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

Abstract

Xeropicta derbentina (Krynicki, 1836), a native of Eastern Mediterranean Europe, was introduced to southeastern France during the 1940s and is now widely spread across Provence. In summer it aggregates on 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 life cycle, this species has been found in Provence to have a biennial life cycle. Moreover, in southeastern France, field studies within a restricted area show variations in demographic structure. In consequence, the life cycle of X. derbentina and the demographic patterns observed require clarification. Five populations with various demographic structures were studied over 1 year in the same location, i.e. under the same climatic conditions. The field study was complemented by laboratory observations on mating, egg-laying and hatching. Xeropicta derbentina appears to be a semelparous species, with an annual life cycle being found on four plots. The reproductive period begins at the end of summer and lasts until the beginning of winter. First egg-laying occurs within 1 week after mating and lasts up to 30 days. Hatching takes place 15–20 days after egg-laying. Xeropicta derbentina possesses multiple mating and egg-laying sessions, involving successive hatching. Populations are mainly characterized by two growth stages, the first in spring when newly-hatched snails evolve into juveniles, and the second in late summer when they reach maturity. However, on the highest density plot, a biennial life cycle is observed for some newly-hatched snails that show an interrupted growth during summer and evolve into juveniles only in the second autumn. Moreover, this life cycle not only varies 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 hatching occurs early or late and whether they survive the first and second winters. Xeropicta derbentina is thus able to have various growth speeds and life spans, and appears to switch from an annual life cycle to a biennial cycle in response to population density or climatic conditions.