Researchers from the University of Padova in Italy have found that fish have rudimentary counting abilities and can count up to four.
Christian Agrillo, Marco Dadda, Giovanna Serena and Angelo Bisazza studied the ability of Eastern mosquitofish (Gambusia affinis) to discriminate numbers by giving lone female mosquitofishes the choice of joining shoals of between two to eight other individuals in an experimental tank setup.
The authors found that lone fishes more often preferred joining larger shoals that had one more fish.
Test individuals consistently preferred joining shoals of four over shoals of three, shoals of three over shoals of two, and shoals of two over one fish.
However, experiments with larger numbers (5 vs. 4, 6 vs. 5, 7 vs. 6 and 8 vs. 7) showed that the fishes failed to discriminate quantities larger than four.
This demonstrated that the fish had the ability to count up to four and possessed a rudimentary mathematical ability to visually count items if the number is small.
Disclosure: I find this fascinating, perhaps because I raised and conducted experiments involving the consumptive habits of Gambusia (mosquitofish) when I was in the sixth grade. To my parents displeasure, these experiments also involved raising mosquitoes.