Comprehensive field and laboratory investigations will be undertaken to determine the local mating system and geographic population structure of the lemon shark, Negaprion brevirostris, a large coastal shark and a tropical apex predator. The research objectives are to: 1) genetically characterize, for the first time, the mating system of a cartilaginous fish; 2) clarify the use of a tropical nursery ground by juvenile sharks and females in paturition; and 3) examine population structure at both a local and extensive geographic scale. The study site at Bimini is unique in that nearly the entire juvenile population in the lagoon can and will be sampled, and genetic analysis of these samples will in large part provide the data for the first two objectives. Specifically, the field component includes a five-year sampling program of which three years have been completed. The data from juveniles will be supplemented by genotyping adults sampled in and around Bimini. The genetic and field data together will provide estimates of the total population size of pups, their growth, survival and movements, the number of females using Bimini as a nursery ground, and the relative reproductive success of males and females within and across years. Finally, the data can document whether individual females are returning to the Bimini nursery areas to give birth and if so, the interbirth intervals for females and the number of males siring their young. The third objective of the study will allow placement of the detailed local genetic structure documented at Bimini within the framework of the species genetic structure as a whole. Population structure at larger geographic scales will be accomplished through sampling 20-30 lemon sharks at four other locations that represent multiple spatial scales and potential barriers to dispersal. These sites include 1) a second Bahamian site (Grand Bahama Island) approximately 100 km across the NW providence channel from Bimini; 2) a second known lemon shark nursery west of the Florida Keys (Marquesas) separated from Bimini by 350 km and the Florida current; 3) a distant Atlantic site off the east coast of Brazil approximately 5000 km from Bimini; and 4) samples from the Pacific coast of Mexico completely isolated from the Atlantic sites by the Isthmus of Panama. Two methods of genetic analysis, microsatellites and mitochodrial DNA (mtDNA), will be employed in this study. Microsatellites exhibit the levels of variability necessary for inferring parent/offspring and sibship relationships at Bimini, especially when supplemented by additional genetic information provided by the maternally inherited mtDNA. The use of unlinked genetic markers having different transmission genetics (mtDNA is maternally inherited and microsatellite loci exhibit Mendelian inheritance) will provide separate but complementary estimates of genetic variation within and among populations, provide estimates of population differentiation and migration rates between the five sites in our study, and distinguish putative differences in patterns of male and female mediated gene flow.
