Zoological Studies

Vol. 60, 2021

(update: 2021.07.27; 10.01)

Histological Evidence of Multiple Spawning in Wild Female Japanese Eel Anguilla japonica

Akio Shimizu1, Shigeho Ijiri2, Hikari Izumi2,3, Koichiro Gen4, Hiroaki Kurogi1, Hiroshi Hashimoto5, Hideki Tanaka6, Tadao Jinbo7, Hiroaki Saito8, and Seinen Chow1,*


1Fisheries Resources Institute, Japan Fisheries Research and Education Agency, 2-12-4 Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan. *Correspondence: E-mail: chow@affrc.go.jp (Chow)
E-mail: aneko5861@outlook.jp (Shimizu); hkuro@affrc.go.jp (Kurogi)
2Division of Marine Life Science, Hokkaido University, Hakodate, Hokkaido 041-8611, Japan. E-mail: ijiri@fish.hokudai.ac.jp (Ijiri); h-izumi@hirosaki-u.ac.jp (Izumi)
3Institute for Regional Innovation, Hirosaki University, 2-1-1 Yanagawa, Aomori 038-0012, Japan.
4Nagasaki Field Station, Fisheries Technology Institute, Japan Fisheries Research and Education Agency, 1551-8 Taira-machi, Nagasaki 851-2213, Japan. E-mail: kgen@affrc.go.jp (Gen)
5Amami Field Station, Fisheries Technology Institute, Japan Fisheries Research and Education Agency, 955-5 Sakiyamahara, Setouchi-cho, Oshima-gun, Kagoshima 894-2414, Japan. E-mail: hhiroshi@affrc.go.jp (Hashimoto)
6Aquaculture Research Institute, Kindai University, 468-3 Uragami, Nachi-katsuura, Higashimuro, Wakayama 649-5145, Japan. E-mail: hidektana@gmail.com (Tanaka)
7Shibushi Field Station, Fisheries Technology Institute, Japan Fisheries Research and Education Agency, 205 Natsui, Shibushi, Kagoshima 899-7101, Japan. E-mail: tjinbo@affrc.go.jp (Jinbo)
8Tezukayama Gakuin University, 4-2-2, Harumidai, Minami-Ku, Sakai-city, Osaka 590-0113, Japan. E-mail: hiroakis@wish.ocn.ne.jp (Saito)

Received 13 May 2021 / Accepted 14 July 2021
Communicated by Pung-pung Hwang

The post ovulatory follicle (POF) is an important and reliable tissue structure used to investigate the spawning history in teleost fish. Fresh POFs shortly after spawning are comprised of cellular (follicular cells) and acellular (basement membrane and fibrils such as elastic fibers) components. The cellular components are quickly disintegrated by means of apoptosis, while the acellular components persist for a longer period. Since cellular components are well visualized by conventional hematoxylin-eosin (HE) staining but acellular components are not stained well, old POFs that have lost cellular components are difficult to identify. In this study, periodic acid-Schiff and Victoria blue staining, which can distinctly visualize acellular POF components, were applied to the ovarian tissues of Japanese eel (Anguilla japonica) (n = 9) captured from June to August of 2008, 2009, and 2013 at the southern West Mariana Ridge, a spawning area for Japanese eels. Only new POFs were observed in seven females caught in June, and these females had ovaries with early- to mid-vitellogenic stage oocytes. Both fresh and old POFs were observed in a female caught in July, and only mid-vitellogenic stage oocytes were observed. Only old POFs and no vitellogenic stage oocyte were observed in a female caught in August. A progressive decrease in muscle lipid content, gonad somatic index, and condition factors was observed from June to August. Thus, the female Japanese eel can spawn at least twice or three times at most during spawning season, depending on energy reserve.

Key words: Anguilla japonica, Wild female, Histology, Ovary, Multiple spawning.

Citation: Shimizu A, Ijiri S, Izumi H, Gen K, Kurogi H, Hashimoto H, Tanaka H, Jinbo T, Saito H, Chow S. 2021. Histological evidence of multiple spawning in wild female Japanese eel Anguilla japonica. Zool Stud 60:61. doi:10.6620/ZS.2021.60-61.

Supplementary materials: Fig. S1Fig. S2