マダイ卵巣卵の成熟過程と産卵数
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概要
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The present report aimed to study the fecundity of sea bream, Pagrus major (Ternminck et Schlegel), by obtaining the basic information on the number and maturation process of its ovarian eggs and comparing the number of spawned eggs by the cultured fish with the fecundity of the wild ones. The materials used in this investigation were collected from the coastal waters of Chikuzenkai region of northern Kyushu, during the period from March, 1969 to July, 1970. Here the study is principally based on the histological observations of the ovarian eggs, related with their diameters as well as their numbers. The ovaries preserved in 10 per cent formalin were weighed, and the egg counts and diameter measurements were made under 50 times magnification using the projector. Egg number was obtained by computing the number in a small portion of the ovary for its total. Histological sections of the ovary were prepared by the paraffin and the celloidin method, and stained with Delafield haematoxylin and eosin. The peak spawning season is during the period from late April to late May. From the egg diameter distribution curves, the growth of the ovarian eggs for maturity were discussed, and the multiple spawning by individual sea bream was suggested by the multiplicity of modes in the distribution curves (Fig. 1). There are shown in P1.1, 2 and 3 the general histological features of the ovary and the ovarian eggs of sea bream in each maturation stage. The eggs in the various maturation stages, newly formed empty follicles and the old ones were found simultaneously within the ovary of a mature sea bream. Therefore, the spawning is thought to occur in many times for long period. Maturation stage and size range of the eggs, maturation phase and type of the ovary were arranged in Table 1 based on the observations of the sections and the egg diameter measurements. Relationship between the number of ovarian eggs developed over peripheral nucleolus stage and fork length of the fish by ovary types is shown in Fig. 2. No significant differences are observed between wild fish and cultured one in the number of ovarian eggs corresponding to the fork length in different areas. Fig. 3 shows the relationship between the number of eggs developed over yolk vesicle stage and fork length in each ovary type. All these eggs increase in number with the increase of the fork length of the body. The total number of spawned eggs during a spawning season by cultured fish is about twice as many as the number of ovarian eggs developed over peripheral nucleolus stage and about quadruple as many as the number of eggs developed over yolk vesicle stage. It may be concluded that the immature eggs are continuously developed into the mature ones throughout the spawning season. Comparing the number of the eggs in the most matured batch with those of spawned by the cultured fish in the same size, it may be estimated sea bream ovulate their eggs into the ovarian cavity from one to four times during a day and spawn once at a time a day.この報告の目的は,マダイ卵巣卵の成熟過程と卵巣内卵数に関する基礎知識を求め,既往の飼育魚の産卵例と対比しつつ,産卵数との関係を検討することである.この調査に用いた材料は九州北部の筑前海域より1969年3月から1970年7月にかけて採集したものであり,主として卵巣卵の組織学的観察と卵数および卵径測定を行なつたものである.10%フォルマリン液で保存した卵巣標本を用いた.卵数および卵径測定は万能投影機を用い,50倍に拡大して行なつた.卵数算定は重量法で行ない,卵巣中の1部分の卵数を卵巣重量比で全卵数にひきのばした.卵巣の組織切片標本はパラフィン法もしくはセロイジン法により作製し,デラフィールド・ヘマトキシリンとエオシンの二重染色をほどこした.産卵盛期は年度により多少異なるが4月下旬から5月下旬にかけてである.卵径組成を求め,卵の発育について論議し,卵径組成が多峯型で多回産卵を行なうことを暗示した(Fig.1).各成熟期の卵巣および卵巣卵の一般的組織像についてはPl.1,2および3に示した.成熟魚の組織学的検査によれば,種々の成熟期にある卵や,新・旧の排卵痕がその卵巣中に同時にみられ,長期にわたつて多数回の産卵がなされることを示している.卵の成熟期,その卵径範囲,卵巣の卵径組成による型とその成熟期については,切片観察と卵径測定を基にしてTable 1にとりまとめた.卵巣型別にフォーク長と卵黄胞期以上に発育した卵の数との関係を求めた(Fig.2).北島の資料(未発表)を用いて養成魚と天然魚の卵巣内卵数のフォーク長に対する関係を,異なつた水域の材料で比較したが,差は認められなかつた.卵黄球期以上に発育した卵の数をフォーク長との関係において卵巣型別にプロットした(Fig.3).これらの全ての卵数はフォーク長の増大にともなつて増加する.養成魚の産卵期中の全産卵数(古賀ら,北島の資料)は,卵黄球期以上に発育した卵巣内卵数の約4倍,卵黄胞期以上のそれの約2倍である.この結果から毎日の産卵を行なうかたわら産卵期中は,常時未成熟期の卵団から成熟期の卵団へと,発育添加がなされていると推定した.養成魚の産卵数とそれと同じフォーク長の魚が有する卵巣内の分離成熟卵団の卵数を比較することによつて,マダイは1日のうちに1～4回排卵すると推定した.なお,それらの産卵はほとんど1回(多くて3回)でなされる.
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