Thermal tolerance characteristics of larvae of the Magellanic intertidal midge, Telmatogeton magellanicus (Diptera: Chironomidae)

Abstract Insects are found in nearly every biome on Earth, but relatively few have colonized intertidal environments, which are characterized by dynamic fluctuations in abiotic conditions, including temperature. In this study, we examined thermal tolerance of the larvae of the Magellanic midge, Telmatogeton magellanicus, an intertidal flightless chironomid native to Tierra del Fuego and the southern Chilean island archipelagoes extending to Cape Horn and including the Magellanic Sub‐Antarctic Ecoregion. Although little is known about the thermal physiology of this species, many polar and temperate species of intertidal invertebrates can survive internal ice formation. Thus, we investigated whether specimens collected in the austral summer were freeze tolerant by assessing their ability to tolerate subzero exposures when inoculated in frozen seawater. Larvae of T. magellanicus collected in late March 2023 readily tolerated exposure to −2.0°C for 2 h, regardless of whether the seawater containing the samples had frozen or remained unfrozen. However, at a lower temperature of −3.9°C, larval survival reduced significantly from 98.5% (95% CI: 92.1–100%) in supercooled water to 32.1 ± 6.2% (20.3–46.0%) in frozen seawater. Together, our results suggest that their cold tolerance is likely promoted primarily through supercooling, but larvae of T. magellanicus also appear to exhibit a weak level of freeze tolerance. Additionally, larvae readily survived a heat exposure to 30°C for 2 h, though their survival progressively decreased at higher temperatures, reaching 0% at 35°C. This level of heat tolerance is consistent with typical temperatures likely experienced for short periods during the austral summer in their native distribution. Southern South America is ~1000 km from maritime Antarctica, and tectonically these land masses were connected prior to the break‐up of Gondwana. Although T. magellanicus is not phylogenetically considered their closest relative, our results may inform future comparative stress‐tolerant studies with chironomid species from Antarctica and the core sub‐Antarctic, helping to better understand the biogeography and evolutionary history of extreme‐adapted insects.