Ocean Currents

Are Sharks Really as Tough as They Seem?

To help us celebrate Shark Week, Sarah discusses surprising new research indicating the negative impact acidification is having on a variety of shark species

Lorenzo Ragazzi
© Lorenzo Ragazzi

Imagine a tropical shark, effortlessly cruising through clear blue water in search of her dinner. Smaller fish flee or try to sneak by unnoticed by this tough top predator. But is this shark really as tough as she seems? Surprising new research shows that ocean acidification, invisible and silent, has the ability to harm many different species of shark by directly affecting body processes.

Like many animals, sharks are most sensitive to ocean acidification and other environmental changes when they are at their youngest. The good news? Sharks that develop in eggs (oviparous species) don’t show signs of decreased embryo survival or slower development under ocean acidification (OA), the way shellfish larvae do. There haven’t been any studies yet on whether ocean acidification changes how live-born shark species (viviparous) develop as embryos. But the outlook is darker for juvenile sharks.

Juvenile sharks have showed an array of responses to ocean acidification, including changes in metabolism, aerobic capacity, and blood chemistry. On one hand, this could mean that sharks, because they are complex ocean animals, can guard against internal changes driven by ocean acidification in ways that simpler animals like shellfish and plankton cannot. On the other hand, this could signal broader changes. Sharks and other top predators are like racecars that need to be precisely tuned for highest performance. With slightly decreased metabolism or aerobic scope, sharks’ speedy hunting and long-distance migration could suffer. It’s difficult to draw conclusions because only less active sharks (buccal ventilators) have been studied regarding OA, but more active sharks (ram ventilators) depend on swimming to fully ventilate their gills.

Lorenzo Ragazzi2
© Lorenzo Ragazzi
Here’s where it gets weird: Sharks depend heavily on their sense of smell and electroreception to navigate and hunt prey. But ocean acidification reduces the sense of smell in several shark species, increasing the amount of time these sharks take to find their prey. One shark species normally found in a shallow reef habitat didn’t show these behavioral changes, possibly because it can tolerate more environmental variability than other species.

There’s still a lot of work to do before we can predict how all sharks will respond to ocean acidification. There haven’t been many studies yet on how ocean acidification affects sharks. And just a few species have been studied, all of them more sedentary species.  Bony fishes, which have some biological similarities to sharks, show changes in brain chemistry under ocean acidification, but we don’t know yet whether sharks show the same behavior. We also know that sharks living among coral reefs could suffer from losing that habitat if ocean acidification and warming degrade reef habitats—while that’s not a direct impact on sharks, it can certainly affect their prospects for a healthy and happy life as well.

What can we do? Certainly more research is needed. And solving ocean acidification in the open ocean depends on cutting atmospheric CO2 levels. But in the meantime, we can make sure that sharks don’t experience other stressors that could decrease their tolerance for environmental changes like ocean acidification, warming, and ocean oxygen loss. As rugged as many sharks seem, they suffer from climate change too.

Federal research funding can help deepen our scientific understanding of this problem. Write to your members of Congress today—urging them to fully fund ocean acidification research projects.

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