Circatidal swimming behaviors of fiddler crab Uca pugilator larvae from different tidal regimes

Abstract

Fiddler crab Uca pugilator larvae are released within estuaries near the time of high tide and are exported offshore to undergo development. Previous studies indicate that zoea-1 larvae use selective tidal-stream transport (STST) to migrate from adult habitats to shelf waters. In areas with semidiurnal tides, this behavior is mediated by a circatidal rhythm that is characterized by upward swimming toward the surface during ebb tide, followed by a descent toward the bottom during flood tide. We tested the hypothesis that U. pugilator zoeae from different tidal regimes possess activity rhythms that match local tidal patterns. Ovigerous crabs with late-stage embryos were collected from areas with semidiurnal, diurnal, and mixed tides and from an area within a microtidal coastal lagoon. Following hatching, swimming activity was monitored under constant conditions for 96 h. Zoeae from semidiurnal and diurnal regimes possessed activity rhythms with free-running periods that matched the tides at the collection sites (≈12.4 and 24.8 h, respectively). Crabs from beaches with mixed tides displayed asymmetrical oscillations in swimming activity with dominant periodicities that corresponded to the semidiurnal and diurnal constituents of the tides. Zoeae from microtidal areas possessed rhythms that were similar to those observed in crabs from areas with semidiurnal tides. At all locations, peaks in activity occurred during the time of expected ebb currents, which is consistent with STST behavior favoring seaward transport. Differences in larval activity patterns suggest that STST behaviors are controlled by separate circatidal and circalunidian clocks or paired circalunidian clocks coupled in antiphase. © Inter-Research 2007.