Supercool or dehydrate? An experimental analysis of overwintering strategies in small permeable arctic invertebrates

Abstract

Soil invertebrate survival in freezing temperatures has generally been considered in the light of the physiological adaptations seen in surface living insects. These adaptations, notably the ability to supercool, have evolved in concert with surface invertebrates' ability to retain body water in a dry environment. However, most soil invertebrates are orders of magnitude less resistant to desiccation than these truly terrestrial insects, opening the possibility that the mechanisms involved in their cold-hardiness are also of a radically different nature. Permeable soil invertebrates dehydrate when exposed in frozen soil. This dehydration occurs because the water vapor pressure of supercooled water is higher than that of ice at the same temperature. The force of this vapor pressure difference is so large that even a few degrees of supercooling will result in substantial water loss, continuing until the vapor pressure of body fluids equals that of the surrounding ice. At this stage, the risk of tissue ice formation has been eliminated, and subzero survival is ensured. Here we show that these soil invertebrates do not base their winter survival on supercooling, as do many other ectotherms, but instead dehydrate and equilibrate their body-fluid melting point to the ambient temperature. They can achieve this equilibration even at the extreme cooling rates seen in polar soils.

Figure 1

Examples of differences in water potential (WP) between the body fluids of a supercooled animal and the surrounding ice at varying ambient temperature. The more intense supercooling becomes, the larger becomes the WP difference. A negative value of WP difference means that the organism loses water to the surrounding ice. The water potential (ψ) of body fluids at a given temperature has been calculated by using Van't Hoff's equation: ψ = Osm⋅R⋅T, where Osm is the osmolality of body fluids, R is the gas constant, and T is absolute temperature (°K). The MP of body fluids is calculated by application of the osmolal MP depression constant (−1.86°C osmol⁻¹ kg). The MP of a solution is defined by the vapor pressure of ice when there is equilibrium between ice and solution in the system. Thus, the “osmotic pressure of ice,” or water potential of ice, can be calculated by transforming the temperature of ice (ambient temperature) to osmolality and then using Van't Hoff's equation.

Figure 2

Body-fluid MPs of groups of three to four specimens of O. arcticus at decreasing temperatures when held in an atmosphere saturated with the vapor pressure of ice. Solid line, ambient temperature in the freezing cabinet; open circles, MP of individual groups of O. arcticus exposed over ice; closed circles, MP of individual groups of O. arcticus exposed over water at +1°C (control); dashed line, mean MP. Arrows at points indicate that the MP was lower than the range of the psychrometer, −7.5°C. In the time period of 4–12 days, the average MP decreased by 0.7°C day⁻¹ (linear regression, R² = −0.87; P < 0.0001).

Figure 3

Body-fluid MP depression in collembolans and earthworm cocoons during a natural temperature fall. Circles, MP of individual O. arcticus; triangles, MP of individual D. octaedra cocoons. The solid line indicates the temperature of the freezing cabinet. Points with MP values at −60°C indicate that freezing or melting was not observed even after exposure to −60°C.

Figure 4

Average MP of O. arcticus at decreasing temperatures when held in an atmosphere saturated with the vapor pressure of ice. Solid line, ambient temperature in the freezing cabinet; circles, average MP of O. arcticus body fluids. MPs are calculated by using data reported in Worland et al. (16).