Coastal marine ecosystems are characterized by dynamic spatial and temporal variability of seawater pH. However, most studies assessing the biological impact of declining ocean pH (i.e., ocean acidification) ignore this aspect of the marine environment. As many marine ecosystems experience diel pH fluctuations, we developed a novel experimental system that mimics diel pH variability in order to study the impact of variable pH exposures on marine invertebrates. As a pH-sensitive species, larvae of the mussel Mytilus galloprovincialis were exposed to varying pH cycles over the course of early development. This developmental period is when the larvae construct their first shell.
Through a series of experiments, exposure to pH variability revealed two distinct and independent pH effects. First, low pH exposure during a specific 12-h developmental period caused an increase in frequency of abnormal larval development and shell structure. This pH-sensitivity was independent of the pH exposures of both preceding and later developmental stages and coincided with development of the chitin-based shell matrix. Second, larval shell growth was a function of mean pH exposure over the shell growing period. This study reveals pH-sensitivity over fine-scale developmental stages that are relevant to the timing of natural pH variability in coastal ecosystems. As ocean acidification progresses, short-lived biological processes could thus benefit or suffer from variable pH exposure and have downstream consequences for ecological processes.