LUNAR AND TIDAL PERIODICITY OF MOLTING AND REPRODUCTION IN STOMATOPOD CRUSTACEA: A SELFISH HERD HYPOTHESIS

Abstract

All crustaceans are susceptible to conspecific or other predators when molting; this vulnerability is considerably magnified in mantis shrimp, which employ intense aggression against conspecifics and congenerics, and which are rapacious predators. This study examines the ecological and evolutionary consequences of timing of molting and reproduction in stomatopod Crustacea. During laboratory maintenance, Gonodactylus zacae from the Gulf of California molts during neap tides; oviposition probably occurs during neap tides. G. falcatus from Hawaii [USA] tends to molt during spring tides; oviposition occurs during neap tides. Strong evidence suggests that Haptosquilla glyptocercus from Eniwetok molts during neap tides. Pseudosquilla ciliata from 3 populations molt during the last lunar phase, providing the only evidence for lunar rather than tidal control of molting in stomatopods. P. ciliata from Hawaii and Florida [USA] oviposit during waxing neap tidal phases; those from Thailand oviposit during waning neap tides. G. falcatus and G. graphurus from Australia molt in opposite tidal cycles; G. graphurus molts during neap tides but reproduces during spring tides. The 5 spp. of Gonodactylidae from Australia oviposit during spring tides. Field data demonstrate significantly more molting during spring than neap tidal cycles in a gonodactylid community coprised of 7 spp. in Phuket, Thailand. Laboratory maintenance increases variation in the pattern of molting across tidal cycles, although molting still occurs significantly more frequently in spring than neap tidal cycles. G. chiragra molts significantly more frequently than expected during spring than neap tides in the field. Field and laboratory data demonstrate significant correlation of oviposition with spring tides among species in 1973 but not 1974. These data support a temporal selfish herd hypothesis that synchrony of molting in the population reduces mortality due to conspecific and congeneric aggression in stomatopods, but not predictions from alternative hypotheses that physical factors, feeding, or predation impose molting and reproductive rhythms. Molting synchrony is more consistent with the idea that stomatopod populations are limited by availability of burrows than by food. The initiation of molting synchrony in the population can be explained by selection for response to environmental cues, by chance, and/or by small, local populations with synchronous hatching of young. Oviposition rhythms may subserve the primary molting rhythm.