Original article Cold hardiness in molluscs Armelle Ansart a, *, Philippe Vernon b a Université de Rennes 1, UMR 6553 Ecobio, Equipe Physiologie et Ecophysiologie, bat.14, 263, avenue du Général-Leclerc, CS 74205, 35042 Rennes cedex, France b Université de Rennes 1, UMR 6553 Ecobio, Station biologique, 35380 Paimpont, France Received 27 November 2002; accepted 6 February 2003 Abstract Molluscs inhabit all types of environments: seawater, intertidal zone, freshwater and land, and of course may have to deal with subzero temperatures. Ectotherm animals survive cold conditions by avoiding it by extensive supercooling (freezing avoidant species) or by bearing the freezing of their extracellular body fluids (freezing tolerant species). Although some studies on cold hardiness are available for intertidal molluscs, they are scarce for freshwater and terrestrial ones. Molluscs often exhibit intermediary levels of cold hardiness, with a moderate or low ability to supercool and a limited survival to the freezing of their tissues. Several factors could be involved: their dependence on water, their ability to enter dormancy, the probability of inoculative freezing in their environment, etc. Size is an important parameter in the development of cold hardiness abilities: it influences supercooling ability in land snails, which are rather freezing avoidant and survival to ice formation in intertidal organisms, which generally tolerate freezing. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Cold hardiness; Freezing tolerance; Freezing avoidance; Mollusc 1. Introduction Molluscs, even if largely marine, form one of the rare animal groups found in all environments: seas, intertidal zones, freshwaters and lands. Originated in the sea, molluscs have progressively invaded terrestrial habitats, as is shown by the succession of species on the rockyshore, from species bearing only short air exposure to species which are only rarely immerged. Terrestrial gastropods consist of the order Stylommatophora (Pulmonata) and certain genera of some prosobranch superfamilies, which constitute the “land oper- culates”. The only classes to have invaded freshwaters are the Gastropoda (Pulmonates Basommatophora and Proso- branchs) and the Bivalvia (Hunter, 1964). Molluscs are found even in the more contrasted environ- ments, in terms of salinity, humidity and temperature, from the Negev desert (Israël) (Heller and Dolev, 1994), to Green- land (Hunter, 1964). If marine molluscs are living in a well- buffered environment and are less threatened by freezing, temperature rarely being below the freezing point of seawa- ter (Murphy, 1983), intertidal and terrestrial molluscs are frequently exposed to freezing, as well as freshwater snails inhabiting temporary ponds or the margin of larger water bodies. Two alternatives exist for organisms living in a habitat that will freeze: move to an unfrozen habitat (either vertically or horizontally); or face freezing conditions. For this last cat- egory, two strategies are classically described: avoid freezing by extensive supercooling (freezing avoidance; see Fig. 1) or survive freezing of the body fluids (freezing tolerance). Although molluscs are largely distributed, relatively few investigations have been carried out in terms of their cold hardiness. Only intertidal molluscs have received good atten- tion. Results are scarce concerning freshwater and terrestrial species. Here is a review of these results, preceded by a short presentation of cold hardiness in ectotherm animals. 2. Cold hardiness principles Studies on cold hardiness in animals, understood as the resistance to subzero temperatures, mainly concern insect. Investigations have also been carried out on other arthropods such as millipedes, collembolan and acari, or on ectotherm vertebrates (frogs, lizards). * Corresponding author. E-mail address: armelle.ansart@univ-rennes1.fr (A. Ansart). Acta Oecologica 24 (2003) 95–102 www.elsevier.com/locate/actoec © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. DOI: 1 0 . 1 0 1 6 / S 1 1 4 6 - 6 0 9 X ( 0 3 ) 0 0 0 4 5 - 6