Postlarval chromatophores as an adaptation to ultraviolet radiation

It is now well established that ultraviolet radiation (UVR) may have detrimental, even lethal effects on zooplankters. Unlike copepods and other holoplankters, which may avoid UVR by undergoing diel vertical migration, larvae of many decapod crustaceans and fishes recruit to adult populations by remaining in near-surface waters during the daytime. Consequently, they are exposed to biologically damaging UVR. A possible adaptation in these larvae is chromatophores, which may absorb UVR by expanding in high light environments. The supposition is that expanded chromatophores more effectively absorb UVR, but there is some fitness cost to having expanded chromatophores in low light environments. Since the ratio of visible light to UVR in the water column changes as result of season, latitude, dissolved organic carbon, and a host of other factors, the benefits of chromatophores would be maximized if they responded specifically to UVR. The purpose of this study was to determine whether the chromatophores of crab postlarvae (megalopae) could expand in response to UVR. Megalopae of two species of crabs (Cancer oregonensis, Telmessus cheiragonus) were collected from large surface-swarms during mid-day as they recruited onshore in early May 1998 at Friday Harbor, Washington, USA. Dark-adapted megalopae (held in the dark for 8 h before experiments) were exposed to UVR (UVBR+UVAR, 280-400 nm), UVAR (320-400 nm), and light (400-1700 nm) in the laboratory. Chromatophores expanded after only minutes of exposure to UVR, UVAR, and light for both species. Two alternative hypotheses may explain why both harmful and comparatively benign wavelengths stimulated chromatophores to rapidly expand. First, larvae may not distinguish among different wavelengths, which, if true, would increase the vulnerability of these larvae to intensifying UVBR due to ozone depletion. Second, chromatophores have functions other than blocking UVR, such as crypsis and thermoregulation, and must respond to light for these other functions to operate.