An introduction to the identification of the pelagic fish eggs and hatched larvae

A number of marine fishes spawn pelagic eggs which are easily collected by a tow net, while the identification of them is not always easy because of their simple structure. Therefore to identify the pelagic eggs we must observe the process of the egg development as well as the hatched larvae. The hatched larvae can be reared as far as the yolk have been consumed, but even at this stage, some of them are still remain unidentifiable. The pelagic fish eggs can be divided into the agglutinated pelagic eggs and isolated ones. The characteristics of both of these groups are as follows : Agglutinated pelagic eggs. The eggs are spawned either imbedded in a gelatinous ribbon or balloon, or they are agglutinated to each other forming an egg mass without gelatinous substance. Isolated pelagic eggs. The spawned eggs are isolated, not forming any mass. The following characteristics are especially important to identify the pelagic eggs. Egg size and shape. The diameter of the pelagic eggs is ranging from 0.5 to 4.8 mm, mostly 0.7-1.5 mm. Most of the pelagic eggs are spherical in shape, but some of them are ellipsoidal or in other shape. Egg membrane. Usually colorless, thin, without any markings, but some have double membrane, coated membrane or membrane with various markings or appendages (Pl. 1, figs. 5, 7, 8, 9, & 11). Perivitelline space is usually narrow, but sometimes considerablly wide. Yolk is mostly colorless and homogeneous, but some have segmented yolk. Oil globule. The pelagic eggs can be divided into three groups according to the absence or presence of an oil globule or oil globules. Coloration, position as well as the number of oil globule must be examined, together with their changes during the course of egg development. Pigmentation. Sorts and distribution of pigments, time and location of their occurrence and their changes during the egg development are also important. Hatching pore. Some eggs have special form in hatching pore. In the hatched larvae, the shape of the yolk, the position of oil globule, the nutnber of myotomes, the position of the anus and its shifting, the nature of the marginal fin, pigmentations, dorsal and ventral fins (if present), and the changes of all these characteristics during the larval development arc indispensable for the identification of species.魚類の初期生活史の研究には,卵およびそれから孵化した仔魚の性質を明らかにすることが必要であるのは云うまでもない.海産の硬骨魚の多くが浮性卵を産み,しかもそれらは容易に採集されるが,種名ないしは所属の判明しないものが多い.我国における浮游性魚卵の研究は19世紀の末から行なわれ,神谷(1916～1925)によつてその当時までの知識がまとめられ,50種の記載と53種についての検索が示された・その後にも浮游性魚卵の研究はかなり行なわれているが断片的なものが多く,それらをまとめた業績は見当らない.筆者は1951年から九州近海に出現する浮性魚卵の採集研究を行い,現在までに200余種を得ている.しかし,当然出現することが期待される種類はなお非常に多く,今後更に研究を続行する必要が痛感される.最近水産資源調査研究の一部として,産卵調査が各地で行われ,神谷以後の知識の整理が必要になつて来た.この報告では浮游性魚卵およびそれから孵出した仔魚の種の同定について解説する.なお本研究に当り終始御指導下さつた内川恵太郎前教授に謹んで感謝の意を表する