Left-handedness probably originated as a species-level character in two common western Atlantic snail groups, whelks ( Busycon) and cone shells ( Conus), during the Pliocene some 3.5–4.0 Myr ago ( Vermeij 2002). Sinistrality is commonly assumed, however, to have no survival-related advantages after hatching, being entirely equivalent in functional terms with right-handedness ( Vermeij 1975, 2002, 2004 Gould et al. ![]() ![]() Handedness is known to affect reproductive success ( Ueshima & Asami 2003) dextral snails often have difficulty in mating with conspecific sinistral individuals, making it likely that the maintenance of handedness is associated with sexual selection for compatibility during mating ( Asami et al. Little attention has been given to the adaptive consequences of these rare reversals in shell coiling. Although left-handed (sinistral) individuals are known in many normally dextral species, characteristically sinistral species are exceptionally rare ( Arthur 2000 Vermeij 2002). The vast majority of marine snails, for instance, have shells that coil dextrally-to the right when oriented with the shell apex pointing upwards and the aperture facing the observer. From the twists of molecules to the internal organs of humans, conspicuous biological asymmetries pervade nature ( McManus 2002).
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