucleus immediate ly abutting the outer limiting membrane. The photoreceptors are arranged in an hexagonal array with their outer segments surrounded by a retinal tapet um which elicits a yellow fundal eyeshine. The tapetum contains both diffus e (vitread spheroidal granules) and specular (sclerad needles) reflectors. The characteristics of the eye of M, mordax are related to the diurnal burr owing of this species during its upstream migration, a feature not found in the other two extant families of lampreys. The absence of cone-like photor eceptors and the presence of a tapetum suggest that although photopic visio n is probably greatly diminished, characteristics have been evolved by the eye to maximize light capture and increase sensitivity in the low light int ensities encountered during this species' nocturnal migration. Copyright (C ) 2000 S. Karger AG, Basel.