The Arctic is impacted by some of the fastest temperature changes observed on Earth, but the impact on terrestrial arthropod fauna is unclear. Acute physiological thermal limits of terrestrial ectotherms from high latitudes often exceed the local air temperatures, suggesting that they may be able to cope with increasing temperatures. However, knowledge of how arctic terrestrial arthropods cope with elevated temperatures for longer periods is lacking. Here we investigate how acclimation temperature and exposure time affect the acute physiological heat tolerance of five terrestrial arthropod species (Neomolgus littoralis, Megaphorura arctica, Nysius groenlandicus, Psammotettix lividellus, and Nabis flavomarginatus) immediately after collection in arctic and sub-arctic habitats. We show that although acute heat tolerances are relatively high, even exposure to moderate (temperature span assessed ca. 3-29°C) acclimation temperatures for 24 hours have strong negative effects on heat tolerance for four of the five species. Similarly, exposure time negatively affected heat tolerance, depending on species and temperature. Together our results suggest that exposure to even moderately elevated temperatures for periods of 24 h or even shorter can lead to lower acute heat tolerance for cold-adapted terrestrial arthropod species from sub-Arctic and Arctic regions. Consequently, climate change leading to extended periods of mildly elevated temperatures may have strong negative effects on these species. We argue that this aspect is currently overlooked when assessing the ability of arthropods from Arctic and sub-Artic regions to cope with climate changes as such predictions are typically based on acute heat tolerance estimates and with the assumption of beneficial acclimation responses.

Microhabitat temperatures

Temperature loggers were placed at the sites where the animals were collected. At Ny Ålesund, two SmartButton loggers (SmartButton, ACR System Inc.) were placed in and on gravel at the beach to record the thermal environment (every 5 minutes) where individuals of N. littoralis were caught, and at a protected and an exposed tundra area, representative. At Narsarsuaq, air temperature and ground-level temperature at the collection site were continuously recorded (every 15 minutes) using TMS-4 dataloggers (TMS-4, TOMST, Czech Republic) to record the thermal environment in which the individuals were caught.

Thermal tolerance

Two assays were used to measure heat tolerance in the present study: 1) critical thermal maximum temperature (CTmax), where the specimens were exposed to gradually increasing temperatures and the temperature at which they entered heat coma was recorded, and 2) heat knockdown time (HKDT), where the specimens were exposed to a constant, stressful temperature, and the time until heat coma was recorded.

Thermal maximum temperature (CTmax)

CTmax was established for the Svalbard species (N. littoralis and M. arctica) using Elara 3.0 (IoTherm, Laramie, WY), a portable fully programmable heating/cooling anodized aluminum stage designed to maintain precision temperatures (Oyen et al., 2016). Individuals were individually placed in a 24-well plate drilled in 5mm acryl (each well had a diameter of 15 mm), with a bottom made of 1mm aluminum, and a lid made of 5mm acryl. The plate was subsequently placed on a thermoelectric plate of Elara 3.0. The plate was insulated from room air within a polystyrene box (16 (W)X 18.5 (L) X 19 (H) cm and 2.5 cm thick). Using a metal frame, a camera with an 8mm lens (Basler USB 3.0) (Basler Inc, Exton, PA) was mounted on top of the cooler allowing recording of the behaviour of animals during thermal exposure using the Basler camera Pylon Viewer (version 5.1.0.12681) software. The individuals were exposed to the following temperature profile: 10 min at 10°C and subsequently the temperature was increased by 0.2 ± 0.01°C min⁻¹. Heat tolerance estimates were based on manual observations of behaviour using video recordings and the time of the last movement was assumed to constitute their CTmax. During each run, a temperature logger (SmartButton, ACR System Inc.) was placed on the thermoelectric plate of the Elara 3.0 and used to calculate CTmax.

Heat knockdown time (HKDT)

For the Greenlandic species (N. groenlandicus, N. flavomarginatus, and P. lividellus), 5 ml vials containing field-collected individuals were mounted to a rack and submerged into a temperature-controlled water bath (PolyScience MX Immersion Circulator: MX-CA12E) maintained at 48.3°C for N. groenlandicus, at 44.4°C for N. flavomarginatus, and at 46.1°C for P. lividellus. The individuals were then observed and stimulated with flashes of light and gentle tapping on the vial caps with a metal rod. The time until movement ceased was recorded for each individual. We used heat knockdown temperatures that resulted in HKDT lower than 60 minutes for each species.

For Svalbard species (N. littoralis and M. arctica), HKDT was obtained using Elara 3.0 (IoTherm, Laramie, WY), where N. littoralis was exposed to a constant temperature of 40.5°C and M. arctica at 30.0°C. The heat tolerance estimates were based on manual observations of behaviour using video recordings and the time of the last movement to constitute their HKDT. During each run, a SmartButton temperature logger (SmartButton, ACR System Inc.) was placed on the thermoelectric plate of the Elara 3.0 to establish realised heat knockdown temperatures.