“Differential survival between visual environments supports a role of divergent sensory drive in cichlid fish speciation”

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Martine E. Maan, Ole Seehausen, and Ton G. G. Groothuis

Fitness consequences of visual adaptation: cichlid fish survive best in their natural light environment

Light-dependent survival

Aquarium system in which fish were raised at the University of Groningen, with light conditions mimicking those at different water depths at Python Islands in Lake Victoria: shallow habitat (left) and deep habitat (right).
(Credit: Martine E. Maan)

Animal sensory systems are highly diverse, as they must adapt to different sensory environments. A recent study shows that even in the laboratory, sensory conditions can have a major impact on fitness.

Researchers from the Netherlands (University of Groningen) and Switzerland (Eawag Institute and University of Bern) investigate the causes and consequences of visual adaptation, and its possible role in species divergence. They collected two species of cichlid fish from Lake Victoria (East Africa), that inhabit different depth ranges with different light regimes: in deeper waters, blue light hardly penetrates and the light environment is dominated by green and yellow light. The two species differ in visual system properties, matching these different visual environments.

To test whether the fish have indeed adapted to their environment, the researchers mimicked the shallow-water and deep-water light conditions in the laboratory and raised both species in both conditions. After one year, they found that both species survived significantly better in the light regime that mimicked their natural habitat. This confirms that the fish’ visual systems have indeed adapted to their natural light environment.

For further confirmation, the researchers also bred hybrids between the two species. Consistent with previous observations on hybrids, these fish survived just as well as the parental species, but most importantly, their survival did not differ between light conditions. This implies that the light-dependent survival observed in the parental species is indeed due to genetic effects, presumably the genetically based differences in visual system properties. Together, these observations suggest that depth-mediated variation in light environments in Lake Victoria generates strong divergent selection on fish visual properties, strong enough to cause major differences in survival.

The same mechanism probably works in other aquatic environments as well, because visual conditions under water can vary dramatically between geographic locations or depth ranges. The results of this study are also relevant for aquaculture: manipulating the light regime may improve fish performance and welfare. Read the Article

Males in manipulated light treatments: Pundamilia pundamilia (full brothers; top) and P. nyererei (full brothers, bottom) in shallow (left) and deep (right) light conditions.
(Credit: D. Shane Wright)