Recent research shows at least one small shark species will be able to cope with the level of ocean acidification predicted for the end of this century.
Dr. Jodie Rummer from the ARC Centre of Excellence for Coral Reef Studies (CoralCoE) at James Cook University (JCU) and her students and colleagues have been studying the epaulette shark Hemiscyllium ocellatum for several years. They have recently been investigating epaulette embryos as they develop in their eggs.
Epaulettes live in shallow coral reefs and intertidal pools that experience large daily fluctuations in oxygen, carbon dioxide levels, and temperatures. This video shows their ability to ‘walk’ using strong pectoral and pelvic fins:
After their eggs are fertilized internally, the females lay them on the sea bed and reef flats.
Before they hatch, epaulette eggs normally incubate on the reefs for 3–4 months. Over the course of the study at CoralCoE, Rummer’s team raised epaulette shark eggs from 10 days after they were laid until 30 days after they hatched. Because the eggs can be transparent if a light is shone behind them (termed ‘candling’), the researchers were able to count gill and tail movements while the embryos were developing. They also measured how much yolk the embryo was consuming and how much the embryo was growing.
Upon hatching, they monitored survival and growth. Surprisingly, they found no differences between growth and survival in sharks reared under current-day conditions when compared with those reared under ocean acidification conditions predicted for the year 2100. See Johnson et al., 2016.
Shark gills play an important role in helping correct pH disturbances. The research team thinks that the risk of death under ocean acidification conditions may be highest in the early embryo stage, before the gills are fully developed. Those that get past this stage will be able to carry on with business as usual.
Other research from Rummer’s team shows that elevated carbon dioxide (ocean acidification) does not affect the metabolic performance of adult epaulettes (Heinrich et al., 2014) or their sheltering and foraging behaviours (Heinrich et al., 2015). However, a 4-degree increase in water temperature during development is likely to interfere with their colour and camouflage patterns. This could impact their ability to hide in the reef, attract mates and deter predators. (Gervais et al., 2016)
Although epaulettes are regarded as a “low risk” species in the context of predicted climate change, they are far from being “no risk”. They are small, vulnerable and adapted to living among coral and in the crevices of reef flats. Ocean warming and acidification pose huge challenges for reef-building corals. If the reefs don’t do well with climate change and cease to provide hiding places, or if epaulettes lose their camouflage, they will be at risk, no matter how physiologically tolerant they are.
Sharks, skates, and rays (elasmobranchii) have existed for over 400 million years. Beyond simply the large size of some elasmobranchii species, other characteristics that have aided their evolutionary success may continue to help them adapt under future climate change scenarios. See discussion in Heinrich et al., 2014. Rummer predicts that some will be winners and some will be losers as they endeavour to keep pace with the rate at which we are changing the planet.