Sharks that hide in coral reefs may be safe from acidifying oceans

Atmospheric CO₂ levels have increased by almost 40% in the last 250 years, and the world's oceans have absorbed more than 30% of the additional CO₂. The resulting rise in seawater CO₂ and associated reduction in pH -- known as ocean acidification -- is a significant threat to marine organisms and ecosystems.

The epaulette shark was already known to be remarkably tolerant to short periods of hypoxia (low oxygen levels in the environment), as this species frequently hides deep within the crevices of coral reefs where oxygen levels can reach very low levels. Given these challenging microhabitats, epaulette sharks may be able to tolerate short periods of elevated CO₂ as well, but nothing was known about the species' response to prolonged exposure.

A team of researchers mainly based at James Cook University in Queensland, Australia, exposed epaulette sharks to either control (390 µatm), medium (600 µatm), or high (880 µatm) CO₂ treatments for 60-90 days and then measured key aspects of their respiratory physiology. They measured resting oxygen consumption rates to see if elevated CO₂ increased the sharks' basic maintenance costs. They also measured the sharks' sensitivity to low oxygen to see if exposure to elevated CO₂ affected this trait, as it may be important to their reef-dwelling lifestyle. Neither attribute was affected by the long-term exposure to the CO₂ treatments. So, the team investigated further by looking at blood parameters associated with uptake and delivery of oxygen as well as a metabolic enzyme responsible for energy production.

Dr. Jodie Rummer, corresponding author of the study, comments: "Our findings suggest this reef-inhabiting animal is indeed making some physiological adjustments to cope with elevated CO₂, and these may be linked to maintaining oxygen transport, energy, and balancing ions and pH, but at no obvious cost to the animal."

The authors emphasize the important link between environment, lifestyle, and physiological tolerance to changing environmental conditions. Dr. Rummer says, "investigating animals that are already experiencing challenging conditions in their environment may help us understand which species will fare well under future climate change conditions. Although the epaulette shark is not an apex predator, it plays an important role in balancing food webs and the overall health of coral reef ecosystems. The next obvious step is to examine predator species that live in the open ocean, as they may be more susceptible to future ocean acidification conditions."