瀬戸内海西部海域のナルトビエイAetobatus narutobieiの胎内仔魚期からの発育段階区分

ナルトビエイは21世紀初頭から本邦西部域で大量出現し，有用貝類等を食害するとして駆除事業が行われる一方，絶滅危惧種ともされている。本種は卵胎生魚であるが，生活史全般の発育に関する研究はない。我々は2004-’12年に瀬戸内海西部域で調査を行い（標本数：胎内仔魚368尾，捕獲個体985尾），本種の発育について，海産硬骨魚類の発育段階区分（卵－仔魚－稚魚－若魚－未成魚－成魚）を参考に検討した。その結果，本種は基本的には硬骨魚類の発育段階を経るが胎内仔魚期に仔・稚魚期を経て，若魚期として誕生することが明らかとなった。胎内では前半は卵黄栄養，後半は組織栄養によって成長し，基本的な体形を完成した。誕生後は雌の方が雄よりゆっくり，大きく発育・成長し長命の性的二形の特性を示した。本種の仔魚発育・胎内保育の様式，および仔魚の成長を支える雌の大型化は，仔魚をより大きく発育・誕生させるための進化的デザインであると考えられる。

Since the beginning of the 21st century, the Naru eagle ray (Aetobatus narutobiei) has been found to be distributed in large numbers in the western sea regions of Japan. Moreover, it has been exterminated as it feeds on beneficial shellfish. It is an ovoviviparous ray, and ecological studies on this species have been carried out around the Japanese coast; however, the developmental stages in the life of the Naru eagle ray have not been reported yet. Based on our research in the western Seto Inland Sea, Japan, from 2004 to 2012 on a total of 368 embryos (uterus larvae) and 985 free-swimming individuals, we identified the developmental stages of this ray according to those of marine teleosts (stages: egg, larva, juvenile, young, immature, and mature). As a result, the Naru eagle ray embryo was found to develop from an egg and pass through the juvenile to the young stage in the uterus of the female parent, where it utilizes the yolk and ingests the uterine milk through the trophonemata. Before birth, the uterus larva completes its body form as a species. This species exhibits sexual dimorphism, where females grow slower, larger, and live longer than males. It could be considered that sexual dimorphism along with the modes of development and maintenance of the uterus larva are evolutionarily conserved mechanisms.