Comparative ocular anatomy in a blind African cichlid fish, Lamprologus lethops.

Objective: Troglomorphic fishes provide excellent comparative models for studying eye evolution. We describe the gross and microscopic anatomy of ocular structures of the depigmented, blind cichlid, Lamprologus lethops, and its putative sister species, Lamprologus tigripictilis collected from the lower Congo River in the Democratic Republic of Congo.

Procedures: Both species were fixed, paraffin-sectioned and stained. Immunohistochemical staining for rhodopsin markers was also performed.

Results: The globe in L. lethops is smaller than its sighted congener and recessed beneath bone and skin. The scleral profile maintains a wrinkled spherical shape, and the choroid is occupied by adipose tissue containing no rete mirabilis. The globe in L. lethops is foreshortened in the anterior-posterior dimension and deviated dorsally toward the midline with no extraocular muscles. At the posterior pole of the globe, there is an open periocular space containing no cell bodies. In L. tigripictilis, no choroidal adipose tissue is seen and a rete mirabilis is present. The retina of L. lethops is thinner compared with L. tigripictilis. Both species have scleral cartilage and fully developed lenses. Rhodopsin is present in the inner and outer segments of both species.

Conclusions: Ocular adaptations evolve over time as a response to a life in darkness. Combining ocular anatomy, developmental data, and genetics will lead to insights about evolution in these fishes and contribute to understanding how ocular evolution works in other vertebrates.