The fine structure of the cornea of the salamanderfish, Lepidogalaxias salamandroides (Lepidogalaxiidae, Teleostei)

Abstract

The salamanderfish, Lepidogalaxias salamandroides, is a small freshwater fish that is endemic to southwestern Australia. Its phylogeny is still uncertain, but it belongs to a monotypic family that is as primitive as any living teleost. Able to survive during periods of drought by burrowing into the substrate and estivating for many months, this species has evolved a unique visual system, which is highly adapted to its changing environment. Thought to use cutaneous respiration during periods of dormancy, it secretes a mucus sheath over the eyes and genital openings to inhibit desiccation. The cornea is split into dermal and scleral components, separated by an iridescent layer and a mucoid layer. The dermal cornea comprises an epithelium containing both goblet cells and mucus-secreting granules in the central region, which overlies a thick stroma of collagen lamellae. The dermal stroma possesses numerous sutural fibers and flattened keratocytes and collections of three types of pigment granulates in the periphery. Posterior to the dermal stroma lies an iridescent layer consisting of five flattened cell processes interspersed with membranous extensions of smooth endoplasmic reticula. The mucoid layer is composed of numerous membrane-bound structures that allow movement of the scleral cornea beneath the dermal cornea or secondary spectacle. The scleral stroma is thin (six to seven lamellae) contains keratocytes, and overlies a relatively thin and irregular Desçemet's membrane and a monolayered endothelium. In the iridocorneal angle and situated between Desçemet's membrane and the endothelium is an "annular ligament," which is composed of flattened cells with lobulated nuclei and a fine granular cytoplasm with a few mitochondria. The numerous corneal inclusions are discussed with respect to their phylogeny and function.