Morphological adaptations on the eye of the golden gray mullet (Chelon aurata): Using light and scanning electron microscopical study.

The present investigations were designed to describe the ultrastructural properties of the eye of the golden gray mullet (Chelon aurata). For this purpose, the eyes were examined grossly, and by light and electron microscope. The external layer consists of the cornea and the sclera. Three layers compose the cornea; the anterior stratified cuboidal epithelial; the anterior limiting (Bowman) membrane; and the thick dermal layer of the stroma. The mucoidal layer has small collagen fiber bundles embedded in the CT layer and located between the anterior portion of the scleral cornea and the dermal stroma, (or "substancia propria"). The iridescent layer is thin at the center and thick at the periphery. It contains a pigmented layer with many ossicles. SEM analysis reveals that the cornea consists of undetermined shaped cells joined together by numerous thread-like micro-ridges, with several micro-tubercles on the external surface. The photoreceptor layer had two types of cells: the rod-shaped and the cone-shaped cells. The cone cells differentiate into two types of cells: single and double cells. SEM analysis of the retina showed that rod cells appear as thin long uniform rod-like, while the cone cells appear as rod cells with ovoid bases. SEM analysis demonstrates that the inner side of the retinal epithelium appears to be wrapped around itself. The morphological appearance of the eye adapts to life in superficial aquatic conditions. In conclusion, the current findings provide morphologic evidence to better understand the mechanism of fish vision adaptation to environmental conditions. RESEARCH HIGHLIGHTS: The transparent cornea composed of three layers; anterior stratified cuboidal epithelial, Bowman's membrane, and a thick dermal stromal layer. The mucoidal layer is formed from small collagen fibers bundles embedded in the CT layer and located between the anterior portion of the scleral cornea and the dermal stroma. There are two types of photoreceptor cells: rod and cone cells. Cone cells can be single and double cells.