The mating system of a species can have great effects on its genetic structure and evolution. We studied the extent of multiple paternity in a gastropod with internal fertilization, the intertidal snail Littorina saxatilis. Paternal genotype reconstruction based on microsatellite markers was performed on the offspring of wild, naturally fertilized females from 2 populations. The numbers of males contributing to the offspring per female were among the highest detected in invertebrates so far, with the exception of social insects. No reproductive skew in favor of males that were genetically more distant from the females was detected, and the pattern of fertilization appeared random. The result fits a hypothesis of indiscriminate mating, with genetic bet hedging as the most likely explanation. Bet hedging may have evolved as a form of inbreeding avoidance, if the snails are not able to recognize relatives. However, nutritional benefits from sperm or sexual conflict with males are additional possibilities that remain to be assessed in this species. Whatever the causes, such high levels of multiple paternity are remarkable and are likely to have a large impact on population structure and dynamics in a species in which migration between populations is spurious. Observations of multiple paternity in single clutches or broods have been commonplace since Darwin’s time (Birkhead 2000). However, a classic paradigm in evolutionary biology state
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