As the tide goes out, every mussel clinging to the shore clamps its shell closed in preparation for the rise in temperature as the sun begins beating down. But the conditions encountered by each individual can vary dramatically, even between neighbours hanging onto the same rock. ‘Individuals that are just centimetres apart can experience temperatures that are more different than individuals separated by hundreds of kilometres in latitude’, says Lani Gleason, from California State University, Sacramento, USA. But what impact might environmental variations within a matter of centimetres have on the animals that experience them directly? Knowing that the temperature that ectothermic (cold blooded) animals experienced in the recent past can radically alter their physiology, Gleason's colleagues, Wesley Dowd, from Washington State University, USA, and Luke Miller, from San Jose State University, USA, decided to compare the responses of mussels perched high up on the shore with those of animals that are rarely exposed to the air to find out how much their reactions differ.
After collecting mussels from the shore just beneath the Hopkins Marine Station, USA, Dowd and Miller allowed the animals to anchor themselves to individual acrylic plates. They then connected 30 individuals to a specialised circuit board to record each mussel's body temperature while also recording when they opened their shells to breathe and feed, before bolting the plates to the rocks at three different locations on the shore: just above the low tide line, in a tide pool and near the high tide line. ‘We knew that the ocean would eventually destroy just about anything we put in it’, laughs Dowd, who recalls getting up at 03:00, 03:30 and 04:00 h to maintain and service the monitors when they were exposed at low tide.
After 23 days, Dowd and Miller retrieved the battered equipment and the mussels to which they were attached to discover that the molluscs perched at the highest location on the shore suffered the widest range of body temperatures, differing by up to 14°C. And when the duo compared how long the mussels’ shells were open to breathe and feed, it varied between 45 and 70% in the two positions lower down the shore. ‘We were quite surprised at how much neighbouring mussels at the same tidal height varied in the amount of time they spent gaping [opening their shells] each day’, says Gleason. And when she investigated the physiological impact on each mollusc of its individual microclimate, it was clear that the mussels that got hotter suffered more DNA damage and produced more of the antioxidant catalase to counteract the damaging effects of oxygen at the higher temperatures. Meanwhile, Jacob Winnikoff, George Somero, Paul Yancey and Dylan Bratz discovered that the hottest mussels produced more of the stabilising molecule taurine – which is known to protect animals from high temperatures – even when they were adjacent to cooler mussels on the same acrylic plate. Explaining that these strong correlations were even more impressive because the mussels were in the real world, instead of being cosseted in the lab, Dowd adds, ‘This suggests that the underlying physiological mechanisms are pretty robust’.
So, mussels that are nestled up against each other have dramatically different experiences of the environment surrounding them: ‘Individual variation matters’, says Gleason. And this could have significant implications for how populations cope as temperatures continue rising. ‘We think that this additional level of complexity in the way the environment varies and in the way that individuals respond to that variation further complicates the prediction of climate change effects’, cautions Dowd.