Lunar and tidal periodicity of molting and reproduction in stomatopod crustacea: a selfish herd hypothesis.

Abstract

One of the constraints imposed upon arthropods by their protective exoskeleton is the necessity to molt. While the physiology and morphogenesis associated with the molt cycle in crustaceans has commanded a great deal of attention (e.g., Pas sano, 1960 ; Drach and Tchernigovtzeff, 1967 ; Stevenson, 1972 ; Aiken, 1973; Davis, Fyhn, and Fyhn, 1973 ; Reaka, 1975a) , few workers have examined the process of molting in an evolutionary perspective. This study examines the eco logical and evolutionary consequences of molting in stomatopod crustaceans. Mantis shrimp are primitive members of the Malacostraca ( Brooks, 1962; Siewing, 1963 ; Holthuis and Manning, 1969 ; Schrani, 1%9a, b, 1973 ; Burnett and Hessler, 1973 ; Reaka, 1975b) , but they possess a highly specialized life style which may be associated with unique molting and reproductive adaptations. Stomatopods frequently live in species assemblages where as many as five con generics occupy indistinguishable microhabitats in one tidepool (Reaka, 1972, and in preparation) and seven congenerics may occupy one tide flat (this study). Protection from predation is afforded by burrows or cavities in mud, sand, rocks or coral. Stomatopods employ intense and sometimes lethal agonistic behavior against conspecifics and congenerics while fighting to obtain or retain burrows and in addition are rapacious carnivores. All crustaceans are susceptible to conspecific or other predators when molting and often possess behavioral adaptations to minimize mortality at this time (Passano, 1960). This vulnerability is considerably magnified in molting stoma topods because of their behavior and frequent sympatry with closely related species; if not killed by another mantis shrimp, a stomatopod may be displaced from its hole and exposed to predation by fishes (Townsley, 1953; Camp, 1973). Large size is an obvious advantage for pursuit and overpowering of prey, defense against predators, conspecific and congeneric fighting, and reproduction (Reaka, 1974, 1975c), but carries the disadvantage of repeated molts. How, then, do stomatopods minimize the effects of molting while maintaining their tactics of predation and contest competition? Reclusive and submissive behavior, as well as sealing the entrance of the burrow, reduce vulnerability at the molt in stomatopods; the stages of morphogenesis associated with incapacitation and exuviation are abbreviated in several aggressive taxa of crustaceans but especially in stomatopods; and asym metrical stages of the molt cycle suggest that the early premolt may be a waiting period, whereupon initiation of the molt progresses rapidly following the correct cue (Reaka, 1975a). The present paper examines timing of molting and repro duction, and suggests that lunar and tidal synchrony of molting in the population 468