In the oceans of the future, some marine species may be, in a different way, big fish in a small pond.
As humans continue to emit carbon dioxide, much of it ends up in the ocean; in 2017, the world’s oceans absorbed 2.6 billion metric tons of carbon, a 36% increase on the previous average. The acidity the extra CO2 creates in the water has consistently proven to be deleterious to the ocean ecosystem, destroying food sources and habitats, and is likely leading to the loss of some species altogether. But scientists have found that some fish are capable of prospering in acidic water and are likely to flourish in future oceans. That’s largely because the CO2 levels are causing their sexual organs to balloon.
In a new study, scientists investigated the common triplefin, a small fish of between four and eight centimeters in length that subsists on a diet of small crustaceans, such as little crabs, snails, and shrimp. Increased carbon dioxide can act as a fertilizer for some sea plants such as turf algae—the weeds of the deep—which provide good cover for their minuscule prey, resulting in more food for the triplefin. As they’re eating more and expending the same effort, they’re conserving energy, which their bodies invest in gonad growth. “The surplus energy from increased feeding supported heavier gonads,” the study states. Reproduction is “energetically costly,” so it’s ideal for a species that only lives about three years to take advantage of the extra libido.
The University of Adelaide research team compared two populations of common triplefin on the coast of New Zealand: one in waters with a normal pH, and one in naturally occurring low pH levels, where volcanic activity has released carbon dioxide. They recorded the organisms’ daily activities with GoPro cameras, then watched the footage to find the frequency of certain behaviors, such as foraging. Then they caught some of the fish and took measurements: liver weight as a gauge of energy levels, and gonad weight, a good proxy for assessing how much energy they’re putting into reproduction. “You’re not going to invest more into bigger gonads, and then fewer eggs are expelled,” says Ivan Nagelkerken, a professor of marine ecology and the study’s lead author. “That would be a very contradictory finding.”
They found that both male and female genitalia were larger in zones with higher CO2 levels. The males had eaten more, because of the higher algae biomass, taking in more energy. While the females had not eaten more, they’d reduced activity to conserve more energy and invest it into growing larger ovaries.
The triplefin are able to adapt to these changes because of their unpicky feeding habits, Nagelkerken says. Like rats, pigeons, and cockroaches on land, they can adjust to eating different foodstuffs, unlike picky specialist eaters (such as koalas, which rely solely on eucalyptus). Triplefin are also unfussy about their shelter, so the algae—like a lawn on the seabed—suits them. That’s not true for other marine species, which can’t adapt to the shallow algae. A lot of these other fish are natural predators of triplefin, so as their populations wither, triplefin populations continue to boom.
But that’s detrimental overall. “If their food disappears, they will disappear too,” Nagelkerken says. “And the whole food web collapses.” So, even if a handful of species can boast about their surplus family jewels—the team has also found similar trends with goby in the Mediterranean and damselfish around Papua New Guinea—the pattern puts the entire ocean’s ecosystem out of joint. “Some [species] will do very well, many will do poorly, and most likely, we will simply lose species,” Nagelkerken says. There’ll be less total biomass—”less kilograms of fish per square meter”—and a loss of biodiversity, which causes a multitude of further problems. “If you erode biodiversity, you’re eroding the capacity of an ecosystem to deal with natural and human disturbances,” he says. It will also have effects on seafood harvesting for human food.
Nagelkerken says his study has broader lessons for the future of marine science: that, in order to study these urgent, wider effects of climate change on the whole ocean, you have to leave the lab. “In a tank, you cannot mimic an ecosystem,” he says. By conducting hands-on experiments in the water, and finding the exact species directly affected by climate change, there’s an opportunity to target particular organisms and manage their populations by reducing stressors such as overfishing and pollution in very specific areas. “We can really do something in the meantime,” he says. “We don’t need to wait decades to help deal with climate change.”