Fine structures of oocyte envelopes of three related cobitid species in the genus Iksookimia (Cobitidae)

Fine structures of the oocyte envelope (zona radiata) in three spined loaches, Iksookimia longicorpus, I. hugowolfeldi, and I. yongdokensis, are examined and compared. The zona radiata surrounding the oocyte is composed of two layers, a zona radiata externa, which comprised the outermost zona radiata (Z₁) of a homogeneous less electron dense single layer and the middle zona radiata (Z₂) of a homogeneous more electron dense single layer, and a zona radiata interna, which comprised the innermost zona radiata (Z₃) of heterogeneous electron-dense multilayers. Especially, the surface of the outermost zona radiata (Z₁) showed a morphological difference in its external appearance among the species. In the surface structures of Z₁, I. yongdokensis had a round granule appearance, and both I. hugowolfeldi and I. longicorpus were villus in morphology. In the villus appearance of these two species, however, there were clear differences between I. hugowolfeldi and I. longicorpus: the former had a few short thin villi, 1.0-1.7 μm in length, but in the latter species the villi were numerous, long, and thick, 2.0-2.5 μm long. The variety of such structures in these three species of Iksookimia may be a useful aid in taxonomic relationships and closely related to their habitats.