This means that they have similar counting abilities to those observed in apes, monkeys and dolphins and humans with very limited mathematical ability.

Christian Agrillo, an experimental psychologist at the university of Padua in Italy said: "We have provided the first evidence that fish exhibit rudimentary mathematical abilities."

Last year, he and his colleagues showed that if a female mosquito fish is harassed by a male, she will try to avoid his attentions by seeking solace in the largest nearby shoal; demonstrating that the fish can tell bigger shoals from smaller ones.

The team first conducted a series of experiments to see whether a lone mosquito fish would prefer to join a shoal of between two and four others.

Larger numbers
A second series of experiments revealed the fish's ability to process larger numbers. The fish were not able to directly count over four, but they were able to distinguish between larger numbers if they differed by a ratio of 2:1.

For example, the fish could distinguish between a shoal of 16, compared to a shoal of eight others. But they could not tell the difference between a shoal of 12 compared to a shoal of eight, a ratio of 3:2. This demonstrates that fish are able to visually estimate larger numbers - but not very accurately.