Elements of cold hardiness in a littoral population of the land snail Helix aspersa (Gastropoda: Pulmonata)
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O R I GI N A L P A P E R
A. Ansart Æ P. Vernon Æ J. Daguzan
Elements of cold hardiness in a littoral population of the land snail Helix aspersa (Gastropoda: Pulmonata)
Accepted: 8 July 2002 / Published online: 21 August 2002 Ó Springer-Verlag 2002
Abstract The land snail Helix aspersa can be considered partially tolerant to freezing, in the sense it can survive some ice formation within its body for a limited time, and possesses a limited ability to supercool. This study aimed at understanding what factors are responsible for the variation of the temperature of crystallization (Tc) in a littoral temperate population. The ability to supercool was maximal (ca. )5 °C) during dormancy periods (hibernation and aestivation) and minimal (ca. –3 °C) during spring and autumn, in relation with the decrease of water mass and the increase of osmolality. Tc decreased in October to remain stable through late autumn and winter; it increased quickly with the awakening of animals in April. Snails with an epiphragm had a significantly higher ability to supercool (ca. )4.8 °C) than snails which did not form an epiphagm (ca. )4.2 °C). The animals’ size had a weak but significant influence on the realization of the Tc. It appeared that there was not a real cold-hardiness strategy in this population; rather a sum of parameters, varying in consequences of the external conditions and of the activity cycle, which are responsible for the enhancement of the supercooling ability during winter. Keywords Helix aspersa Æ Land snail Æ Cold hardiness Æ Temperature of crystallization Æ Haemolymph osmolality
Communicated by: G. Heldmaier A. Ansart (&) Æ J. Daguzan Universite´ de Rennes 1, UMR 6553 Ecobio, Equipe Ecophysiologie, Baˆtiment14, 263 Avenue du Ge´ne´ral Leclerc, CS 74205, 35042 Rennes Cedex, France E-mail: [email protected] Tel.: +33-2-23236865 Fax: +33-2-23235054 P. Vernon Universite´ de Rennes 1, UMR 6553 Ecobio, Station biologique, 35380 Paimpont, France
Abbreviations DM dry mass Æ INAs ice-nucleating agents Æ Tc temperature of crystallization Æ WC water content Æ WM water mass
Introduction During cold periods, ectotherms are confronted with the problem of subzero temperatures. Classically, two main strategies are described to survive such environmental conditions (see reviews, e.g. Sømme 1982; Zachariassen 1985; Duman et al. 1991; Block 1995; Storey and Storey 1997; Ramløv 2000). Freezing-tolerant animals are able to survive freezing of their body fluids below their temperature of crystallization (Tc), the temperature at which the body fluids spontaneously freeze. In contrast, freezing-intolerant species cannot bear ice formation and survive subzero temperatures by extending their supercooling ability, i.e. by decreasing their temperature of crystallization. Different mechanisms are implicated in both strategies, including synthesis of cryoprotectants of a colligative type such as sugars and polyols, or noncolligative antifreeze proteins, and ice-nucleating agents (INAs), which can be alimentary particles, microorganisms,
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