-1- ABSTRACT The sex status of Anguilla bicolor McClelland is difficult to be distinguished between males and females. Thus, we evaluated 309 fishes from The Serayu River, with length and weight range around 9 - 81 cm and 0.59 - 1260 g respectively to access the anatomical and histological characteristics of their gonads for each sex. Param- eters assessed were body sizes, eye diameter, fin length, and gonad weight. The results showed that body length, weight, eye index, and gonadosomatic index of the male were significantly smaller (p<0.01) than those of the females. Fins index was not significantly different (p>0.05) between males and females. The histologi- cal structure of gonad showed that the eel gonads can be classified as either indif- ferent gonads, testis, or ovary based on the length of body size. Collecting all the data together it can be concluded that A. bicolor is a gonochoric. Copyright: © 2022, J. Tropical Biodiversity Biotechnology (CC BY-SA 4.0) Short Communication Anatomical and Histological Characteristics of Gonad of Tropical Eel Anguilla bicolor McClelland, 1844 in Different Length Body Size Farida Nur Rachmawati 1 *, Ridwan Affandi 2 , Yulia Sistina 1 1) Faculty of Biology, Universitas Jenderal Soedirman, Jl. Dr Soeparno No 63 Karangwangkal Purwokerto 53122, Central Java, Indonesia. Tel./Fax. +62-281-637894 2) Fisheries Faculty Bogor Agricultural Institute. Jl. Raya Dramaga Kampus IPB Dramaga Bogor 16680 West Java, Indonesia * Corresponding author, email: farida.rachmawati@unsoed.ac.id Journal of Tropical Biodiversity and Biotechnology Volume 07, Issue 02 (2022): jtbb68824 DOI: 10.22146/jtbb.68824 Anatomical and physiological parameters are essential factors in determining the maturity of eel gonads (Arai & Abdul Kadir, 2017a; Ismail et al. 2017; Ching et al. 2019). Mature male eels Anguilla bicolor are rarely found in fresh- water (Rachmawati & Susilo 2011; Arai & Abdul Kadir 2017b; Rachmawati et al. 2017) because they prefer to live in brackish water or seawater. The im- mature ones, soon migrate to the sea to complete the phase of spermatogen- esis (Aida et al. 2003; Tesch 2008). The length of Anguilla japonica males is found in fresh water and saltwater ranged between 42.5 - 63.4 cm, whereas the length of female eels is ranged between 46.1 and 85.6 cm (Sudo et al. 2013). Body length is a sex-determining factor (Sugeha et al. 2009; Arai & Abdul Kadir 2017), but body length always overlaps in different sexual sta- tuses, thus is it challenging to distinguish the male and female eels. Previous studies reported that male eel is shorter in length of body size than that of female eel (Frisch 2004; Bark et al.2007; Tesch 2008; Côté et al. 2015). Ob- servations of A. bicolor's body length was varied among researchers. Rovara et al. (2008) studied that the length of females is over 50 cm, while the lengths Keywords: anatomical and histological characters Anguilla bicolor determination sex status eel gonad Submitted: 01 September 2021 Accepted: 21 April 2022 Published: 24 June 2022 Editor: Ardaning Nuriliani