They might appear dull and staid, but don't be misled by their looks. Rough periwinkles (Littorina saxatilis) are wanton. So wanton that an average of 7.6 (± 2.1) males can claim paternity of young in a brood. As promiscuous as that sounds, it pales next to behavior of the (very) social insects — female army ants (Eciton burchellii) mate with an average of 15.5 males and queen honeybees with 80 or more. Still, as molluscs go, Littorina saxatilis tops the league table.
So what benefit does multiple mating confer in this species?
In such cases, all males contribute to the brood. There is no selection. Females don't choose between males before mating. Nor is there sperm competition (where sperm of some individuals are more successful at fertilisation than those of others) or cryptic female choice (where selection takes place inside the reproductive tract).
So if females are indiscriminate and don't distinguish between males, there is a possibility of inbreeding. Rough periwinkles not only occur in dense aggregations but also have limited mobility, which together increase that likelihood. It may be that multiple mating is a bet hedging strategy that reduces the negative impact of breeding between closely-related individuals. The more fathers, the lower the chances of banjo-pluckin' offspring. At least, the banjo-players will be in the minority. (Compare twenty offspring from one father versus twenty offspring from ten fathers.)
Because of this, multiple paternity also preserves genetic variability in fluctuating populations. A single female may carry with her sperm from as many as ten males — so preventing a genetic bottleneck in the case of a crash or the founding of a new population.
But mark my words, it won't be long before we see a periwinkle being interviewed on A Current Affair.
Mäkinen, T, Panova, M & André, C. (2007). High levels of multiple paternity in Littorina saxatilis: hedging the bets? Journal of Heredity 98(7): 705 – 711. Doi: 10.1093/jhered/esm097