PLASTICITY OF THE LIFE CYCLE OF XEROPICTA DERBENTINA (KRYNICKI, 1836), A RECENTLY INTRODUCED SNAIL IN MEDITERRANEAN FRANCE

Xeropicta derbentina (Krynicki, 1836), a native of Eastern MediterraneanEurope, was introduced to southeastern France during the 1940sand is now widely spread across Provence. In summer it aggregateson plants, making its populations clearly visible. However,its life cycle within the Mediterranean basin is poorly documented.While X. derbentina in its native area exhibits an annual lifecycle, this species has been found in Provence to have a bienniallife cycle. Moreover, in southeastern France, field studieswithin a restricted area show variations in demographic structure.In consequence, the life cycle of X. derbentina and the demographicpatterns observed require clarification. Five populations withvarious demographic structures were studied over 1 year in thesame location, i.e. under the same climatic conditions. Thefield study was complemented by laboratory observations on mating,egg-laying and hatching. Xeropicta derbentina appears to bea semelparous species, with an annual life cycle being foundon four plots. The reproductive period begins at the end ofsummer and lasts until the beginning of winter. First egg-layingoccurs within 1 week after mating and lasts up to 30 days. Hatchingtakes place 15–20 days after egg-laying. Xeropicta derbentinapossesses multiple mating and egg-laying sessions, involvingsuccessive hatching. Populations are mainly characterized bytwo growth stages, the first in spring when newly-hatched snailsevolve into juveniles, and the second in late summer when theyreach maturity. However, on the highest density plot, a bienniallife cycle is observed for some newly-hatched snails that showan interrupted growth during summer and evolve into juvenilesonly in the second autumn. Moreover, this life cycle not onlyvaries among plots but also at a 1-year interval within plots.Hence, the life span of X. derbentina is between 12 and 20 months,but can be extended up to 30 months according to whether hatchingoccurs early or late and whether they survive the first andsecond winters. Xeropicta derbentina is thus able to have variousgrowth speeds and life spans, and appears to switch from anannual life cycle to a biennial cycle in response to populationdensity or climatic conditions. (Received 8 October 2004; accepted 15 December 2004)