The morphology of the retina and lens of the sandlance, Limnichthyes fasciatus (Creeiidae).

The sandlance, Limnichthyes fasciatus (Creeiidae), is a small teleost (30 mm in length), which lives beneath the sand. It has minute dorsally placed eyes (1.04 mm in diameter), which move independently of one another. The structure of the retina and lens was examined by both light and electron microscopy. A deep convexiclivate fovea lies on the visual axis of the eye, and regional increases in photoreceptor and ganglion cell densities occur within the area surrounding the foveal depression. The sandlance possesses a pure cone fovea, with a regular square mosaic of a single cone bordered by four equal double cones distributed over most of the retina. Rods are rare and are distinguishable from cones on ultrastructural morphology. A pigmented choriocapillaris extends behind the retina, closely apposing the retinal pigment epithelial cell layer and Bruch's membrane. Surrounding the optic nerve, and adjacent to the choriocapillaris, is a vascularised, horseshoe-shaped choroidal gland, or rete mirabile. A small system of vitreal blood vessels from the hyaloid artery near the optic nerve, supplies the large number of ganglion cells, arranged in up to five sub-laminae, within the ganglion cell layer. The retina is jacketed by an uveal argentea within the sclera. This argentea contains plates of guanine crystals, oriented with their flat surfaces approximately perpendicular to the incident light path, and discrete bundles of melanosomes apposing the sclera. A non-spherical lens, previously described only in deep-sea teleosts, was found, and its refractive properties are discussed in relation to the deep pit fovea.