Vertical migration behavior and horizontal distribution of brachyuran larvae in a low-inflow estuary: implications for bay-ocean exchange

Rhythmic vertical migration has been well documented as a behavioral mechanism of brachyuran larvae that are actively exported from or retained within estuaries with net tidal residual flows, such as partially mixed estuaries. However, the effectiveness of vertical migratory behavior in mediating net larval transport in a low-inflow estuary (LIE), which is characterized by little or no freshwater inflow for extended periods of time, has not been addressed. Larval transport hypotheses, developed using a validated hydrodynamic model of San Diego Bay (SDB; an LIE), predicted that vertical migratory behavior was necessary to transport larvae out of the bay, while larvae lacking vertical migratory behavior would be retained within the bay. These predictions were tested in a field study that examined the temporal and spatial (vertical and horizontal) distribution of 2 brachyuran families, Pachygrapsus crassipes (Grapsidae) and Lophopanopeus spp, (Panopeidae), sampled at 2 sites within SDB. P. crassipes zoeae exhibited vertical migratory behavior, which enhanced their net export from the bay. Stage I P. crassipes zoeae were concentrated in surface layers (0 to 2 m) during nocturnal ebb tides, but exploited low-velocity conditions associated with the benthic-boundary layer, which retarded transport back into the bay during flood tide. The lack of Post-Stage I P. crassipes zoeae within SDB suggests that the observed vertical migratory behavior was an effective means for exiting the bay, Stage I and Post-Stage I Lophopanopeus spp. larvae were sampled within SDB, but exhibited no clear rhythmic migration patterns. The lack of vertical migratory behavior and the presence of all larval stages of development suggest that Lophopanopeus spp. larvae were retained within the bay throughout meroplanktonic development. Heterogeneous horizontal distributions of Stage I P. crassipes and Lophopanopeus spp, zoeae reflect hatching sites and physical circulation features which concentrate larvae at specific areas. Differences in larval behavior observed for grapsid and panopeid larvae in SDB had a significant effect an their dispersal and retention.