Control of Egg-Hatching Time in Crabs from Different Tidal Heights

Control of egg-hatching time was investigated in the crabs Neopanope sayi, Uca pugilator, and Sesarma cinereum, which occupy a gradient in adult habitats from sublittoral to supralittoral zones, respectively. Hatching time was monitored in the laboratory for eggs attached to females and for eggs removed several hours prior to eclosion. Embryos of all species hatched into larvae swimming independently of the female; however, those of U. pugilator and S. cinereum hatched several hours later and with lower viability than attached eggs. Mean hatching times of attached and detached eggs were correlated for N. sayi, but not for U. pugilator or S. cinereum. These results strongly imply that the embryos are the principal controllers of hatching time in N. sayi but not in U. pugilator or S. cinereum. For all species, removed eggs hatched over a longer time interval than attached eggs, indicating that the female is responsible for the synchrony of release. Increases in the readiness of attached eggs to hatch prior to actual time of larval release were relatively moderate in N. sayi, but more rapid and dramatic for U. pugilator and S. cinereum. These changes are likely the result of enzymatic degradation of the egg membranes. We postulate that, for N. sayi, enzyme release is controlled by rhythms in the embryos, but, for U. pugilator and S. cinereum, females signal the embryos to release enzymes, resulting in rapid degradation of the membranes. Selection for control of larval release time by the female may be especially important in terrestrial and semiterrestrial crabs. Brachyurans carry their fertilized eggs attached to abdominal appendages for various lengths of time depending upon species and environmental conditions. Release of larvae by many crabs occurs rhythmically with respect to exogenous cycles such as phase of the moon, tide, and time of day (for reviews see DeCoursey, 1983; Forward, 1987). For warm temperate and tropical brachyurans, egg hatching is a short duration event, with all eggs present on a female usually hatching within 15-30 min (DeCoursey, 1979; Forward et al., 1982). Exceptions occur especially among the Xanthidae which occasionally release a group of larvae each night for two or three consecutive nights (Forward et al., 1982; Christy, 1986; De Vries and Forward, 1989). Synchronous hatching of an egg mass is the result of synchronous development of the embryos and factors which control the precise time of hatching. Synchronous development of the embryos within a mass is due to an unexplored interaction between the female and the embryos (Forward and Lohmann, 1983; De Vries, unpublished data). Previous work on control of the precise time of hatching in lobsters produced a confusing picture. For Homarus gammarus, Pandian (1970) and Ennis (1973) asserted that the controlling mechanism is in the embryos, while Branford (1978) contended that it is in the female. To date, control of hatching time in decapods has been well-studied only in the subtidal xanthid crab Rhithropanopeus harrisii (see Forward and Lohmann, 1983). Forward and Lohmann (1983) concluded that the clock that sets the precise time of hatching in this species is located in the embryos. In their model, a chemical cue is released upon the hatching of several eggs. This substance induces abdominal pumping by the female, which then causes the remaining larvae to be liberated. Presumably, changes in the integrity of the egg membranes occur sometime prior to egg hatching, since abdominal pumping at other times does not result in hatching (Forward and Lohmann, 1983). These changes may be attributable to eggmembrane degradation by proteolytic enzymes secreted by the crab embryos near the time of egg hatching (De Vries and Forward, in preparation). For terrestrial and semiterrestrial crabs, water into which larvae must be released may not be immediately accessible. It would, therefore, seem maladaptive for embryos to determine the time of egg hatching, since they cannot directly assess proximity of, or