Live-bearing fish in sulphur-rich springs give birth to fewer, larger young than counterparts in non-toxic waters.

Rüdiger Riesch at the University of Sheffield, UK, and his colleagues studied nine species of fish, including the guppy (Poecilia reticulata), which have independently flourished in sulphur springs in the United States, the Caribbean and South America.

The researchers show that the toxic waters do not directly damage fish fertility. Instead, parents have fewer offspring as an inevitable trade-off of investing their energy in producing larger offspring, which can more easily detoxify hydrogen sulphide gas.

The discovery illustrates a widespread pattern of predictable evolution, they say.

Ecol. Lett. http://doi.org/p5h (2013)