The intertidal sessile mussel Mytilus californianusaggregates to form dense beds along the eastern shores of the northern pacific. Mussel beds are ecologically important because they effectively remove phytoplankton from the near-shore water column and release waste materials that serve as nutrients to surrounding organisms. Tidal fluctuations pose a challenge to mussels because they can only acquire food during high-tide submergence and are subjected to temperature stress during low-tide aerial exposure. Submergence time decreases from low to high shore-positions (tidal-gradient), and wave splash decreases from wave-exposed to wave-sheltered-positions of the shore-line (wave-exposure-gradient). Hence, the intertidal zone is a mosaic of dynamic microhabitats that affect feeding intervals and intensity of temperature stress of its inhabitants. To understand the strategies that mussels use to acquire nutrients in the face of costs associated with daily thermal stress, we measured digestive enzyme activity within the digestive gland and the mRNA levels of heat-shock protein 70 under field conditions. We found that mussels high on the shore appear to partially compensate for costs associated with temperature stress by up-regulating trypsin and cellulase activities. To test whether these activity patterns are driven mostly by temperature and aerial exposure, we subjected mussels to a benign (no thermal stress) tidal regime and compared their digestive enzyme activities to mussels acclimated to a daily-thermal-stress tidal environment in the laboratory.
Results/Conclusions
The results revealed robust biological patterns, suggestive that mussels have an acutely flexible digestive system that is tuned to the environment. For example, the activity of amylase, which breaks down starch, was negatively affected by thermal stress, while cellulase activity, which breaks down plant material and detritus, was consistently elevated between treatment groups. Therefore, mussels may perform nutrient-balancing activities as a function of microhabitat parameters which in turn is modulated by shore position. These data will be discussed in the context of the functional complexity of the mussel-digestive gland. Lastly, a conceptual model will be presented that describes the relationships between food resources, shore position, digestion, and growth in M. californianus.