Abstract
Ceratophysella sigillata (Collembola, Hypogastruridae) has a life cycle which may extend for >2 years in a temperate climate. It exists in two main morphs, a winter-active morph and a summer-dormant morph in central European forests. The winter-active morph often occurs in large aggregations, wandering on leaf litter and snow surfaces and climbing on tree trunks. The summer-dormant morph is found in the upper soil layers of the forest floor. The cryobiology of the two morphs, sampled from a population near Bern in Switzerland, was examined using Differential Scanning Calorimetry to elucidate the roles of body water and the cold tolerance of individual springtails. Mean (SD) live weights were 62 +/- 16 and 17 +/- 6 mug for winter and summer individuals, respectively. Winter-active springtails, which were two feeding instars older than summer-dormant individuals, were significantly heavier (by up to 4 times), but contained less water (48% of fresh weight [or 0.9 g g(-1) dry weight]) compared with summer-dormant animals (70% of fresh weight [or 2.5 g g(-1) dry weight]). Summer-dormant animals had a slightly greater supercooling capacity (mean (SD) -16 +/- 6degreesC) compared with winter-active individuals (-12 +/- 3degreesC), and they also contained significantly larger amounts of both total body water and osmotically inactive (unfrozen) water. In the summer morph, the unfrozen fraction was 26%, compared to 11% in the winter morph. The ratio of osmotically inactive to osmotically active (freezable) water was 1 : 1.7 (summer) and 1 : 3.3 (winter); thus unfrozen water constituted 59% of the total body water during summer compared with only 30% in winter. Small, but significant, levels of thermal hysteresis were detected in the winter-active morph (0.15degreesC) and in summer-dormant forms (0.05degreesC), which would not confer protection from freezing. However, the presence of antifreeze proteins may prevent ice crystal growth when feeding on algae with associated ice crystals during winter. It is hypothesised that in summer animals a small decrease in freezable water results in a large increase in haemolymph osmolality, thereby reducing the vapour pressure gradient between the springtail and the surrounding air. A similar decrease in freezable water in winter animals will not have such a large effect. The transfer of free water into the osmotically inactive state is a possible mechanism for increasing drought survival in the summer-dormant morph. The ecophysiological differences between the summer and winter forms of C. sigillata are discussed in relation to its population ecology and survival.
Highlights
Mass aggregations of insects and other arthropods occur rarely in winter, but some Collembola form colo nies, often in the coldest part of winter (e.g. Uchida & Fujita, 1968), which behave as superorganisms, moving over the forest floor as entities in a highly synchronised manner
The results enable a closer definition o f the survival strategy of this species to be made and they attempt to answer the question “Does a winter-active arthropod jeopardise its survival through its unusual life cycle?”
The ratio o f unfrozen : frozen water o f 1 : 3.3 in the winter-active morph o f C. sigillata is comparable to values found by W orland (1996) and Worland et al, (1998) for the Arctic springtail Onychiurus arcticus and for other, larger, cold-adapted arthropods (Block et al, 1998; W orland et al, 2000)
Summary
Mass aggregations of insects and other arthropods occur rarely in winter, but some Collembola form colo nies, often in the coldest part of winter (e.g. Uchida & Fujita, 1968), which behave as superorganisms, moving over the forest floor as entities in a highly synchronised manner. The springtail Ceratophysella sigillata (Collem bola, Hypogastruridae) is one such species in European forests (Fig. 1a). It emerges from beneath the snow cover (Fig. 1b), forming dark columns or fronts, which migrate across the snow surface and it feeds on algae on tree bark in great numbers (Fig. 1c). Few invertebrates exhibit such winter activity followed by a dormant state in summer Summer dormancy is triggered by temperature and under natural conditions it is obligatory
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