Light, Biological Receptors

Biological receptors used to detect underwater light vary in structure, from simple clusters of cells that only detect light intensity to complex organs that form detailed images. While most aquatic organisms perceive some portion of the visible spectrum, UV photoreceptors have recently been found in a wide diverse of species, from bacteria to fish. These UV photoreceptors are thought to aid in communication, foraging, and avoidance of damaging sunlight. However, they are not without costs as seeing in the UV can cause damage to the eye and image degradation. Given that the spectral composition of underwater light can directly affect an organism's physiology and behavior, environmental stressors that threaten to alter the color and clarity of fresh waters are a concern. These include nutrient and sediment loading associated with changes in climatic patterns and anthropogenic activities. This chapter discusses: (1) the underwater UV environment, (2) the structure and function of photoreceptors and the methods used to measure their spectral sensitivity, (3) the distribution of UV photoreceptors among freshwater organisms, (4) the adaptive significance of UV vision, and (5) potential effects of environmental stressors on UV vision.