Microhabitat choice and high temperature tolerance in the land snail Theba pisana (Mollusca: Gastropoda)


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    Department of Genetics and Biometry, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, U.K.


Adults of the land snail Theba pisana near the species’northern limit (Tenby. South Wales) spend the daytime inactive attached to stems of tall vegetation, avoiding higher temperatures near the ground, while juveniles remain low down. This may be related to the lesser ability of juveniles to withstand desiccation for long periods, and/or to their requiremcnts for an easily accessible source of suitable food during growth. In the field and laboratory, adults (at least) respond to light and wind direction, avoiding the hottest and most exposed sides of stems on which they rest.

The upper lethal temperature of snails from S. Wales lies between 42 and 46 °C, depending on exposure time. Juveniles are more tolerant than adults, probably because of their greater ability to use evaporative cooling for short periods. Spanish T. pisana are more tolerant (upper lethal limit 46–50 °C) than Tenhy snails; this difference may have a genetic basis. Aestivating Spanish snails are more tolerant than active ones.

Climbing and orientation are essential for both adults and juvenilcs in the Mediterranean habitats where Theba pisana evolved, since ground temperatures can exceed the lethal limit. It is less important at the northern edge of the range, but is still retained by the adults.