Intertidal mussel microclimates: Predicting the body temperature of a sessile invertebrate

To elucidate the determinants of intertidal invertebrate body temperatures during aerial exposure, I developed deterministic models using the environmental inputs of solar radiation, air temperature, ground temperature, and wind speed 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 (beds). On average, the model accurately predicted the body temperatures of solitary mussels in the field to within similar to 1 degrees C. Steady-state simulations (using constant environmental conditions) predicted that, under conditions where evaporative water loss is limited, smaller (5 cm) mussels experience lower body temperatures than larger (10 cm) mussels exposed to identical environmental parameters. When evaporative cooling is limited only by intolerance to desiccation, the trend in body size reversed due to a disproportionately greater amount of tissue (per unit length) in larger mussels, which provides them with a greater reservoir of water available for evaporative cooling. In both scenarios, larger mussels display a greater "thermal inertia" (time constant of change), which buffers them against rapid changes in environmental conditions. No one environmental factor controls body temperature, and thus measurements of single environmental parameters such as air temperature are very unlikely to serve as accurate indicators of mussel body temperature. Results of unsteady simulations (using fluctuating environmental conditions) further indicated a significant effect of the spectral characteristics of the physical environment on body temperature. In many cases predictions of body temperature based only on daily means or extremes of environmental parameters are off by 6 degrees C or more due to the time dependence of the system. Models of body temperature must therefore be based upon repeated measurements of multiple environmental parameters, rather than simple statistical measures such as daily mean, maximum, or range. Significantly, several parameters in the model presented here are modified by the proximity of neighboring organisms, including predators and competitors. During extreme environmental conditions (using steady-state conditions), mussels living in beds are predicted to experience substantially lower (4 degrees-5 degrees C) body temperatures than those living in gaps. Furthermore, living within an aggregation also augments a mussel's thermal inertia, which dampens the effects of rapid temporal changes in the physical environment. In contrast to most previous studies in rocky intertidal habitats, results thus suggest that "physical factors" are not immutable boundaries imposed by the environment, but may be significantly altered by the organism itself through its size, morphology, and interactions with neighbors, which may create feedback loops between abiotic and biotic controls.