INTERTIDAL MUSSEL MICROCLIMATES - PREDICTING THE BODY-TEMPERATURE OF A SESSILE INVERTEBRATE

To elucidate the determinants of intertidal invertebrate body temperat ures during aerial exposure, I developed deterministic models using th e environmental inputs of solar radiation, air temperature, ground tem perature, and wind speed to predict the body temperatures of intertida l mussels (Mytilus spp.). Combined with field studies, these models we re 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 accura tely predicted the body temperatures of solitary mussels in the field to within similar to 1 degrees C. Steady-state simulations (using cons tant environmental conditions) predicted that, under conditions where evaporative water loss is limited, smaller (5 cm) mussels experience l ower body temperatures than larger (10 cm) mussels exposed to identica l environmental parameters. When evaporative cooling is limited only b y intolerance to desiccation, the trend in body size reversed due to a disproportionately greater amount of tissue (per unit length) in larg er mussels, which provides them with a greater reservoir of water avai lable for evaporative cooling. In both scenarios, larger mussels displ ay a greater ''thermal inertia'' (time constant of change), which buff ers them against rapid changes in environmental conditions. No one env ironmental factor controls body temperature, and thus measurements of single environmental parameters such as air temperature are very unlik ely to serve as accurate indicators of mussel body temperature. Result s of unsteady simulations (using fluctuating environmental conditions) further indicated a significant effect of the spectral characteristic s of the physical environment on body temperature. In many cases predi ctions of body temperature based only on daily means or extremes of en vironmental 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 he re are modified by the proximity of neighboring organisms, including p redators and competitors. During extreme environmental conditions (usi ng steady-state conditions), mussels living in beds are predicted to e xperience substantially lower (4 degrees-5 degrees C) body temperature s than those living in gaps. Furthermore, living within an aggregation also augments a mussel's thermal inertia, which dampens the effects o f rapid temporal changes in the physical environment. In contrast to m ost previous studies in rocky intertidal habitats, results thus sugges t that ''physical factors'' are not immutable boundaries imposed by th e 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.