IS THERE A SURVIVAL-RELATED ADVANTAGE OF SINISTRAL COILING IN MARINE SNAILS?

The vast majority of marine snail species have shells that coil dextrally, or to the right when oriented with the shell apex pointing upwards and the aperture facing the observer. Although left-handed, or sinistral, individuals are known in many normally dextral species, establishment of left-handedness at the species level is exceptionally rare.

Sinistrality is associated with intense sexual selection for mate compatibility; however, the trait is not thought to have any significant survival-related advantages. A reason to suspect this assumption might not hold is that the majority of crab predators that feed on snails are laterally asymmetric, typically having the claw they use to crush or peel the shell of their prey on the right-hand side of their bodies. Ng and Tan (1985) hypothesized that this right-handedness of crabs would result in a disadvantage in breaking sinistrally coiled snails.

We used the trace-fossil record of unsuccessful crab predation on the shells of ecologically and morphologically similar pairs of dextral and sinistral Busycon and Conus species from several southeastern United States Plio-Pleistocene deposits to test Ng and Tan’s (1985) hypothesis. We predicted that frequencies of shell repair should be higher in dextral snails than in sinistral snails, because sinistral prey are thought to be more difficult for right-handed crabs to manipulate (and thus likely abandoned prior to inflicting any shell damage that would later have to be repaired). Our preliminary results support our prediction, with dextral whelk and cone species typically having higher frequencies of shell repair than sinistral species.

Our data suggest that sinistrality is selectively advantageous in increasing survivorship against laterally asymmetric crab predators. In this way, natural selection and sexual selection likely are reinforcing each other in the establishment and maintenance of sinistrality within these groups. We suggest that this advantage is at least as important as selection for mate compatibility, especially in highly competitive environments for long-lived species, where individuals are exposed to their crab enemies for a significant amount of time before reaching sexual maturity. It’s not all about sex all of the time.