Ontogenetic variation in fin ray segmentation between latitudinal populations of the medaka, Oryzias latipes

Fin rays of ray-finned fishes are composed of multiple bony segments, and each fin ray elongates by adding a new segment to the tip. Therefore, fin ray length is determined by the number of segments and the length of each segment. A comparison of the anal fin rays of a northern and southern wild population of the medaka, Oryzias latipes, revealed that southern fish had more segments per fin ray, resulting in longer anal fins than the northern fish. When fish were reared in a laboratory common environment, segmentation of the fin rays started earlier with respect to body size in the southern fish. In the southern males, moreover, the rate of segment addition accelerated after a certain body size, indicating sexual maturity. These patterns of segment addition during ontogeny were consistent with the patterns of fin ray elongation. Although distal segments tended to be longer, except for the most proximal segment, in both populations, the southern fish had shorter segments than the northern fish at any position on fin rays. These results indicate that the interpopulation variation in fin length is largely due to genetically-based differences in the control of segment addition, and that the length of each segment does not contribute to it. We suspect that fin ray segmentation is regulated by thyroid and sex hormones that differ between populations. We also found that some segments fuse with each other at the base of each fin ray, the functions and mechanisms of which remain unclear.