Angelfish, like those pictured here, can tell the difference between larger and smaller groups of fish. At least, in certain situations.
Gerlai and Luis Gomez-Laplaza of the University of Oviedo in Spain exploited the previously determined tendency of angelfish to seek protection in unfamiliar environments by joining the largest possible fish group, called a shoal. To rule out possible confounding effects arising from sexual interactions, the researchers only used juvenile angelfish for their experiments.
Test fish placed in special compartmentalized tanks were given a simultaneous choice between shoals containing different numbers of fish. The angelfish were always able to select the larger of two groups so long as the ratio between the shoals was 2:1 or above. Below that ratio, their choices were less predictable, suggesting a limit to their quantity estimation abilities.
After the findings were published, the researchers, according to Gerlai, "have already collected new data suggesting that angelfish can discriminate much more precisely than this. That is, angelfish can tell the difference between 3 and 2, for example."
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