High and dry oysters at most risk from climate change

While it may be fashionable in some circles to doubt the provenance of climate change, there is no doubt that our oceans are warming and becoming more acidic. And even though a pH drop of 0.1 units may not sound that drastic, the reality is that ocean acidity has increased by 26% since the beginning of the Industrial Revolution, and the change is only going to accelerate. Although many fish appear to be able to tolerate the acidity shift physically, it is not clear how molluscs that cling to life on the wave-battered shoreline will cope. Elliot Scanes from the University of Sydney, Australia, explains that oysters that are routinely exposed to the air as the tide recedes may be better prepared for more acidic conditions than shore-mates that live beneath the tide: when the tide goes out, bivalves exposed above the waves close their shells, which restricts water flow over the gills and leads to a natural accumulation of CO₂ – and acidification – of their tissues. Could this regular exposure to more acidic conditions leave high-and-dry Sydney rock oysters better prepared for future climate change than their perpetually immersed cousins, or could the additional stress tip them over the edge?

Scanes, Laura Parker and Laura Stapp headed by boat along the shore at Port Stephens, Australia, to prise free clumps of oysters before testing their resilience in Wayne O'Connor's laboratory at the Port Stephens Fisheries Institute. ‘It was difficult to set up the experimental system; we needed to try and recreate a natural tidal cycle in the laboratory’, says Scanes, who teamed up with Pauline Ross to design a system of pumps and timers to pump seawater into the laboratory tanks twice a day over 3 weeks to simulate the natural tidal cycle that oysters high up on the shore would experience in current (390 μatm CO₂) and future climate scenarios (1000 μatm CO₂). Then, the pressure was on to measure the effects of the different tidal and CO₂ regimes on the molluscs’ ability to maintain their delicate pH balance, their metabolic rate and growth: ‘We had to make sure that oysters weren't out of the water for longer than necessary, because this would have affected the results’, says Scanes.

Worryingly, oysters that had been exposed to the air each day for two 9 h periods in the high CO₂ conditions (to simulate the experience of molluscs at the upper end of the tidal range at the end of the century) had significantly higher metabolic rates than the molluscs that had remained immersed. Meanwhile, the concentration of CO₂ in their tissues was higher and the pH of their tissues had fallen an enormous 0.35 pH units. In addition, the molluscs that lived higher up the shore were in poorer shape and had grown more slowly than the oysters that were living below the tideline.

‘Ocean acidification is likely to make life more difficult for oysters high on the shore and in the future there are likely to be fewer oysters on the rocky shore’, says Scanes. And the loss could have implications for other species clinging to life in the turbulent tidal zone. ‘Oysters are one of the most important habitat-forming animals on the rocky shores of Australia’, says Scanes, explaining that the layers of shell built up by subsequent generations of the filter feeders provide a rich environment for other coastal species, in addition to the molluscs cleansing the water. ‘Fewer oysters means less habitat for other organisms’, warns Scanes.