Photo: Getty

Thanks to carbon dioxide emissions, Earth’s seas are getting both warmer and more acidic. Until now, the consequences of ocean acidification were thought to be mostly limited to marine animals that make hard, calcium carbonate shells, but new research suggests that fish will feel the impact too. Acidic conditions can screw with how their skeletons develop, which could have ramifications for everything about their fishy lives.

As an ever-increasing concentration of carbon dioxide in our atmosphere bleeds into the oceans, the resulting chemical reaction is lowering the water’s pH. This is already hurting oysters and other shellfish, and eating away at coral reefs. Recently, scientists have also determined that acidic seas can blunt the sensitive senses of smell of some fish, making it difficult for them to sniff out prey or to avoid becoming a meal.

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But no one had investigated whether the internal skeletons of fish might be altered by acidification in a manner similar to what’s happening to the shells and exoskeletons of other animals. To find answers, Valentina Di Santo, an ecophysiologist at Harvard University turned to the little skate (Leucoraja erinacea), a small, flattened relative of sharks. Like their shark and ray cousins, skates have a skeleton made mostly of cartilage instead of bones mineralized with calcium phosphate. But “elasmobranchs” like skates also have a layer of highly-mineralized tiles on their skeletons that could potentially be susceptible to the ravages of acidification.

A swimming little skate.
Photo: Valentina Di Santo

Di Santo took newly-laid skate egg cases and placed them in a tank setup under one of four different treatments that either had a modern or hotter, “future” temperature (as forecast for the year 2100) as well as a current or more acidic pH. She let the embryos develop over the next several months, and as soon as the baby skates hatched, Di Santo scanned their skeletons using a special x-ray technique.

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So, did exposure to an acidic ocean corrode the baby skates’ mineralized cartilage? Not at all. In fact, as described in the paper published today in Proceedings B of the Royal Society, the skeleton actually got denser and more mineralized.

“This is a surprising result because most studies on invertebrates show a decrease in mineralization of the exoskeleton with acidification,” Di Santo told Earther.

Di Santo speculated the fish may be beefing up their skeletons as a consequence of other physiological processes related to dealing with the acidic conditions. The skates may be buffering their bodies by increasing the phosphate in their blood and other tissues in an attempt to keep their pH at equilibrium. And pumping the body full of phosphate might boost the calcium phosphate mineralization of the skeleton. However, more research will be needed to determine if this is actually what’s going on.

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Things got even weirder when Di Santo accounted for higher future temperatures alongside acidification. The skates’ broad fins were particularly susceptible to these combined effects, becoming less mineralized. Di Santo thinks this may be because baby skates grow faster width-wise in warmer water, and the mineralization process might not be able to keep up with skeleton growth.

Underside of a little skate
Photo: Valentina Di Santo

A skeleton that’s denser in some places and less dense in others will likely have profound impacts on how affected fish can get around. That’s particularly true for fish without a swim bladder to help them float, including skates, stingrays, and sharks.

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“I expect costs of swimming to increase dramatically for these fishes, thus impacting potential migrations to more favorable conditions,” Di Santo said.

Meanwhile, a loss of skeletal rigidity in the wing can also present problems for swimming skates. All of this is compounded by the expectation that bottom-dwelling fish like skates will seek cooler, deeper waters as oceans heat up, pushing them right into depths that generally have higher carbon dioxide concentrations, and thus lower pH.

Di Santo said a key next step will be quantifying just how much more difficult swimming is for fish when faced with warming and acidification. Given how important efficient swimming is for saving energy, acquiring food, and escaping predators, encumbered fish may suffer serious ecological repercussions.

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And of course, more research is needed to figure out just how much other fish will have their skeletons modified by acidification. It’s not clear what the impact will be on fish with bony skeletons, for example, or fish that swim and keep buoyant with the help of a swim bladder (unlike skates).

But whatever the magnitude of acidification’s impact on fish, the realization that fish skeletons can be just as susceptible to acid warping as the shells of mollusks and corals is definitely eyebrow raising. Even if acidification is only tweaking the skeletons in a fraction of species, that could still amount to a global fiddling with the physiological dials on animals that support entire ocean food webs.

Jake Buehler is a science writer living on Washington’s Olympic Peninsula with an adoration for the Tree of Life’s weird, wild, and unsung. Follow him on Twitter or at his blog.

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