Modulation of digestive physiology and biochemistry in Mytilus californianus in response to feeding level acclimation and microhabitat

The intertidal mussel Mytilus californianus is a critical foundation species that is exposed to fluctuations in the environment along tidal- and wave-exposure gradients. We investigated feeding and digestion in mussels under laboratory conditions and across environmental gradients in the field. We assessed whether mussels adopt a rate-maximization (higher ingestion and lower assimilation) or a yield-maximization acquisition (lower ingestion and higher assimilation) strategy under laboratory conditions by measuring feeding physiology and digestive enzyme activities. We used digestive enzyme activity to define resource acquisition strategies in laboratory studies, then measured digestive enzyme activities in three microhabitats at the extreme ends of the tidal- and wave-exposure gradients within a stretch of shore (<20 m) projected sea-ward. Our laboratory results indicated that mussels benefit from a high assimilation efficiency when food concentration is low and have a low assimilation efficiency when food concentration is high. Additionally, enzyme activities of carbohydrases amylase, laminarinase and cellulase were elevated when food concentration was high. The protease trypsin, however, did not increase with increasing food concentration. In field conditions, low-shore mussels surprisingly did not have high enzyme activities. Rather, high-shore mussels exhibited higher cellulase activities than low-shore mussels. Similarly, trypsin activity in the high-shore-wave-sheltered microhabitat was higher than that in high-shore-wave-exposed. As expected, mussels experienced increasing thermal stress as a function of reduced submergence from low to high shore and shelter from wave-splash. Our findings suggest that mussels compensate for limited feeding opportunities and thermal stress by modulating digestive enzyme activities.