The taxonomy of some British Littorina species, with notes on their reproduction (Mollusca: Prosobranchia)

The rough periwinkle Littorina saxatilis is considered, in current literature, to consist of various sympatric “subspecies” and “varieties”. These, it has been suggested, are incipient species and are evidence for sympatric speciation. This paper shows that in Wales the “saxatilis” aggregate consists of four separate, fully sympatric species: L. rudis, patula, nigrolineata and neglecta. Differences between these species include proportions, sculpture and size of the shell, range of colour patterns, the anatomy of the genitalia, and isozyme patterns. The possible evidence for incipient species is discussed and rejected. The diversity of penis form amongst the British winkles is remarkable, and it is suggested that in these sympatric species the structure of the penis could be of importance in species recognition. The four species occupy different zones on the shore and three certainly differ also in their methods of reproduction: L. rudis is ovoviviparous and contains embryos throughout the year, patula is ovoviviparous but contains embryos only in winter, nigrolineata is oviparous. The reproduction and dispersal methods of all British winkles are reviewed. The evidence available supports the view that non-planktonic dispersal is an adaptation which, in species whose shell colour varies considerably from one locality to another, enables each population to become adapted to its local environmental conditions. Ovoviviparity occurs in high-shore non-planktonic species, and may be an adaptation to avoid the dangers of egg desiccation. Planktonic dispersal is found in species whose shell colour does not vary as much as in non-planktonic ones. In these species, it is argued that a common planktonic pool enables a rapid re-colonization in the case of local decrease in population numbers, and also the rapid dispersal of successful mutations.