The breeding migration of Littorina arcana Hannaford Ellis, 1978 (Prosobranchia: Littorinidae)

Two species of rough winkle, the oviparous Littorina arcana and the ovoviviparous L. saxatilis are very similar in their outward appearance and are commonly syntopic. This has led some researchers to regard these species as conspecific and to suggest that reproductive modality is not a fixed characteristic. However, the syntopy of these species is much more complex than has been supposed and data presented here show how the seasonally reproducing L. arcana migrates downshore during its breeding period, into areas where L. saxatilis is comparatively sparse. This results in a separation of the breeding populations of the two species and consequently a spatial separation of their juveniles, at least in their early stages. Selection pressures on the breeding animals and young juveniles will therefore differ.