Cold hardiness, adaptive strategies, and invasion of slugs of the genus Deroceras (Gastropoda, Pulmonata) in northeastern Asia

Abstract

Abstract-Four species of slugs have been identified in the vicinity of Magadan: Deroceras laeve, D. altaicum, D. reticulatum, and D. agreste. They exemplify three different life cycle schemes, with D. reticulatum and D. altaicum wintering at the egg phase; D. laeve, at the slug phase; and D. agreste, at either phase. The D. altaicum and D. reticulatum slugs and D. laeve eggs are intolerant of subzero temperatures. D. laeve’s tolerate freezing and survive at temperatures below −28°C. The eggs of other species, which lose up to 35% of water upon cooling, can withstand temperatures as low as −15 to −17°C (D. altaicum), −25°C (D. agreste), and −35°C (D. reticulatum). According to preliminary data, D. agreste slugs survive at temperatures down to −10°C. The almost ubiquitous distribution of D. laeve in regions with cool summers (including zonal tundras) is accounted for not so much by the high rate of ontogeny as by its significant cold hardiness and ability to winter at different phases of the life cycle (except for the egg phase), which allows the period of development to be prolonged for the next seasons. The last is confirmed by the fact that the slugs collected before and after hibernation proved to have identical patterns of distribution by body weight. Three species of slugs introduced in the vicinity of Magadan fail to spread inland. In the case of D. reticulatum, this is explained primarily by the fact that the frost-free season in inland areas is too short to allow these slugs to complete ontogeny and lay eggs. The barriers to their expansion appear to be insuperable, since this process remains unsuccessful over no less than 75–80 years.