Divergence of Visual Morphology and Opsin Expression in an African Cichlid Linked With Turbidity

Turbidity is increasing in freshwaters globally due to human activities and is known to affect visually-mediated behaviours in fish. As anthropogenic impacts continue to degrade aquatic environments, it is critical to determine how sensory systems are affected and what this might mean for population persistence. We investigated visual morphology and opsin gene expression in an African cichlid fish (Pseudocrenilabrus multicolor) that experiences environmental extremes across its East African range. We examined visual morphology (i.e., size of eyes and brain) and quantified retinal opsin gene expression of adult wild-caught fish from two sites representing extremes of turbidity and oxygen (a high turbidity, high dissolved oxygen river and a low turbidity, low dissolved oxygen swamp). Further, we reared offspring of wild-caught parents from each population in a full-factorial high/low oxygen, high/low turbidity design to test for plasticity and tease apart the influence of each stressor individually and in combination. For wild-caught fish, population drove variation in eye morphology and brain mass such that the river population had larger visual traits than the swamp population. Sex was an important driver for brain morphology with males having larger brains and optic tecta than females. All fish expressed the same cone opsin palette, comprised of sws2a, rh2a, and lws; however, river fish expressed a higher proportion of lws opsin compared to swamp fish. Further, population significantly explained variation in lws opsin for males and trended toward significance in females. When major environmental stressors were examined in the rearing experiment, oxygen was a strong driver of eye and brain size. Cone opsin trends were less clear in the rearing experiment; the proportion of lws opsin in females significantly varied by the interactive effect of oxygen and turbidity, while sws2a opsin in males varied significantly by turbidity. This research improves our understanding of the effect of elevated turbidity on African cichlid vision and contributes to growing knowledge of how animals respond to human-induced environmental change and multiple stressors.