Species adapt to their environment through the process of natural selection. In aquatic animals, visual systems generally are sensitive to the wavelengths of light that are most abundant in their environment. Light is absorbed by visual pigments in the retina, initiating the process of vision. However, the background light spectrum in water changes with angle of view, and these distinct spectra project onto different regions of the retina. As a result, the optimal visual pigments for detecting objects are predicted to differ by retinal region. In this study, the spatial distributions of visual pigments will be mapped in retinas of several cichlids from Lake Malawi that differ in light habitat, foraging mode, and genus. Retinas will be obtained from both wild-caught and lab-reared cichlids. Some cichlids will be reared under inverted light conditions, and results from different species will be compared to determine the effects of photic habitat, foraging mode, and genetics upon visual pigment distributions.
This study is among the first to quantitatively examine the retinal distribution of visual pigments in relation to viewing backgrounds. If retinal distributions of pigments have diversified among fishes, these differences might drive visual signals to diverge also. Coevolution of visual sensitivity and color signals could help explain how nearly 1000 cichlid species evolved in Lake Malawi just in the last 1-2 million years. In addition, this dramatic example of evolution will be used to teach adaptation and speciation at urban high schools.
Vision allows animals to locate critical objects such as food, predators, and mates. The eye's photoreceptor cells must respond to the light present in the environment in order to accomplish these tasks. Our research has uncovered a novel way in which the eye may tune its photoreceptors to the light environment. The process of vision begins when a visual pigment inside the retina’s photoreceptors absorbs light. Most photoreceptors contain a single visual pigment, composed of an opsin, a type of protein, bound to a molecule called a chromophore. The specific combination of opsin and chromophore determine the part of the light spectrum to which the photoreceptor is sensitive. Few exceptions have been documented in which a single photoreceptor mixes opsins to form spectrally distinct visual pigments. How these exceptions affect vision is generally unclear. We show that opsin mixing can tune a photoreceptor to the light environment by shifting and broadening its absorbance spectrum. Photoreceptors of the cichlid fish, Metriaclima zebra, mix different opsins in regions of the retina that view distinct backgrounds in the environment. In the cichlid's environment, and that of many animals, the background changes with angle of view. The mixing of visual pigments increases absorbance of the cichlid's backgrounds, thereby increasing the contrast of dark objects. Increased contrast may aid in the detection of silhouetted animals, such as potential prey, predators, or mates. This is the first documentation of photoreceptors mixing opsins in a manner that tunes sensitivity to the environment. Thus, opsin mixing may be a novel mechanism of spectral tuning. Our calculations show that mixing some combinations of opsins can hinder color discrimination, creating a trade-off between visual functions. This trade-off may explain why cichlids mix some opsins more frequently than others. Our research also demonstrates that this opsin mixing within retinal regions can vary to match the backgrounds of the environment in which a cichlid develops. The cichlid fishes in Africa's great lakes are forming new species at an extremely rapid rate. While doing so they have evolved a diversity of visual sensitivities, color patterns, and behaviors used in communication. Not only does this make cichlids popular pets, but also a rich resource for understanding how new species are formed. Visual sensitivity biases the evolution of communication signals because they must be conspicuous to the intended viewer. Our research reveals an additional dimension in which visual sensitivity can vary: not only between species, but also within a single retina. Sensitivity differences between regions of the retina may affect the perception of signals in different parts of the visual field, placing an emphasis on the relative positions of the signaler and viewer. During the course of this project we developed activities for undergraduate biology instruction based on Lake Malawi cichlids. In completing the activities, students apply math and computer skills to predict population parameters and growth under different conditions. We developed an assessment to evaluate the effectiveness of these class activities. The activities will be made available free to the public through a website. We also taught Lake Malawi National Park (LMNP) officials and local students about the role of vision in the ecology of the lake's cichlids. Instruction covered topics that include the spectral filtering of light by water, light absorption in the retina, spectral tuning of photoreceptors, visual communication in fishes, and the ways in which new species are formed. In addition, our laboratory work provided substantial research experience and mentoring to an undergraduate student who is a member of underrepresented groups based on gender and ethnicity.
