Pre-adapted to the maritime Antarctic? – Rapid cold hardening of the midge, Eretmoptera murphyi

During the 1960s, the midge, Eretmoptera murphyi, was transferred from sub-Antarctic South Georgia (55 degrees S 37 degrees W) where it is endemic to a single location on maritime Antarctic Signy Island (60 degrees S 45 degrees W). Its distribution has since expanded considerably, suggesting that it is pre-adapted to the more severe conditions further south. To test one aspect of the level of its pre-adaptation, the rapid cold hardening (RCH) response in this species was investigated. When juvenile (L1-L2) and mature (L3-L4) larvae of E. murphyi were directly exposed to progressively lower temperatures for 8 h, they exhibited Discriminating Temperatures (DTemp, temperature at which there is 10-20% survival of exposed individuals) of -11.5 and -12.5 degrees C, respectively. The mean SCP was above -7.5 degrees C in both larval groups, confirming the finding of previous studies that this species is freeze-tolerant. Following gradual cooling (0.2 degrees C min(-1)), survival was significantly greater at the DTemp in both larval groups. The response was strong, lowering the lower lethal temperature (LLT) by up to 6.5 degrees C and maintaining survival above 80% for at least 22 h at the DTemp. RCH was also exhibited during the cooling phase of an ecologically relevant thermoperiodic cycle (+4 degrees C to -3 degrees C). Mechanistically, the response did not affect freezing, with no alteration in the supercooling point (SCP) found following gradual cooling, and was not induced while the organism was in a frozen state. These results are discussed in light of E. murphyi's pre-adaptation to conditions on Signy Island and its potential to colonize regions further south in the maritime Antarctic.