Stenotherms at sub-zero temperatures: thermal dependence of swimming performance in Antarctic fish

We examined the burst swimming performance of two Antarctic fishes, Tremato mus bernacchii and T. centronotus, at five temperatures between -1 degreesC and 10 degreesC. As Antarctic fishes are considered one of the most cold s pecialised and stenothermal of all ectotherms, we predicted they would poss ess a narrow thermal performance breadth for burst swimming and a correlati ve decrease in performance at high temperatures. Burst swimming was assesse d by videotaping swimming sequences with a 50-Hz video camera and analysing the sequences frame-by-frame to determine maximum velocity, the distance m oved throughout the initial 200 ms, and the time taken to reach maximum vel ocity. In contrast to our prediction, we found both species possessed a wid e thermal performance breadth for burst swimming. Although maximum swimming velocity for both T. bernacchii and T. centronotus was significantly highe st at 6 degreesC, maximum velocity at ah other test temperatures was less t han 20% lower. Thus, it appears that specialisation to a highly stable and cold environment is not necessarily associated with a narrow thermal perfor mance breadth for burst swimming in Antarctic fish. We also examined the ab ility of the Antarctic fish Pagothenia borchgrevinki to acclimate their bur st-swimming performance to different temperatures. We exposed P, borchgrevi nki to either -1 degreesC or 4 degreesC for 4 weeks and tested their burst- swimming performance at four temperatures between -1 degreesC and 10 degree sC. Burst-swimming performance of Pagothenia borchgrevinki was unaffected b y exposure to either -1 degreesC or 4 degreesC for 4 weeks. Maximum swimmin g velocity of both acclimation groups was thermally independent over the to tal temperature range of -1 degreesC to 10 degreesC. Therefore, the loss of any capacity to restructure the phenotype and an inability to thermally ac climate swimming performance appears to be associated with inhabiting a hig hly stable thermal environment.