Changes in oocyte and blood plasma osmotic components of ayu, Plecoglossus altivelis Temminck & Schlegel during oocyte maturation

Oocyte maturation in teleosts is characterized by a complex series of cytoplasmic and nuclear changes. The purpose of this study is to present sequential changes in the oocyte constituents and regulatory mechanisms involved in the hydration and osmotic regulation of oocytes during final maturation of ayu, Plecoglossus altivelis Temminck & Schlegel. Oocyte volumes and water contents in ayu (P. altivelis) markedly increased from 0.372 mm³ and 60.5% at the mature stage to 0.712 mm³ and 79.4% at the time of oviposition. In the same period, net increases in total free amino acids and electrolytes were noted, and involvements of these constituents in the water uptake by oocytes are discussed. Moderate oocyte hydration in ayu is accompanied by steady increases in Na⁺, but not K⁺, and these observations are in contrast to results for all other marine teleosts reported elsewhere. In the present study, a steady increase in oocyte Na⁺ during maturation until ovulation pre‐dominantly results from diffusional leakage due to concentration gradients, while at the time of oviposition, both active transport and diffusional pathways play important roles in osmotic regulation in ayu oocytes.