For most species, a stressor like a parasitic infection compounds the threat of environmental change. But not for sea snails.

Researchers at the University of Otago in New Zealand found sea snails infected by parasitic flatworms were better able to survive acidified seawater.

The discovery was unexpected. Scientists were mostly interested in understanding the affects of pH levels on the snails.

When they compared survival rates at today's pH levels and those predicted for the years 2100 and 2300, they found the changes had little affect on the snails infected by flatworms. The survival rates of uninfected snails declined as the pH levels rose.

Scientists believe infected snails, counterintuitively, have more energy to adapt to the harsher habitat.

"Snails infected by flatworms are invariably 'castrated' by these parasites, meaning that they no longer expend energy on reproductive activities," Colin MacLeod, a zoologist at Otago, said in a news release. "This leaves them with more energy to maintain their acid-base balance and shell integrity in the acidified conditions."

MacLeod is the lead author of a new paper on the discovery, published this week in the journal Biology Letters.

"Our study strongly supports the growing consensus that parasitic infection must be carefully taken into account in attempts to accurately assess the impact of ocean acidification on ecologically and commercially important marine species," MacLeod concluded.