UC Davis

Unlike most zooplankters, larvae of decapod crustaceans and fish often are pigmented and may hatch and ascend to the sea surface during the day. Chromatophores possibly protect these larvae from ultraviolet radiation (UVR), but may increase their visibility to planktivorous fish. We exposed larvae of four species of tropical crabs to sunlight and to planktivorous fish in the field to address this paradox. Most lightly pigmented larvae of three species died from exposure to sunlight within 1 d, and 94- 97.5% of them died after 2 d. The more darkly pigmented Pachygrapsus transversus larvae survived significantly better; only 57% of them died after 2 d of exposure to sunlight. These darkly pigmented larvae survived encounters with fish as well as or better than larvae of two other species that did not have large melanophores. Larval chromatophore systems may block UVR without greatly increasing the visibility of larvae to fish. Larvae that migrate from adult habitats to nursery grounds in surface waters throughout the day likely have chromatophore systems that are effective against both of these countervailing selective pressures; however, increasing UVR may stress larvae. The timing of larval release relative to the diel cycle was not related to the susceptibility of larvae to photodamage.