Postlarval chromatophores as an adaptation to ultraviolet radiation

It is now well established that ultraviolet radiation (UVR) may have detrim ental, even lethal effects on zooplankters. Unlike copepods and other holop lankters, which may avoid UVR by undergoing diel vertical migration, larvae of many decapod crustaceans and fishes recruit to adult populations by rem aining in near-surface waters during the daytime. Consequently, they are ex posed to biologically damaging UVR. A possible adaptation in these larvae i s chromatophores, which may absorb UVR by expanding in high light environme nts. The supposition is that expanded chromatophores more effectively absor b 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 wate r column changes as result of season, latitude, dissolved organic carbon, a nd a host of other factors, the benefits of chromatophores would be maximiz ed if they responded specifically ro WR. The purpose of this study was to d etermine whether the chromatophores of crab postlarvae (megalopae) could ex pand in response to UVR. Megalopae of two species of crabs (Cancer oregonen sis, Telmessus cheiragonus) were collected from large surface-swarms during mid-day as they recruited onshore in early May 1998 at Friday Harbor, Wash ington, USA. Dark-adapted megalopae (held in the dark for 8 h before experi ments) were exposed to UVR (UVBR + UVAR, 280-400 nm), UVAR (320-400 nm), an d light (400-1700 nm) in the laboratory. Chromatophores expanded after only minutes of exposure to WR, WAR, and light for both species. Two alternativ e hypotheses may explain why both harmful and comparatively benign waveleng ths stimulated chromatophores to rapidly expand. First, larvae may not dist inguish among different wavelengths, which, if true, would increase the vul nerability of these larvae to intensifying UVBR due to ozone depletion. Sec ond, chromatophores have functions other than blocking UVR, such as crypsis and thermoregulation, and must respond to light for these other functions to operate. (C) 2000 Elsevier Science B.V. All rights reserved.