Observations on the shape of the lens in the eye of the silver lamprey, Ichthyomyzon unicuspis

Abstract

The shape of the lens in the eye of the silver lamprey, Ichthyomyzon unicuspis was examined in live, frozen, and fixed material. Contrary to other reports, the lens was found to be nonspherical with a cone-shaped posterior. The egg-shaped lens, which contains horizontal sutures on both the anterior and posterior surfaces, is also asymmetric in the nasotemporal axis. Its equatorial diameter exceeds its axial diameter (thickness) and the radius of curvature of the lens in the dorsoventral axis is greater than the radius of curvature in the anterioposterior axis. The lens is surrounded by a thick basement membrane with the anterior lens surface covered by a single layer of cuboidal epithelial cells. Juxtaposed to the lens capsule is a dense layer of lens fibres, which stain more darkly and surround an ill-defined lens nucleus. The shape of the lens is discussed in relation to that in aquatic gnathostomes and compared with the putative multifocal lenses of some mesopelagic teleosts. It is also hypothesized that the previously reported active focussing ability of the lamprey eye may have been misinterpreted, owing to failure to take into account the nonspherical lens shape, and may reflect measurements taken of the eye and lens at different angles.