UC Davis

Timing of larval release by many shore crabs is cued by environmental cycles to occur during nocturnal spring ebb tides, when larvae are transported away from high densities of planktivorous fishes in the dark. However, a recent laboratory study indicated that larval release may be weakly synchronized relative to this safe period in upwelling regions, potentially increasing fish predation. We determined the timing of larval release and predation in marshes in an upwelling region by sampling plankton and fishes during flood and ebb tides on either side of high slack tide. Larval release was weakly synchronized, peaking during spring and intermediate ebb tides in twilight and darkness. Almost all larvae (99.8%) were eaten at twilight during peak release, when they likely were more visible than at night. However, larvae comprised only 4.1% of the diets of the 3 fish species that ate them. These fish species were often absent when conditions would make larvae most vulnerable to predation, and they preferred other prey to welldefended larvae. Larvae released outside the safe period were eaten more than those that were released during the safe period, providing selection for the timing of larval release. However, despite the large numbers of larvae present outside of the safe period, predation by fishes was much lower than expected. Thus, the selective effects of fish predation may be relaxed, raising the possibility that the strength of fish predation as a selective force varies among coasts and other selective forces that may affect the timing of larval release. © Inter-Research 2013.