Within-site variation of growth rates and terminal sizes in Mytilus californianus along wave exposure and tidal gradients.

Mytilus californianus is a foundation species of rocky shores of western North America. Its dominance depends on rapid growth to large sizes, which confers an advantage in size-dependent species interactions. Initial rates of growth and final (terminal) sizes of the mussels depend on environmental factors. Prior comparisons of growth made over large spatial scales (tens of meters to hundreds of kilometers) indicate that temperature, submergence time, and wave exposure affect growth. However, there are few studies quantifying variation in temperature, wave force, and mussel growth parameters at small scales within local populations-that is, meter-level increments. Such measures are necessary to better understand the consequences of the complex spatial mosaic of physical factors in the intertidal zone. We measured variation in temperature, wave force, size-specific shell growth, and terminal size at 3-4-m intervals along horizontal contours within two mussel beds separated by 15 ds of latitude. Both mussel beds showed the same general trends: growth rates attenuated along gradual clines from low and wave-exposed to high shore and sheltered. For example, young adults from low and wave-exposed microhabitats grew 9- and 6-fold higher than those from high-shore-wave-sheltered points. While higher flow may promote growth by enhancing feeding, it also appears to exert a positive effect by moderating energetically costly temperature stress. Consistent with the growth rate findings, cumulative degree-hours explained 83% and 69% of the variation of terminal sizes in regressions for the two locations.
(© 2015 Marine Biological Laboratory.)