Morphology of the oviparous rough winkle, Littorina arcana Hannaford Ellis, 1978, with notes on the taxonomy of the L. saxatills species-complex (Prosobranchia: Littorinidae).

Littorina arcana is an oviparous species which is often collected with the related but ovoviviparous L. rudis, both species occurring apparently in the same zone on many British beaches. The shells of the two species are frequently indistinguishable and since the species are sympatric they cannot be reliably separated in the field.
Dissection of freshly killed animals and identification from the characteristics of the pallial oviduct and the ciliated field (which lies between the genital tract and the columellar muscle) is the only sure way to separate these two species. The ciliated field of L. arcana is comparatively small and inconspicuous. The pallial oviduct is distinguished by the relatively large so-called albumen and capsule glands and the development of a jelly gland in a position homologous to the brood pouch in L. rudis. The penis of L. arcana is typically relatively shorter, has more penial glands, a tapering tip and is often almost triangular in shape. However, these penial differences are not diagnostic. The shell is often patulous in character but many other shell varieties can be produced.
Circumstantial evidence suggests that the report of oviparity in rough winkles by Seshappa (1947) was based on observation of L. arcana, rather than the other oviparous rough winkle, L. nigrolineata. Facts that suggest that the description of L. patula given by Heller (1975) was based on a mixture of specimens of L. arcana and L. rudis are considered, and a revision of the taxonomy of the rough winkles is proposed whereby the name L. patula is regarded as synonymous with L. rudis.
L. arcana appears to be a widespread species, so far, known to occur in Eire, France, Norway and Britain.