飼育実験におけるマアナゴ葉形幼生の形態変化と行動について

[Author abstract]The leptocephalic larvae of metamorphosis stage of the conger eel, Conger myriaster (BREVOORT), caught from off Hamakaida, Nangokushi, Kochi Pref. were reared in the aquarium of our laboratory until they grew up to the early immature stage, and were used in the study of their morphological change. The measurings of the body parts were carried out in the six living fish at intervals of about two days. The movement and the feeding habit were also observed during the experimented period. The results obtained were summarized as follows. (1) The absolute growth curves in the metamorphosis stage varied remarkably among the measured characters. Especially in the post-dorsal origin length (PD) and the caudal length (CL), each curve had a maximum value at the time of about the half way in the period of metamorphosis stage. At the time, the origin of anal fin moved forward nearly to a vertical line through the last vertical blood vessel, the region being recognizable in most apodal fish as to be the boundary of abdominal and caudal vertebrae. (2) The relative growth curves, represented by the common logarithms, can be classified into four types: <-type was indicated by the TL-HL relation, -f-type by the TL-PD and TL-CL, r-type by the TL-SD and TL-SA, and the linear type by the TL-SLV, respectively. (3) The somatic adult-form melanophores began to appear on the dorsal region of the head, immediately after the time when the anal fin origin moved to a vertical line through the last vertical blood vessel. About one hundred days after this time, a series of white spots on the lateral line was recognized, which is a remarkable feature in the adult form of the present species. (4) When the body form was becoming roundish, the larvae of metamorphosis stage changed from the bathypelagic to benthic habitat. (5) The reared fish began to take food, immediately after the end of metamorphosis. The cognition of food seemed to be largely depended on the sense of touch and the olfaction.[著者抄録]高知県南国市浜改田地先で採集したマアナゴの変態期葉形幼生を飼育し,活魚測定により初期変態期から初期末成魚期までの形態変化を調べ,また遊泳・摂餌行動について観察した。(1)変態期の絶対成長は,測定形質によって変化の状態が著しく異なっていた(Fig. 1 )。特に,後背鰭始部長(PD)と尾部長(CL)では,変態期の途中に極大値を示した。この時期は,臀鰭始部が腹椎骨と尾椎骨とのばぼ境界部に当る最終垂直血管の位置まで前進する時期とほぼ一致した。(2)全長に対する各測定形質の相対成長は,対数グラフで示すと(Fig. 2),く型(TL-HL),イ型 (TL-PD,TL-CL),γ型(TL-SD,TL-SA,TL-AD)および直線型(TL-SLV)の4型に大別される。(3)成魚型黒色素胎は臀鰭始部が最終垂直血管の位置に達する直後からぃ頭部背面に出現し,また側線上の白色点列はこの時期から約100日後頃に確認された。(4)変態が進み,体形が丸味をおびる頃に,底生生活に移向した。(5)摂餌は変態完了直後頃から始まり,また餌の感知はおもに嗅覚と触覚に依存すると思われる行動がみられた。

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