Comparative morphology and morphometry of the micropyle of two Korean rice-fishes, Oryzias latipes and Oryzias sinensis (Pisces, Adrianichthyidae)

The morphology and morphometry of the micropyle of two Korean rice fishes, Oryzias latipes and O. sinensis, were investigated by light microscopy and scanning electron microscopy. Obtained from adult gravid females during the spawning season, the full-grown eggs of each species have a single micropyle at the animal pole. For the two species, the micropyles are similar in appearance, but there exist some differences in detail. The egg is larger in O. latipes than in O. sinensis (1,123.3-1,465.5 μm vs. 823.1-1,152.1 μm in diameter), and the outer diameter of a single micropyle for O. latipes is nearly two times larger than that of O. sinensis (18.5-22.4 μm vs. 10.0-12.5 μm). The micropylar diameter to egg diameter ratio (1.6 ± 0.1% vs. 1.0 ± 0.1%) is also larger in O. latipes than O. sinensis. In regard to micropyle structure, the two showed distinguishing characteristics: O. latipes has a funnel-shaped micropyle consisting of two regions, an outer gradual pit and an inner narrow canal, while O. sinensis has a conical-shaped micropyle having only a deep narrow canal. Consequently, these differences in micropylar structure between the two Korean rice-fishes may be considered as a useful taxonomic characteristic in closely related taxa and seem to be structural adaptations to shape and control entry velocity of spermatozoa into the micropyle.