Shark Survival in Acidic Waters

Sharks and their kin, skates and rays, have been on this planet for over 400 million years, surviving mass extinctions, fluctuating temperatures, and more. As tough as they are, sadly they’re one of the most vulnerable marine vertebrates today, with nearly a quarter of species threatened by extinction due to overfishing and habitat degradation. Scientist Jodie Rummer wondered if ocean acidification also poses a threat to these important predators.

“We know that sharks, even the small ones, play an important role in balancing ecosystems as predators,” she explains. “Healthy ecosystems need healthy predators. But what about when water quality becomes challenging, such as what is happening with climate change?”

Approximately 30% of the carbon dioxide, or CO₂, that we humans emit ends up in the oceans, causing ocean acidification. By the year 2100, scientists predict that if we don’t reduce our carbon emissions, the ocean's acidity will increase by 150% from pre-industrial times. For calcifying animals, such as corals and shellfish, the consequences are clear: more acidic waters limit their ability to produce shells. But what effect does ocean acidification have on sharks?

It depends on the species. Recent studies have demonstrated that some shark species are unable to respond to odor cues in water with elevated levels of CO₂, which could reduce their ability to forage for food. Other species experience decreased growth and metabolic efficiency in more acidic waters. Then there’s the epaulette shark (Hemiscyllium ocellatum).

Rummer and her team have found that these sharks are not affected physiologically—in terms of metabolic performance—or behaviorally—in terms of food finding—after prolonged exposure to high CO₂ conditions simulating ocean acidification. The scientists think the reason behind this is that these sharks are adapted to the fluctuating oxygen and CO₂ conditions in their hiding places on shallow coral reef platforms in the Great Barrier Reef.

In a recent study, the team looked at embryonic epaulette sharks, to see how they would fare. Like their adult shark counterparts, the embryos—in an egg case that provides little protection—fared just as well in highly acidic waters (adjusted to potential 2100 levels) as they did in current conditions. In testing two groups of shark eggs, Rummer’s group found no significant differences in growth, nutrient uptake, tail movement in the egg case, or survival after hatching. The team believes that the only higher risk of death under acidic conditions may be before the embryo’s gills are fully developed. Shark gills play an important role in helping correct acidic disturbances, but once the gills are fully developed, the sharks will likely survive as usual.

While this is great news overall for the species, ocean acidification is predicted to pose huge challenges for reef-building corals that provide such critical habitat and shelter for this small shark. “No matter how tough this species seems,” Rummer warns, “if climate change takes away its shelter, it will be just as vulnerable as any other.”

Image(s): C. Gervais and T. Nay, M. Johnson/James Cook University