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

Oxford University Press

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IMAGE: This image shows an epaulette shark in the wild. view more

Credit: M. Heupel

A study published online today in the journal Conservation Physiology has shown that the epaulette shark (Hemiscyllium ocellatum) displays physiological tolerance to elevated carbon dioxide (CO2) in its environment after being exposed to CO2 levels equivalent to those that are predicted for their natural habitats in the near future.

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."

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