Stage-specific distribution of barnacle larvae in nearshore waters: potential for limited dispersal and high mortality rates

The stage-specific spatial distribution and mortality of Balanus glandula and Chthamalus spp. larvae were assessed with a series of daily vertical plankton tows collected from inner-shelf waters in La Jolla, Southern California, in March 2003. Sampling stations were located within 1.1 km of the shoreline, at depths of 10 to 45 m. For both groups, we observed a spatial segregation of naupliar stages and cyprids, although this pattern was statistically significant for Chthamalus spp. only. Early nauplii (NII and NIII) were more abundant at the inshore stations, whereas later stages (NIV to NVI) occurred in greater numbers offshore. Cyprids were consistently more abundant at the inshore station. Such striking differences in the horizontal distributions of late nauplii and cyprids suggest limited dispersal of barnacle larvae in nearshore waters. Particle trajectories computed from current velocities measured in the area indicated that changes in vertical distribution may indeed affect dispersal, and, in some cases, enhance the retention of larvae in shallow, inner-shelf waters. Vertical life tables were used to estimate naupliar mortality rates from pooled daily stage distributions. Average estimates (+/- SE) for the instantaneous rate of larval mortality in B. glandula (0.33 +/- 0.05 larvae d(-1)) and Chthamalus spp. (0.23 +/- 0.03 larvae d(-1)) were substantially higher than previously assumed for these species. We discuss the implications of limited dispersal and high mortality rates for the exchange of larvae among disjunct populations of intertidal barnacles and other coastal benthic invertebrates.