Visual versus climatic selection of shell banding in the landsnail Theba pisana in Israel

The helicid landsnail Theba plsana in Israel has a four‐banded shell in which five morphs occur: 1234, 0234, 0034, 0004 and 0000. Upon each of these bands dark, delicate bandlets may sometimes appear. To examine whether (as is claimed in literature) in the north, where it is more humid, shells are darker than in the south, where it is more arid, a 160‐km‐long north (humid) to south (arid) transect was made along the coastal plain, and 32 samples were collected. Each sample was scored for relative frequency of the darkest morph, the relative frequency of effectively banded shells and the mean number of bands in the sample. In addition, the relative frequency of shells with a pale third band, and of shells with many bandlets (four or more) in the third band was scored. No evidence was found that snails in the north are darker than those in the south. In contrast, however, four of the five variables scored in each snail showed a very highly significant association with the extent of perennial vegetation at the locality. The fifth variable, namely the relative frequency of shells with many bandlets, showed a significant association only in the southernmost part of the transect. Dark shells are cryptic and are probably favoured by visual selection; light snails are conspicuous (but for the exceptional case when they occur among the white flowers of the Retama shrub), but are favoured by climatic selection. The environmental factor which ultimately decides between these two contrasting selective forces is the nature of the vegetation: by shielding snails from solar radiation, perennial vegetation may enable the snails to be cryptic, i.e. dark, without suffering overheating by solar radiation. Annual vegetation, in contrast, does not offer such shading so that snails here must be light‐coloured.