Blind Fish Still Able to Count

Luca Scapoli

Blind Fish Still Able to Count

A fish doesn’t need eyes to count. Somalian blind cavefish (Phreatichthys andruzzii) choose the larger of two groups of sticks when they’re taught to associate food with the bigger set, scientists have discovered. Previous studies have shown that fish—as well as mammals and birds—can be trained to discriminate between quantities. To test whether blind fish—which evolved for millions of years in the perpetual darkness of caves under the desert—still have this ability, researchers arranged sets of plastic sticks on opposing sides of their tanks and put food flakes only among the larger assembly of sticks. The sightless fish, they report online this week in The Journal of Experimental Biology, learned to choose four or six sticks over two, even when the food was no longer present, although they couldn’t discriminate between two and three. Even when four sticks were arranged to cover the same overall area as two sticks, or the sticks were arranged to have the same density, the fish correctly chose the larger set. Blind cavefish sense objects in their environment by detecting subtle changes to water flow around them, and the ability to differentiate quantities likely helps them snap up the largest amount of insect larvae or plankton to survive, or join the largest group of other cavefish for protection.

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