Ocean acidification may be good for thriving marine snails

These tiny snails might make a meal out of ocean acidification

Brendan Kelaher and Amanda Beasley

By Andy Coghlan

Tiny marine snails have challenged doomsday assumptions that ocean acidification driven by global warming will inevitably render the oceans sterile.

While there’s no doubt that many species will suffer, the fate of the tiny snails (Eatoniella mortoni), nicknamed turfwinkles by researchers, living near carbon dioxide-rich ocean vents suggests that at least some species may thrive.

The research is the latest to show that the impacts of acidification may produce winners as well as losers, depending on whether secondary, indirect benefits override the negative direct effects.

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The primary negative effect is that water acidified through absorption of extra carbon dioxide dissolves carbonate, the basic ingredient in shells. As a result, shell building species are expected to be particularly affected.

However, the new findings show that even though carbon dioxide produced by the vents may have made it harder for turfwinkles to build shells, they still thrived overall. This is because it also fueled the growth of algal “turf” on which they feed.

The researchers took advantage of a natural gradient in ocean acidity near carbon-dioxide vents in shallow waters off New Zealand.

This gave them an insight into what an acidified world might mean for some marine ecosystems, says team member Brian Helmuth of Northeastern University Marine Science Center in Massachusetts.

“We found that the total plant mass of algal mats at vent sites was more than twice that at control sites, and that correspondingly there were almost 2.5 times more snails at the vent sites,” says Helmuth.

Coral coup de grâce

Helmuth stressed that it’s not possible to say how such findings may apply to other animals and ecosystems. Generally, species that make hard shells will be directly stressed by acidification, which also has a devastating impact on coral reefs.

“It seems likely that the net response to acidification will be highly variable, and there are plenty of examples of overall negative effects of acidification and ocean warming,” he says.

Algal mats could put more pressure on coral because they compete for space on reefs. “Corals are already severely stressed by ocean warming and acidification, so the fact that their competitors are getting a boost may be the coup de grâce,” he says. “If you prefer diving on reefs full of oozing algae rather than colourful corals, then climate change may not be so bad.”

“Acidification can lead to winners and losers, but in the case of rapid acidification – as with any abrupt change – the number of losers will far outweigh the winners,” says Jim Zachos of the University of California at Santa Cruz. “Typically, it’s opportunistic species that proliferate and dominate, leading to lower diversity overall, and that’s not good news.”

Other researchers also say we shouldn’t leap to the conclusion that higher carbon dioxide levels throughout the ocean would be good for these snails.

“They just congregate around the vents because food is more abundant there, like a fast-food restaurant for gastropods,” says Andrew Watson of the University of Exeter, UK. “But perhaps their success in reproduction is reliant on there being low-carbon dioxide areas nearby as well.”

Most vulnerable

Helmuth says that studies like those around the vents could be valuable for identifying and helping species most vulnerable to acidification. “Having some idea how different ecosystems are likely to respond gives us some hope of preparing for those impacts,” he says.

Terry Hughes, director of the Australian Research Council’s main reef study centre in Townsville, Queensland, welcomes the study, but warns that the levels of carbon dioxide around the vents are much higher than those anticipated through global warming, and so could give unrealistic insights. “We need to be careful that vents really are a realistic analogue for the future,” he says.

Helmuth fears the results could be seized on and exaggerated by sceptics of climate change. “We do worry about this quite a bit,” says Helmuth. “In our experience, results like these tend to be taken out of context by those desperately searching for anything to justify their denial that human-induced climate change is happening,” he said.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2016.12.004

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