Abstract
ABSTRACTThe 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.
Highlights
The intertidal mussel Mytilus californianus aggregates to form dense beds along the western shores of North America
Its contribution to biogeochemical processes and trophic cascades of nearshore regions is proportional to its demography (Prins et al, 1997), which in turn is modified by environmentally sensitive physiological processes
Thermal stress leads to increased use of ATP in order to reassemble affected proteins (Lindquist and Craig, 1998)
Summary
Investigations of environmental-physiological interactions that potentially modify growth rate in Mytilus, such as the positive effects of resource acquisition (e.g. dietary composition and quality, ingestive processes, digestive strategies) (Bayne et al, 1987, 1988; Bracken et al, 2012; Dowd et al, 2013; Navarro and Winter, 1982; Riisgard and Randløv, 1981), as well as the negative effects of environmental stress (e.g. high body temperatures) (FitzgeraldDehoog et al, 2012; Jimenez et al, 2016; Schneider, 2008) can elucidate the mechanistic factors that potentially modify patterns of mussel distribution and abundance over intertidal landscapes
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