瀬戸内海燧灘におけるセトダイHapalogenys mucronatusの生殖について

瀬戸内海燧灘におけるセトダイの生殖生態について, 2000年3月から2001年11月まで同海域から採集した標本により調査した。
GSIは雌雄共に10月から翌年の4月までは一定の低い値をとるため生殖休止期と推定した。その後の6月から徐々にGSIは高くなり雄では6月(3.3%), 雌では7月(17.3%)に最大値を示す個体が出現した。雌雄共に6月から9月まではGSIは高い値を保つため産卵期と推定した。組織切片標本観察によると5月の標本の卵巣内はほとんどが未成熟卵であり, 産卵準備期であると推定した。6, 7, 8月の標本の卵巣内はそのほとんどを成熟卵が占有していた。特に7, 8月の標本には排卵後濾胞が存在しており, 産卵盛期と推定した。産卵盛期の7, 8月にHSIは低い値をとるが, その時期直前の5, 6月に高い値をとっており, 肝臓内に生殖に必要なエネルギー物質を蓄積していることが考えられる。9月は一部の標本に排卵後濾胞が確認されたが, 大部分の卵巣が卵の崩壊吸収過程であったので, 産卵期でも終期にあたると推定した。
卵径組成では調査した全試料で卵径0.7㎜辺りの成熟卵のモードが高くなっているが, 卵径0.7㎜未満の小さい卵のモードも極端に小さくはなっていなかった。さらに組織切片標本観察により排卵後濾胞の確認された標本には未成熟卵と成熟卵の中間の大きさの卵径0.4~0.5㎜の卵が多数確認された。これらより, 卵径組成は多峰型で, 産卵期間中, 放卵しては未成熟卵団から成熟卵団へと次々に卵の成長添加が行なわれる多峰連続産卵型の成熟産卵様式をもつものと推定した。

The reproductive ecology of the Japanese grunt, Hapalogenys Mucronatus, setodai in Japanese, inhabiting Hiuchinada Sea, the west part of the Central Seto Inland Sea, was investigated from March 2000 to November 2001. The changes of GSI (gonadsomatic index), HSI (hepatosomatic index) and the histological observation in the gonad in the Japanese grunt were examined. GSI for both sexes changed low from October to the next April when it was estimated as the resting season. From the histological observation of the ovaries in May, they had immature eggs, so this period was estimated as the pre-spawning season. In this season HSI values were high. In June GSI began to increase, and then GSI attained to the maximum value, 3.3 for male in June and 17.3 for female in July. GSI of both sexes fluctuated at high level from June to September. This period was estimated as the spawning season. Especially, most ovaries were occupied by matured egg tissue with empty follicles after ovulating during from July to August when it was estimated as the full spawning season. HSI of both sexes changed low. It was inferred that energy resource was saved in liver for reproduction. In September, empty follicles were observed in some ovaries. Then, egg degeneration occurred in most ovaries, so it was estimated as the post-spawning season. The examination of ovarian oocytes distribution presented that most matured oocytes appeared at about 0.7 mm in diameter, and then many 0.4-0.5 mm oocytes in diameter were observed in ovaries which had empty follicles. Accordingly, it was concluded that the Japanese grunt has the poly-modal distribution in ovarian oocytes and spawns them continuously immediately after maturing immature oocytes.