Variation in owl limpet Lottia gigantea population structures, growth rates, and gonadal production on southern California rocky shores

The size structures of Lottia gigantea populations were studied at 8 southern California sites, 4 of which were longstanding Marine Protected Areas (MPAs). Greater mean sizes and higher frequencies of larger individuals occurred at sites with lowest human visitation. Mean L, gigantea shell lengths (SLs) were negatively correlated with the number of visitors and collectors per 10 m of shoreline. Limpets never achieved SLs greater than 79.0 mm at any of our 8 sites, a size well below the maximum for this species. Mean SLs were comparable to sizes at other sites where L. gigantea is intensely collected. The presence of collectors, small mean SLs, and the absence of large limpets suggest that L. gigantea populations are affected by humans at our sites. Lower densities and higher frequencies of larger limpets were found in a subpopulation living on open-rock surfaces compared with a subpopulation occupying smaller, patch habitats within mussel beds. Larger limpets were mostly female and gonadal mass increased exponentially with SL in both subpopulations. Limpets <40 mm SL exhibited highly variable growth rates; however, limpets in patch habitats grew slower and produced less gonadal mass than limpets inhabiting open rocks, despite the availability of more microalgal food. Ages of the largest limpets in the open-rock subpopulation were estimated to be >8 yr. Given the age of larger limpets and the potential for collectors to remove larger individuals and shift populations towards smaller size structures, effective MPAs may take decades to reverse the effects of human impacts on L. gigantea populations in southern California.