High-shore mussels, Mytilus californianus, have larger muscle fibers with lower aerobic capacities than low-shore conspecifics

Abstract

The rocky intertidal zone is a dynamic and stressful habitat, dominated by strong vertical gradients of emersion time. Marine organisms that settle high on the shore are exposed to greater temperature stress, increased desiccation risk, and reduced access to their suspended food compared with their low-shore neighbors. The ribbed mussel Mytilus californianus is one such species that occupies a broad vertical intertidal range. The present study asked whether high-shore mussels might conserve energy by employing larger, energetically “cheaper” adductor muscle fibers. This conjecture is based on the optimal fiber size hypothesis, which posits in part that the reduction in surface area (i.e., cell membrane area) to volume ratio of large-diameter muscle fibers also reduces the ATP costs of maintaining resting ion gradients across the sarcolemmal membrane. The evidence suggests that high-shore mussels collected in Pacific Grove, California, USA, in 2015 do have fibers with larger cross-sectional areas when controlling for body mass. This morphological trait is paired with a lower mass-specific activity of the aerobic enzyme citrate synthase in adductor muscle. A common-garden treatment revealed that enzyme activity, but not muscle fiber area, changed relatively quickly (4\xa0weeks). Previous studies have supported the optimal fiber size hypothesis by documenting hypertrophic growth of muscle fibers during ontogeny, but few studies have examined whether fiber sizes in ectotherms might be adjusted based on an individual’s environmental context. Although a definitive link to organismal costs remains to be demonstrated, this mechanism may mitigate some of the costs for organisms living in energy-constrained habitats.