Climate change could make sexier fish

Of all the impacts climate change could have on marine life, scientists have discovered that some fish adapt by developing bigger gonads to make more sperm and eggs.

Investing in reproduction could help ensure the survival of their lineage. But it only occurred in one of four species – common fish that have been identified as the most dominant and best adapted to ocean acidification from rising carbon dioxide (CO₂) levels.

“This suggests that generalist – ‘weedy’ – species that perform well under climate stress are most likely to prevail in a future ocean,” says Ivan Nagelkerken from the University of Adelaide, lead author of a paper published in the journal PLOS Biology.

That could have broader ramifications, he explains.

“So, for some species, climate change provides opportunities to expand their populations, but for a whole lot of other specialist or more sensitive species this might mean trouble. If some species start dominating future oceans this will reshuffle communities as we know them, and is likely to decrease biodiversity and ecosystems health.”

For their research, Nagelkerken’s team used natural laboratories at White Island, New Zealand, where volcanic activity releases CO₂ into sections of the seawater through vents, emulating what a future ocean could look like with climate change.

To explore the critical issue of reproduction, they used GoPro cameras to observe fish behaviour over three years at sites on the island’s north-eastern coast with normal and elevated CO₂ levels and collected samples to study the specimens’ physiology – including gonads, energy storage and body condition – in the lab.

The fish they studied were homebodies that hung around the same area – the common triplefin (Forsterygion lapillum), the more specialist blue-eyed and Yaldwin’s triplefins, and the crested blenny (Parablennius laticlavius).

They found that both males and females of the common triplefin were more abundant near vents with elevated CO₂ and had larger gonads compared to controls, but by using different trade-offs.

“We found males were eating more. They showed intensified foraging on prey – which was more abundant because of the increased biomass of algae that grows under the elevated CO₂,” says Nagelkerken.

“The females, on the other hand, did not eat more. They instead reduced their activity levels to preserve energy and then invested this in larger ovaries.”

The authors note that enhanced reproductive success on its own might not necessarily lead to greater species survival as young larvae are especially vulnerable to environmental disruption.

However, they say this could be complemented by greater food abundance, protection from predators and adaptation, as well as the parental protection that was observed in the current study.

“We conclude that energetic trade-offs by egg-producing females and increased energy budgets and abundances of male caregivers can together stimulate reproductive success of some species in a high CO₂ world.”