Ontogenetic shift in stress tolerance thresholds of Mytilus trossulus: effects of desiccation and heat on juvenile mortality

We examined the sensitivity of newly settled Mytilus trossulus to heat and desiccation, as well as the ontogeny of sensitivity through the early benthic phase. Laboratory experiments were conducted to determine the sensitivity of mussels to the full range of temperatures and desiccation levels experienced in the field. Mussels of 1 to 2 mm shell length (SL) experienced a threshold of heat tolerance at 34 degrees C and a threshold of desiccation tolerance at vapour pressure deficit levels of 1.01 kPa. Extended periods of temperatures reaching or exceeding lethal levels for newly settled M. trossulus occurred relatively rarely in Barkley Sound, British Columbia, Canada, whereas lethal levels of desiccation occurred often during the recruitment season and were usually sustained for several hours. Desiccation, therefore, appears to be a substantially greater threat to recently settled M. trossulus than heat. A final laboratory experiment characterized the changes in sensitivity to desiccation that occur as mussels increase in size. Mussels became highly tolerant to desiccation when they reached a size of 2 to 3 mm SL. This size closely corresponds to the size at which juvenile M. trossulus relocate from protective filamentous algal habitat to adult habitat, suggesting ontogenetic shifts in habitat use by juvenile M. trossulus are a response to changing sensitivity to desiccation. If so, the future survival of newly settled mussels, and thus possibly the local persistence of mussel populations, may depend upon the persistence of protective algal microhabitats under changing climate conditions.