Weakly synchronized larval release maintained in the presence of predatory fishes

Abstract

Many marine species minimize predation during hatching or spawning by releasing larvae or gametes synchronously during nocturnal spring ebb tides. Propagules are then rapidly transported away from high densities of predatory fishes into deeper waters under the cover of darkness. Females also suspend foraging in the presence of predators, but it is unknown whether they are able to delay releasing larvae. In a previous study, we found that larval release is only weakly synchronized to the safe period in a cold upwelling region, although the study was conducted on outdoor seawater tables in the absence of tides that reinforce endogenous rhythms. In this study, we experimentally determined whether larval release 1) is better synchronized in the field and 2) delayed in the presence of predatory fishes. Larval release was weakly synchronized to tidal amplitude, tidal and diel cycles, occurring from late flood to late ebb tide on both intermediate and spring tides during twilight as well as darkness. Weak synchrony likely arises because cold temperatures extend incubation of externally brooded embryos, increasing exposure to environmental variation. Larval release was not delayed in the presence of predatory fishes; nor was refuge use or other behaviors of females altered by fishes. Behaviors were not affected by predators presumably because larvae are already being released near the safe period from refuges. Our results likely apply to other cold regions of the world, but it remains to be determined whether predators alter the activities of nonovigerous female and male shore crabs, which may not spend as much time in refuges as ovigerous females, potentially resulting in cascading effects on communities.