Field observations on Theba pisana (Müller) (Gastropoda: Helicidae) at St Ives, Cornwall

Two colonies of Theba pisana (Müller), of 6–700 and 75–100 individuals, are established at St. Ives, Cornwall, where formerly more extensive areas were colonized by the species. T. pisana is the dominant snail within the colonies which occur on the lower slopes of ‘The Island’ where soil pH is 6–7.5. Food plants include varieties of plantain (Plantago), sea carrot (Daucas carota gummifer) and, in the smaller colony, sea beet (Beta vulgaris maritima). Factors determining activity within the colonies include wind force and direction, but rainfall and the amount of moisture on vegetation were dominant. T. pisana is a nocturnal species, with daytime activity strictly controlled by the prevailing weather conditions. The area in which an individual snail moves within a colony is small, which correlates with the poor ability of the species to disperse by natural means. The spread of the colonies is also restricted by building developments. T. pisana at St. Ives is not apparently subject to mammalian or avian predation, nor to parasitic infestation. It was found buried in the soil in December, February and March, down to 3 cm below surface. Factors causing burial remain uncertain. Polymorphism of T.pisana is limited here; only four varieties and sub-varieties have been recorded.