© 2019 Academia Sinica, Taiwan Open Access Age, Growth, and Sex Ratios of the Giant Mottled eel, Anguilla marmorata, in Freshwater Habitats Near Its Northern Geographic Limit: A Comparison to Tropical Regions Ryoshiro Wakiya 1, *, Hikaru Itakura 2 , and Kenzo Kaifu 3 1 Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan. *Correspondence: Tel: +818056758725. E-mail: ryoshiro.wakiya@gmail.com 2 Graduate School of Sciences, Kobe University, 1-1 Rokkoudaichou, Nadaku, Kobe, Hyogo 657-8501 Japan, E-mail: itakurahikaru@gmail.com 3 Faculty of Law, Chuo University, 742-1 Higashinakano, Hachioji-shi, Tokyo 192-0393 Japan, E-mail: kaifu@tamacc.chuo-u.ac.jp Received 19 March 2019 / Accepted 18 September 2019 / Published 12 November 2019 Communicated by Benny K.K. Chan To understand the latitudinal diference in the basic ecology of the giant mottled eel Anguilla marmorata, we measured its age, growth, and sex ratios in freshwater areas of Amami-Oshima Island, Japan (28.223°N–28.332°N and 129.329°E–129.439°E), near the northern geographic limit of its range and compared these biological characteristics with those observed in the tropical regions of Indonesia. A total of 109 A. marmorata were captured from three rivers on Amami-Oshima Island. The total length (TL) and age of sampled individuals varied across an order of magnitude (TL range: 119–1320 mm, mean: 385.5 ± 172.6; age range: 3–30 years, mean: 12.8 ± 4.9 years). Neither TL nor age difered among rivers. Male A. marmorata accounted for 88.5% of sexually diferentiated individuals, which it contrary to previous results for males living in Indonesian watersheds where males were only found in small rivers and areas close to estuaries. Moreover, we found no males larger than 700 mm in TL and few males that were older than 20 years. The growth rates of male A. marmorata were signifcantly lower than those of females, and growth rates of sexually undiferentiated fsh were signifcantly less than those of males. The mean growth rate of all individuals was 25.9 ± 6.6 mm/y, which is considerably lower than what has been found at lower latitudes, suggesting that growth diferences occur along a latitudinal cline. Key words: Age, Anguilla marmorata, Freshwater, Growth rate, Sex ratio. Citation: Wakiya R, Itakura H, Kaifu K. 2019. Age, growth, and sex ratios of the giant mottled eel, Anguilla marmorata, in freshwater habitats near its northern geographic limit: a comparison to tropical regions. Zool Stud 58:34. doi:10.6620/ZS.2019.58-34. BACKGROUND Species in the genus Anguilla consist of catadromous fish that contain leaf-like leptocephalus larvae that migrate from open oceans to freshwater and estuarine habitats (Han et al. 2016; Chen et al. 2018; Higuchi et al. 2018). After metamorphosing into yellow eels, the fish spend most of their life in continental waters until they begin their downstream migration toward spawning grounds in open ocean. The anguillid eel consists of 19 species and subspecies distributed globally, with the exception of polar regions and the west coast of North and South America (Ege 1939; Aoyama 2003; Watanabe et al. 2004). The giant mottled eel A. marmorata is the most broadly distributed species in the genus, with a range from the south-east African coast to Asia and Polynesia (Ege 1939; Marquet and Galzin 1991; Marquet 1996). It is also one of the largest species in the genus, growing up to 2 m and 21 kg (Castle 1984). Despite this prominent ecology, to date, little basic biological information is available on the giant mottled eel, likely because compared with the Japanese Zoological Studies 58: 34 (2019) doi:10.6620/ZS.2019.58-34 1