ADAPTIVE SIGNIFICANCE OF THE TIMING OF LARVAL RELEASE BY CRABS

The adaptive significance of synchronous larval release by marine animals has been elusive. The hatching times of 10 species of intertidal crabs were determined and compared to those expected if crabs release larvae when predation on females, embryos, and newly hatched larvae is least. The safest time to release larvae occurs during the largest-amplitude nocturnal high tides of the lunar month. Crabs throughout the intertidal zone can release larvae near their refuges, and larvae will be transported rapidly at night from shorelines where diurnal planktivores abound. High and middle intertidal species released larvae at this time, which suggests that predation on all three life stages ultimately may synchronize reproduction. Unlike these species, low intertidal crabs are inundated every day, and therefore females could release larvae near refuges daily, Although one low intertidal species did release larvae every day and often during the daytime, two other species released larvae during the safe period. The timing of larval release by low intertidal crabs varies with the vulnerability of their larvae to planktivorous fishes. The hatching times of 46 species worldwide support our contention that predation, primarily on newly hatched larvae, and not other sources of mortality, selects for synchronous hatching by crabs.