Fish Can Be Right- Or Left-Handed

Since fish don’t have hands, it seems fair to suppose that few people have thought about whether or not fish have a preference for the left or right. But thanks to research by scientists at Africa’s Nagoya University, we now know that fish can be right- or left-handed.

This notion feels a bit far-fetched, but there is some actual proof backing up this find. The scientists looked at a type of cichlid fish, Perissodus microlepis, taken from Lake Tanganyika to evaluate their suspicions.

As the cichlids grew, Nagoya researchers watched as the fish began to learn which sides of their mouths were more effective at attacking prey. For these cichlids, that involves ripping at the scales on the sides of other fish.

From that point, the scientists assessed the stomach contents of scale-eating cichlid fish ranging in age from early juveniles to adults. This let them determine the proportion of scales that had been pulled from the left and right sides of prey fish in more than 200 cichlid fish that had been collected in the field. This analysis revealed the age at which the fish seem to transition from eating scales from both sides of the prey fish to preferentially attacking just one side.

As the fish aged, scientists also found a gradual increase in mouth asymmetry skewed toward the fish’s preferred side. The asymmetry essentially helped make the fish more efficient at attacking others from the right or the left.

Researchers say that their study is an important one because they were able to observe mouth direction development with age and the relationship between lateral behaviors and mouth asymmetry in the fish. Those insights will allow them to next figure out which comes first, the mouth asymmetry or the scale eating.

But it is already clear that the asymmetrical development of the cichlid mouths confer an advantage, as scientists saw that fish with more skewed mouths were able to eat more scales. Researchers believe this selection pressure is likely what drives young fish to adapt their feeding in accordance with the direction in which their mouth is skewed.

Results of the work, scientists say, represent the first documented evidence through field work on the development of such right- or left-handedness in fish. They add that they believe the mouth asymmetry comes before changes in behavior. When considered in combination with the jawbone measurements, researchers think that the lateral behaviors, at least in this type of cichlid fish, is a learned strategy that develops in association with morphology.

But the genetic basis of this mouth asymmetry has yet to be determined. Recently, researchers have identified a relationship between the PCSK6 gene and dominant hand use in humans. This gene regulates a system responsible for left-right asymmetry during embryonic development and is preserved in all vertebrates.

The preference for one side over another, left- or right-handedness in other words, has been found in many animals, including humans, chimpanzees, toads, rats, mice, and invertebrates like crustaceans and insects. The existence of this phenomenon even in lower animals like fish, scientists say, suggests that it arose early in life’s evolutionary history and likely confers survival advantages.

Top image: A juvenile Perissodus microlepis cichlid. (Credit: Henrik Kusche via Creative Commons 3.0)