CHRISTY, J.H.*; RISSANEN, J.R.; BACKWELL, P.R.Y.; Smithsonian Tropical Research Institute; Smithsonian Tropical Research Institute; Australian National University: A One-day Pause in the Biweekly Courtship Cycle of a Tropical Fiddler Crab Allows Females to Avoid Releasing Larvae During Twilight

Predation on adults or young in the intertidal zone may vary predictably with the daylight, tidal height and tidal amplitude cycles and favor reproductive timing for predator avoidance. A novel pattern in the reproductive cycle of the fiddler crab Uca terpsichores supports this idea. Fiddler crab larvae are vulnerable to predation by diurnally feeding planktivorous fish that abound in shallow habitats where adults live. Estuarine fiddler crabs usually release larvae at night near the time of the evening high tides on the few consecutive days every two weeks with larger amplitude nocturnal ebb tides. Consequently, newly released larvae move rapidly seaward on ebbing tides under the cover of darkness and thus avoid falling prey to diurnal planktivors. Daily monitoring for 20 months of the intensity of courtship by U. terpsichores, which lives on protected sand beaches in the tropical Eastern Pacific, shows they have a typical biweekly courtship and mating cycle but with two modes split by a day when reproductive activity is greatly reduced. Given that embryonic development takes about two weeks, this mid-day pause predicts a decline in larval release two weeks later on the days with crepuscular high tides. Plankton sampling showed that larvae are released throughout the night up until dawn. As predicted, there was a decline in the density of newly hatched larvae on days with twilight high tides and also on days with moonlit high tides. Avoiding light-dependent visual predators at the time of larval release is paramount for this species. Apparently less important is extensive seaward dispersal at night perhaps because this species lives relatively close to deeper, safer waters.