The relationship between water content and cold tolerance in the arctic collembolan Onychiurus arcticus (Collembola: Onychiuridae)
The arctic collembolan Onychiurus arcticus is freezing intolerant and experiences temperatures below -25°C during winter periods of low air temperatures and only light snow cover. Summer-collected individuals have a mean (±SE) supercooling point of -6.1 ± 0.1°C. This study was designed to measure the desiccation resistance and subsequent recovery of O. arcticus from partial dehydration and relate these to its cold-hardiness in terms of changes in the supercooling point (SCP) and solute concentration. Drying curves measured with a recording microbalance showed two distinct phases characteristic of the loss of free and chemically bound (osmotically inactive) water. Rates of water loss at 0°C and low relative humidity (< 5%) were similar to those measured for Antarctic Collembola (5% h-1 of the initial total water content). O. arcticus survived losses of 40% of its total body water content and recovered within 36 h but could not survive losses of 50% of its original water content. Differential scanning calorimetry was used to investigate the nature of the body water, i.e. the proportion of freezable to unfreezable water and the nucleation temperature. The melt onset temperature correlated positively with the body water content. But no clear relationship was seen between the water content and the SCP, either because the springtails had low levels of cryoprotectants or because the ice nucleation activity was unaffected. However, long periods (7 months) at -2.5°C reduced the water content from 74 ± 10.1 to 43 ± 7.2% of fresh weight and lowered the SCP from -6.1 ± 2.1 to -15.5 ± 2.3°C. When given access to water these individuals re-gained their body weight within 24 h. During periods of desiccation water losses were attributed to the loss of freezable water with the unfreezable portion remaining almost constant at 16.5 ± 2.0%. It appears that O. arcticus may experience a reduction of body water during winter periods of sub-zero temperatures, which may lower its SCP and enhance its cold tolerance but that it can rapidly return to summer levels given access to free water during the spring melt.