Estuarine immigration by crab postlarvae: mechanisms, reliability and adaptive significance

Most estuarine crab larvae emigrate from estuaries soon after they hatch, develop to the postlarval stage in the coastal ocean and then immigrate to estuaries and settle in adult habitats. We studied the patterns and mechanisms of immigration by postlarvae of 9 crab taxa in the small, high-salinity North Inlet estuary (South Carolina, USA) by sampling the plankton daily for 32 d over the length of the estuary. A concurrent study of larval production and flux showed that all taxa rapidly emigrate, thereby establishing empirically the importance of immigration in this estuary and giving us the rare opportunity to compare the relative species abundance of emigrating larvae and immigrating postlarvae. Contrary to the popular view that planktonic development in the sea uncouples larval recruitment from production, the community composition of emigrants and immigrants were comparable. Postlarvae immigrated at night and primarily on large amplitude flood tides. We show that this pattern is common in other estuaries and we argue that it reduces fish predation on immigrating postlarvae. Saltatory, up-estuary immigration was evident in 5 species that moved from the lower to the upper estuary (4 km) in 1 to 3 d. Although immigration rates have not been measured directly in other estuaries, a study of postlarval settlement in Mobile Bay suggests that rates may vary geographically in relation to variation in the tidal regime.