Six populiiiioii> 01' Li/forinn r u d \ and three or I.. arrnna were screened for rlectl-ophol-ctically tlcrcc riil~lc vat-iiition at 2 1 enzyinr loci. These species repl-odure by ovoviviparity a n d oviparicy respectively, and both are members of the L. saxatilis species complex. The mean coefficient of genetic identity (7) between the two species was high (0.957) and no diagnostic loci were observed. t\llcdc, I k q t i c t i ( y clilli~t-cncrs arc inaintained i n syinpatl-ic population5 or lhr two Fpecics. Intrasprcilic w l c i (~~ 01. / t.;iiigctl ii.otn 0.944 to 0.995 and both sprcirs showrd markrcl intrt--p(~piila'ion geneticIic~rc~wgciiciry, ii li~.itut.c~ consistent with their liinited powers or dispersal. Hetrrozygosity is high i n all populations of both species. Populations of L. rudij have higher mean heterozygosities (I? = 0.153)

Acceppled for publication 25 May I982 Forty-one mixed samples of winkles containing the closely related species, Littorina rudis and L. arcana, were collected from different parts of the British Isles. Littorina rudis was the more ubiquitous species, with L. arcana being more-or-less confined to vertical clifi and rocks. The frequencies of different shell d o u r patterns were determined for both species in each sample. Several colour morphs were diagnostic of one or other of the species over large areas, which confirms that these are separate species. The frequencies, in the two species, of the two commonest morphs, brown and fawn, were strongly correlated; in several other morphs their presence and absence in the two species was significantly associated; the levels of phenotypic diversity in the two species were also correlated. These patterns could not have arisen if the variation between shores was the result of genetic drift or founder effects, so some form of selection is implicated. In the brown morph there is some evidence for frequency-dependent selection, and it is suggested that the polymorphism may be maintained by visual predators through apostatic selection.

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This paper details attempts to breed members of the L. saxatilis species complex in the laboratory and to construct interspecific hybrids between them. Success in reciprocal crosses between animals from the type localities of the 'species' of Littorina saxatilis (Olivi) and Littorina rudis (Maton) indicates that these two taxa are synonymous. Six of the twelve possible reciprocal hybridisation crosses between the four species (L. saxatilis, L. arcana Hannaford Ellis, L. nigrolineata Gray and L. neglecta Bean) have been attempted, with only one proving successful, that between male L. saxatilis and female L. arcana. This hybrid cross produces viable offspring, although at a lower frequency than either of the parental crosses in the laboratory, while the reciprocal cross has, as yet, proved unsuccessful. Limited work on the F1 hybrids shows them to be inter-fertile and also capable of backcrossing with male L. saxatilis, but not with female L. saxatilis or L. arcana. Details are also given of attempts to find natural hybrids in sympatric populations of the species of the complex, using gel electrophoresis.

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