Abstract
The rice fish medaka has been genetically differentiated into the Southern and Northern populations. Both populations have parapatric contacts in the Yura River, Kyoto Prefecture, Japan. However, each population has genetically remained almost pure. This phenomenon suggests the reproductive isolation between the Northern and Southern populations. The ecological differentiation between both populations could be demonstrated by fish schooling behavior. They showed a significant difference in fish school radius, with the Northern Population forming a fish school with a larger radius than the Southern Population under conventional behavioral conditions. When a small number of individuals were analyzed, fish school radius did not show a significant difference between both populations. Moreover, the Southern Population congregated near the edge of a circular aquarium, which is a behavior caused by a stimulus, while the Northern Population moved actively. In addition, the Northern Population showed changes of behavior in December. These findings suggest that both populations have unique characteristic fish schooling behaviors, thereby the endemism of local populations may be threatened if they are released outside their original range.
