Intertidal invertebrates are marine ectotherms that must regularly contend with a terrestrial environment, and the resulting fluctuations in body temperature can have profound impacts on organismal performance and survival. However, we know surprisingly little of what these temperatures are under normal field conditions. I developed deterministic models using environmental inputs to predict the body temperatures of intertidal mussels (_Mytilus_ spp.). Combined with field studies, these models were used to determine the effects of body size on body temperature, and to compare the heat budgets of mussels living as solitary individuals vs. those living in aggregations (1998 Ecological Monographs 68(1):51-74). On average the model predicted the body temperatures of solitary mussels in the field to within ~1*C. Steady-state simulations (using constant environmental conditions) predicted significant effects of body size and aggregation behavior on body temperature. No one environmental factor controlled body temperature, and thus measurements of single parameters such as air temperature are very unlikely to serve as accurate indicators of mussel body temperature. I then predicted the body temperatures of mussels during aerial exposure using historical climate data to estimate the body temperatures of mussel aggregations during a 'typical' climatological year (1999 Ecology 80(1): in press). 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 and perhaps even latitudinal differences in climatic conditions in determining body temperatures.

Like intertidal organisms, subtidal invertebrates must contend with the rigors of their physical environment. The reef-building coral Agaricia tenuifolia is one of the most common constituents of the barrier reef of Belize, Central America, and grows almost exclusively in aggregations. I quantified patterns in colony morphology, light levels and mainstream flow over a range of physical habitats (fore reef, patch reef and lagoon locations). Using physical scaling arguments, I measured water loss rates from scale models in air as proxies for gas flux to corals in water, and found that mass flux is significantly lower in aggregations with more tightly-spaced branches, and decreases with distance from the branch tips (1997 JEMBE 209:233-259). I further examined the effects of branch spacing on light and photosynthesis. Light levels decreased significantly with tighter branch spacing and with distance from the branch tips. Total cellular pigment concentrations, pigment ratios and zooxanthellar population densities all indicated very localized differences in photoacclimation within individual branches (1997 Marine Biology 130:1-10). These results show that a host's morphology can strongly determine the microhabitat of its symbionts over very small spatial scales, and that zooxanthellae can in turn display steep gradients in concordance with these altered physical conditions.