An ocellated wrasse collects algae for a nest.UNIVERSITY OF PALERMO, NATASCIA TAMBURELLOHow fish will respond as global carbon dioxide (CO2) emissions climb and the oceans become more acidic remains an open question. Previous studies have shown that some fish can show altered sensory behaviors in acidified waters. Now, for the first time, scientists have found that some fish also change their reproductive behaviors in CO2-rich, acidified waters. Their results appeared today (July 26) in Proceedings of the Royal Society B.

Researchers from the University of Plymouth, U.K., and the University of Palermo, Italy, studied the ocellated wrasse (Symphodus ocellatus), a Mediterranean, rocky subtidal fish that lives—among other places—near natural CO2 seeps off Italy’s Vulcano Island.

These fish have intricate reproductive behavior, with three kinds of males vying for access to a female’s eggs. There are dominant males (who build nests, court...

To investigate reproductive behavior, the researchers shot one 10-minute video for each of 18 nests in acidic waters and 14 nests in ambient waters. They observed that dominant males spawned with females alone (without sneaker males) less frequently in acidic conditions: these males had three spawns in highly acidic waters versus nine spawns in waters with ambient acidity. Spawning events involving other impinging males were equally frequent in both acidic and ambient waters.

While spawning was less frequent in more acidic conditions, the researchers noted that, from the dominant male’s perspective, it was more productive. By checking the offspring’s paternity, the scientists found that dominant males were able to fertilize a larger fraction of the eggs: from 38 percent in normal ocean waters to 58 percent in acidic ones.

“We had predicted that dominant males would lose out due to increased competition, but our genetic paternity tests showed that they did not. In fact, they fertilized more of the eggs than the other types of males and it was the sneaky males that lost out,” study coauthor Jason Hall-Spencer of Plymouth said in a statement. “Even though the sneakers produced more sperm and spawned on the eggs more often, they did not end up fertilizing more eggs.” The acidity might affect the sperm of distinct types of males differently, the authors wrote in the study.

Given the role of fish in food security and ecosystem stability, the results give a glimpse into the effects of future ocean acidification and warrant further investigation, study coauthor Marco Milazzo of Palermo said in the statement.

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