Based on the belief that marine larvae, which can spend days to months in the planktonic stage, could be transported considerable distances by ocean currents, it has long been assumed that populations of coastal species with a planktonic larval stage are demographically open and highly ''connected.'' Such assumptions about the connectivity of coastal populations govern approaches to managing marine resources and shape our fundamental understanding of population dynamics and evolution, yet are rarely tested directly due to the small size and high mortality of marine larvae in a physically complex environment. Here, we document a successful application of elemental fingerprinting as a tracking tool to determine sources of settled invertebrates and show that coastal mussel larvae, previously thought to be highly dispersed, can be retained within 20 -30 km of their natal origin. We compare two closely related and co-occurring species, Mytilus californianus and Mytilus galloprovincialis, and determine that, despite expected similarities, they exhibit substantially different connectivity patterns. Our use of an in situ larval culturing technique overcomes the previous challenge of applying microchemical tracking methods to species with completely planktonic development. The exchange of larvae and resulting connectivities among marine populations have fundamental consequences for the evolution and ecology of species and for the management of coastal resources.elemental fingerprinting ͉ in situ larval culturing ͉ larval retention ͉ larval transport ͉ Mytilus

The diverse fauna and flora of rocky intertidal ecosystems are being impacted by the activities of rapidly increasing coastal populations in many regions of the world. Human harvesting of intertidal species can lead to significant changes in body sizes of these taxa. However, little is known about the temporal trajectories of such size declines and more importantly, the long-term effects of chronic human impacts. Furthermore, it is unclear whether sizes of species not directly targeted for harvesting are also declining through indirect effects. Here we use historical (extending back to 1869) and field survey data covering 200 km of mainland southern California coast to show that human activities have led to significant and widespread declines in body sizes of rocky intertidal gastropod species over the last century. These declines, initiated several decades ago, are continuing and contrary to expectation, they are not restricted to species harvested for human consumption. Data from the only national park in this area, where conservation laws are strictly imposed, demonstrate that negative ecological impacts can be ameliorated if existing laws are enforced.

Elements incorporated into developing hard parts of planktonic larvae record the environmental conditions experienced during growth. These chemical signatures, termed elemental fingerprints, potentially allow for reconstruction of locations of larvae. Here, we have demonstrated for the first time the feasibility of this approach for bivalve shells. We have determined the spatial scale over which we are able to discriminate chemical signatures in mussels in southern California and characterized the temporal stability of these signals. Early settlers of Mytilus californianus and Mytilus galloprovincialis were collected from eight sites in southern California. Shells were analyzed for nine isotopes using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). We discriminated among mussels collected in two bays and the open coast using Mn, Pb, and Ba shell concentrations. Shell concentrations of Pb and Sr were sufficiently different to discriminate between mussels from the northern and southern regions of the open coast, each representing approximately 20 km of coastline. These signals were relatively stable on monthly and weekly time scales. These results indicate that trace elemental fingerprinting of shell material is a promising technique to track bivalve larvae moving between bays and the open coast or over along-shore scales on the order of 20 km. Identification of spatial variation in elemental fingerprints that is stable over time represents a crucial step in enhancing our ability to understand larval transport and population connectivity in invertebrates.As marine biologists began to recognize the existence of planktonic larval stages of benthic adults during the first half of the 19th century, they began to evaluate the role of early life history in determining the abundance and distribution of benthic populations (e.g., Young 1990). Over time, marine ecologists have become increasingly concerned with the role of prerecruitment processes in structuring populations (e.g., Prytherch 1929;Roughgarden et al. 1988;Caley et al. 1996). 1Corresponding author (bjbecker@ucsd.edu). AcknowledgmentsThis work was funded by the California Environmental Quality Initiative (CEQI, Graduate Research Support Fellowship), the National Science Foundation (OCE-0327209), the Office of Naval Research (N00014-00-1-0174 and N00014-01-1-0473), the Switzer Environmental Fellowship, the Link Foundation, and the Cabrillo National Monument Foundation. B.J.B is supported by the United States National Park Service. Species identification using PCRbased methods were conducted by R. Byrne in the laboratory of R. Burton. Thermistor temperature data were provided by J. Largier, who is supported by California SeaGrant, with assistance from T. Kacena. Significant laboratory and field assistance was provided by L. Fajardo, V. Cannon, T. Bernhardt, and numerous volunteers. LA-ICP-MS analyses were conducted in the Scripps Institution of Oceanography Analytical Facility; K. Walda contributed valuable assistance in ICP-MS te...