Development of Mixed-feeding Stage Larvae of Japanese Flounder, Paralichthys olivaceus (Pisces: Paralichthyidae)

ヒラメ仔魚の成長,卵黄と油球の吸収様式,形態発育および初期摂餌量の変化を,鮮化直後から158時間(158HAH)調べた。その結果,栄養転換過程は以下のように分けられた:1)内部栄養期:艀化～90HAH(全長2.2～3.6mm),2)混合摂餌期:90～140HAH(3.6～4.2mm),3)外部栄養期:140HAH～(4.2mm～)。また,50～60HAH(3.4～3.5mm)には内部栄養の吸収速度に変曲点が認められ,何らかの生理的な変化が起こることが示唆された。この時期は開口の時期とほぼ一致することから,内部栄養期をさらに前開口期と後開口期に分けた。ヒラメ仔魚の栄養転換過程を,これまでに知られているその他の海産魚種と比較したところ,ヒラメでは摂餌開始時の内部栄養量が少なく,これは初期仔魚の生残には不利な形質と考えられた。一方で,内部栄養の吸収完了時に摂餌量が多いこと,摂餌開始から100%摂餌までの時間が短いこと,および摂餌開始時の全長が大きいことなど,初期摂餌に関しては,円滑な栄養転換をサポートする特徴をもつことも明らかとなった。Early growth, yolk and oil exhaustion, early morphological development and initial feeding were examined in laboratory-reared larvae of the Japanese flounder, Paralichthys olivaceus. Based on nutritional transition, the early larval stage of Japanese flounder was divided as follows: the endogenous nutritional phase from hatching (mean total length 2.2 mm) to 90 hours after hatching (HAH) (3.6 mm); a mixed-feeding phase from 90 to 140 HAH (4.2 mm); and an exogenous feeding phase after 140 HAH. Furthermore, the endogenous nutritional phase was divided into two sub-phases at around 50-60 HAH (3.4-3.5 mm), at which time flexion points were apparent on the semi-logarithmic plots of yolk and oil globule absorption. This border coincided with the first observations of mouth opening. In the pre-mouth-opening phase from hatching to 50-60 HAH, a rapid increase in larval growth rate was recorded, while growth rate was slower in the post-mouth-opening phase. Based on comparisons with early-life-stage features of other marine fish species, Japanese flounder larvae retained very few endogenous nutritional resources at the onset of feeding, a characteristic that is disadvantageous for survival in spite of a long mixed feeding period. However, the high rate of feeding at the exhaustion of endogenous nutrition, the reduced time from initial feeding to 100% feeding, and the large larval size at the onset of feeding found in this study are considered to advantageous features supporting the nutritional requirements of larvae