Bettas show left-and right-hand preferences


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The Siamese fighting fish, Betta splendens, not only shows right- and left-hand preferences but also has accompanying body asymmetries according to research published in a recent issue of Behavioural Brain Research.

Lead researcher Yuichi Takeuchi aimed to answer three separate questions:

  • In a display of aggression, would fish would present one side to an opponent in preference to the other side?
  • Are there are differences in appearance between the left and right side of the fish?
  • Are differences in body shape linked to which side the fish presented?  

Bettas prove ideal fish for such experiments, as Takeuchi explains: "Bettas show intensive aggressive behavioural patterns" and "remarkably erect the operculum during these social situations".

In the first phase of the experiment, Takeuchi found that when placed in a mirrored, hexagonal tank  just over half of the fish showed a left or right-sided preference for displays of aggressions: "lefty" bettas were more likely to present and flare their left gill cover at the mirror image of themselves, while "righty" fish would do the opposite.

In the second stage of the experiment, the researchers measured tiny differences in the body shapes of the fishes, specifically looking at the angle at which the spine met the head.  The overwhelming majority of fish had a slight left- or right-sided bend in their backbone.

Finally, Takeuchi compared the two sets of results and found that fish that were already identified as lefties had a subtle bend in their spine to the left while righties would most likely bend to the right, suggesting a link between body shape and left/right preference.

Takeuchi admits that the test may lead to more questions than answers, agreeing that "The reason for the relationship between behaviour laterality... and morphological asymmetry is not clear", and that perhaps these results actually mean that it is time to "re-examine the widespread behavioural laterality in fish".