As the mercury rises and cold-adapted species are forced to move northward, their survival depends on their ability to adapt swiftly to changing environmental conditions. Peter Fields from Franklin & Marshall College, USA, and colleagues Marcus Zuzow and Lars Tomanek from California Polytechnic State University, USA, say, ‘In order to predict which organisms will be affected most by anthropogenic temperature increases, we need to better understand the mechanisms by which temperature affects the physiology of organisms, and specifically to identify the cellular processes that are most sensitive to acute and chronic heat stress.’ Explaining that cold-adapted endemic Mytilus trossulus (Californian) mussels are being gradually displaced from their southernmost Pacific coast territory by their more thermally tolerant – and invasive – cousins, Mytilus galloprovincialis, Fields and his colleagues decided to find out how both species respond at the cellular level to long-term heat stress (p. 1106).
Immersing mussels at 7, 13 and 19°C for 4 weeks, the team analysed the protein expression patterns in the mussels’ gills to identify differences in the two species’ physiological responses. They found that both species changed their cytoskeletal composition and energy metabolism protein expression in response to higher temperatures, and showed signs of stress at lower temperatures. However, the Californian mussels showed more signs of stress than their heat-tolerant cousins. The invasive M. galloprovincialis mussels also responded more strongly to cold acclimation than the Californian native, with M. galloprovincialis changing their protein expression patterns at 7 and 13°C while the M. trossulus expression patterns were almost the same at the two temperatures.
Comparing these new data with the results from experiments where the mussels were acutely stressed – simulating the experience of exposed mussels at low tide – the team says, ‘The combination of cold acclimation (or acclimatisation) followed by acute heat exposure could represent a particularly severe level of stress and be a major limiting factor in setting distribution ranges.’