Effect of inbreeding on reproduction and juvenile performance in two marine gastropods with contrasting reproductive patterns

Many species avoid inbreeding through dispersal of individuals away from their natal groups or sites. Benthic marine invertebrates with long free-living larval stages generally possess great dispersal potential and are therefore expected to have a small chance of inbreeding in the field. Species that lack such dispersive larval stages should have a higher likelihood of inbreeding. If such direct-developing species do often mate with close relatives, we expect them to exhibit less inbreeding depression than those with dispersive larvae, since frequent inbreeding can lead to the purging of deleterious alleles through natural selection. To test this, we compared the effects of inbreeding in 2 Crepidula species with contrasting reproductive patterns: C. fornicata, which has a long dispersive larval stage; and C. convexa, a direct developer without a free-living larval stage. In laboratory studies, we compared reproductive output and several fitness traits of juveniles from snails that were forced to mate with full siblings with those from snails forced to mate with unrelated individuals. In contrast to our expectations, C. convexa showed much stronger inbreeding depression than C. fornicata. For example, inbreeding decreased mean juvenile growth rate for C. convexa by 20 to 44%, but did not affect mean growth rate for C. fornicata. It appears that larval dispersal potential alone can be a poor predictor of the amount of inbreeding that occurs in natural populations and of the effect of inbreeding on offspring fitness.