Effects of geophysical cycles on the rhythm of mass mate searching of a harvested mangrove crab

The harvested mangrove crab Ucides cordatus exhibits conspicuous cycles of searching for mates en masse. This phenomenon, called andada, always occurs at the syzygies, but the particular moon phase, full, new or rarely both, varies for unknown reasons. The presence and absence of andada activities of a Northeast Brazilian crab population was assessed by boat between 2006 and 2011. In 2008 crabs were additionally collected in sampling plots to determine the duration and intensity of andada events and in 2010 the timing of andada with respect to the light:dark cycle and tidal cycle was studied by observations from platforms. We found that andada occurred during the day and night and that the rhythm of mate searching was linked to the syzygy tide inequality cycle (STIC). Andada shifted between new and full moon, depending upon which moon phase had the higher amplitude tides. The ultimate cause of andada is likely to be increased larval survival after synchronous release at highest amplitude spring tides 1 month later. Such anticipatory behaviour is probably under endogenous control. The results of this study can help to improve temporal placements of capture bans for this harvested species and reduce current conflicts between fishers and regulatory agencies.