The accurate description of marine dispersal is an ongoing challenge. Recent efforts have led to an improved understanding of larval dispersal, but there remains a paucity of studies that describe average dispersal patterns for marine species or examine the effects of dispersal on large-scale biogeographical patterns.

This work first reviews literature on what role of physical features play haping species distributions through dispersal limitation in terrestrial and marine species. The review suggests that while there is some evidence that marine species distributions can be shaped by physical barriers to dispersal, there is far less work devoted to the issue than in terrestrial systems.

In the third chapter, we present a new tool to measure larval abundance. We adapted existing protocols that label larvae using fluorescent in situ hybridization (FISH), and combined them with recent cell sorting technology (CS) so that larvae could be quickly and accurately sorted from plankton samples. Between chapters III and IV, we present DNA probes for three bivalve taxa: Musculista senhousia, Mytilus edulis/galloprovincialis/trossulus, and Mytilus californianus/Septifer spp, and one barnacle grouping of Tetraclita rubescens/ Chthamalus spp.

In the fourth chapter, we employ FISH-CS to monitor the abundance of barnacle and mussel larvae in the Santa Barbara Harbor (SBH), California. Using these abundance data, we determined that the barnacle T. rubescens likely arrives at SBH from more distant sites while the mussels M. californianus and Septifer remain near SBH throughout development. In addition, we found that early- and late-stage barnacles were concentrated onshore by internal tides but mussels were not.

Finally, we tested the theory that ocean currents can sufficiently limit larval dispersal to establish species' range limits, and examined the effects of a leaky barrier. We found the settlement of Septifer was significantly lower on the northern side of a potential current-driven dispersal filter at Point Conception, California. This decreased settlement corresponds with the species' northern range boundary. Current reversals were found to be correlated with increased settlement beyond the species' range edge, but the settlement was significantly lower than that observed within the adult distribution.