"ENVIRONMENTAL COMPLEXITY INFLUENCES VISUAL ACUITY IN CICHLID FISHES"
ENVIRONMENTAL COMPLEXITY INFLUENCES VISUAL ACUITY IN CICHLID FISHES. Adam P. Dobberfuhl, Jeremy Ullman, Jessica Hunter, Elizabeth Higgins, Maggie Allen, and Caroly A. Shumway Dept. of Research, New England Aquarium, Boston, MA How do environmental forces shape neural evolution? The highly visual cichlids of the African Great Lakes, renowned for their explosive radiation, lend themselves to exploring how environmental pressures influence the evolution of brain and behavior. In this study, we behaviorally measured the visual acuity of three closely-related Tanganyikan cichlid species from the Ectodini clade differing solely in habitat preference. The fish were of the same size, lens size was the same, and social behaviors (mating and parental care) were identical. One species, Xenotilapia flavipinnis, lives in sandy, simple habits. A second species, Xenotilapia spilopterus lives in a habitat classified as intermediate, between rocky and sandy environments. A third species, Asprotilapia leptura, prefers rocky, complex habitats. We compared the visual acuity of these phylogenetically close, yet ecologically different cichlids to better understand how environment shapes brain and behavior. Visual acuity is the minimum angle formed at the eye by two objects that appear as separate (Douglas and Hawryshyn, 1990). To assess the visual acuity of these fish, the optomotor response and optokinetic responses were measured at different square-wave gratings. After a ten minute period of acclimation in the experimental tank, a fish was presented with a rotating drum of a given square-wave grating (black and white), rotated around a round experimental tank at 4Hz. If a fish could detect the separate black and white lines, it would interpret the drum as moving, responding with either an optomotor (swimming with the stimulus) or optokinetic response (visual following: both pursuit and saccades); if it could not distinguish the black from the white lines, it would interpret the drum as stationary and not move. Each trial was videotaped, and each fish was examined three times for two minutes at each grating size, ranging from 10.11mm to .32 mm. A gray background was presented as a control. Using ANOVA and post-hoc tests, the significance between each species at the fifty percent response was calculated. We found a statistically significant difference between the visual abilities of the rock-dwelling A. leptura and the sand-dwelling X. flavipinnis. We also found a significant difference with respect to social behavior. X. flavipinnis provides biparental care, whereas E. melanogenys is a polygamous lek-forming breeder. The lek-forming species was found to have better spatial acuity than the monogamous species. X. spilopterus had a visual acuity in between that of the species from complex and simplistic habitats; however, the difference was not statistically significant. These results demonstrate the importance of habitat complexity on the visual capabilities of cichlids from Lake Tanganyika. Our next step is to compare the density of retinal ganglion cells to determine whether the enhanced visual processing is occurring in the retina or in the brain. Funding provided by the National Science Foundation: grant number 0218005.