Survival of Marine Larvae Under the Countervailing Selective Pressures of Photodamage and Predation

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 die1 cycle was not related to the susceptibility of larvae to photodamage. Visual predation in heterogeneous terrestrial and benthic habitats has resulted in distinctive variously colored prey that are inconspicuous in their native surroundings (McFall-Ngai 1990). In contrast, predators in the pelagic realm view their prey against a more homogeneous background, and most zooplankters look and behave somewhat alike. They are small and transparent, and migrate from productive surface waters during the day (McFallNgai 1990). These traits are believed to reduce predation by planktivorous fish (Hobson and Chess 1978; Zaret 1980; Lazarro 1987). Despite the prevalence of transparency, pigmented zooplankters occur in most marine habitats and deep lakes, where they descend from bright productive surface waters to dimmer deeper waters. This refuge is not available in shallow ponds, where bright red copepods often are replaced by inconspicuous pale green or blue morphs (Hairston 1979; Luecke and O'Brien 198 1; Byron 1982). Pale morphs may not be widespread because they are more susceptible to photodamage from ultraviolet (UVR) radiation. In lakes that do not contain fish, heavily pigmented zooplankters remain in productive near-surface waters during the day and have higher reproductive outputs (De Meester and Beenaerts 1993; Duncan et al. 1993). Thus, zooplankters may minimize both predation and photodamage by avoiding brightly illuminated waters, but at the cost of reduced reproductive output. Decapod crustaceans and fish are exceptional because their pigmented larvae often develop in sunlit surface Current address: Marine Sciences Research Center, SUNY, Stony Brook, New York 1 1794-5000.