Most shallow-water crabs release larvae during nocturnal maximum amplitude high tides, when larvae are rapidly transported by strong ebb tides to deeper waters at night. Larvae released at this time may avoid being eaten by planktivorous fishes, which abound in shallow water and feed during the day. Eurypanopeus planus (Smith, 1869), Uca beebei (Crane, 1941) and Pachygrapsus transversus (Gibbes, 1850) live on the Pacific coast of Panama and often release larvae during the day. Eurypanopeus planus also releases larvae without regard to tidal amplitude. Unlike most crabs, larvae of these three species will encounter, not avoid, planktivorous fishes. The predator avoidance hypothesis predicts that these larvae should be relatively well protected from predation compared to larvae of species that are released when predators are avoided. We tested this prediction experimentally by exposing larvae of these species and the sympatric crab, Cataleptodius taboganus (Rathbun, 1912), which exhibits the common hatching pattern, to silversides, Membras gilberti an abundant diurnal planktivore that feeds where all species release larvae. The rank order of silversides' prey preferences matched that predicted from the timing of larval release. Larval vulnerabilities to predation were related to the apparent conspicuousness of larvae, which is determined by the color of yolk and chromatophores. A survey of 30 other species of crabs further indicates that conspicuously colored larvae are released when they best avoid predators and that inconspicuously colored larvae may be released at other times.
Morgan, Steven G. and Christy, John H. 1997. Planktivorous fishes as selective agents for reproductive synchrony. <i>Journal of experimental marine biology and ecology<i>, 209(1-2): 89-101.