The physiological and behavioural response of Mytilus mussels to salinity in a changing ocean

Abstract

Mytilus mussels are a widely distributed group which plays an important role in the structural development and maintenance of ecosystems, and represents an important aquaculture species. They live in both coastal and estuarine environments where they are exposed to wide ranging environmental conditions, particularly salinity fluctuations. They also form hybrid zones in the areas where species overlap, and in the Southwest of the UK there is a hybrid zone between Mytilus edulis and Mytilus galloprovincialis. Following an evaluation of current literature in chapter one, chapter two aimed to examine the physiological response of estuarine and coastal mussels from the Southwest hybrid zone to acute salinity changes, in addition to comparing the genetic differences between sites. Mussels exposed to salinity declines of 27 or 19 ppt had significantly lower metabolic rates than those exposed to 35 ppt, but there was no difference in metabolic rate between mussels collected from adjacent coastal and estuarine sites. Genetic analysis using a novel custom designed 60K mussel SNP array subsequently showed that all three sites had mainly M. edulis ancestry, although the upper estuarine site had a M. galloprovincialis frequency greater than that of the coastal site, contrary to previous analysis for this area. In chapter 3, valve movements of M. edulis were measured in response to different rates of salinity decline, using a custom-built hall sensor-based gape system. Mussels adducted their valves when external salinity reached 21 ppt in all rates of salinity change, but the salinity of retained pallial fluid was greater for faster rates, and was always higher than the salinity at which valves closed. Additionally, heart rate and metabolic rate were lower under declined salinities, with heart rate closely linked to gape, demonstrating periods of significantly reduced heart rate coinciding with periods of valve closure. This study is the first to simultaneously measure physiology and gaping behaviour of mussels in response to salinity changes, thus paving the way for future experiments on this subject. Climate change is predicted to cause an increase in flash flooding and ocean freshening, and therefore these results have significant implications for predicting the future ranges of Mytilus species.