Thermal Biology of Rocky Intertidal Mussels: Quantifying Body Temperatures Using Climatological Data

Despite numerous studies demonstrating the importance of body temperature to the fitness of intertidal invertebrates, surprisingly little is known of what these temperatures are under normal field conditions. Using environmental data collected at semi-exposed rocky intertidal beaches in the northeastern Pacific, I compared predictions of the body temperatures of mussels (Mytilus californianus) based on small-scale (microclimatelevel) environmental data averaged over a range of time scales (5 minutes to 1 hour) to those generated using continuous measurements. The error in predicting maximum and mean body temperatures introduced through the use of hourly environmental averages was generally less than 18–28C. I then predicted body temperatures of mussels during aerial exposure using historical, hourly weather data from federal meteorological databases. I used environmental data collected over a 30-yr (1961–1990) period on the Olympic Peninsula of Washington State to estimate the body temperatures of mussel aggregations at two tidal heights under conditions of aerial exposure during a ‘‘typical’’ climatological year. Estimated body temperatures commonly fluctuated by 208C or more during a period of #12 h and did not correlate well with air temperatures. The data indicated that mussels were, on average, hotter at this site during spring (April–June) than in summer (July– September) and colder in autumn (October–December) than in winter (January–March), primarily due to the effect of tidal cycles in determining exposure time. Results of simulations where the effects of climate and tidal cycle were decoupled suggested that the timing of aerial exposure, which can vary consistently over relatively small spatial scales (,100 km), can be more important than seasonal (spring vs. summer) differences in climatic conditions in determining body temperatures. Because of the interactive effects of climate and tidal cycle on body temperatures, populations of mussels in this region living at the same tidal height but separated by only tens of kilometers are predicted to experience very different thermal regimes. As a result, the intertidal environment represents a unique habitat for investigating the thermal biology of invertebrates over a range of spatial and temporal scales.