INTL J BONOROWO WETLANDS P-ISSN: 2775-8052 Volume 13, Number 1, June 2023 E-ISSN: 2775-8044 Pages: 1-8 DOI: 10.13057/bonorowo/w130101 Population density and distribution patterns of Austruca annulipes (Ring-legged fiddler crab) in the mangrove of Bogowonto Estuary, Kulon Progo, Yogyakarta, Indonesia RIZKY NUR FADILAH 1 , INGGRID TRIFENA YULIA 1 , ZHALZABILLA SHAFA ALFITRA 1 , WINDA SAGITA ARMADHAN 1 , RATNA WIDYANINGTYAS 1 , DITA RAHMAYANI 1 , DINDA PUTRI PERMATASARI 1 , IGUSTITA 1 , DIAN KUSUMA 1 , SATRIA AJI PRAMBUDI 1 , GAVRIEL ENOS BERLIN 1 , ADITYA TRIYANTO 2,3 , SUTARNO 2 , MUHAMMAD INDRAWAN 1 , MUH. SULAIMAN DADIONO 4 , KHAIRUL ADHA BIN A. RAHIM 5 , AHMAD DWI SETYAWAN 1,6,♥ 1 Department of Environmental Science, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret . Jl. Ir. Sutami 36A, Surakarta 57126, Central Java, Indonesia. Tel./Fax.: +62-271-663375,  email: volatileoils@gmail.com. 2 Department of Biology, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret. Jl. Ir. Sutami 36A, Surakarta 57126, Central Java, Indonesia 3 Biodiversity Study Club, Faculty of Mathematics and Natural Sciences, Universitas Sebelas Maret. Jl. Ir. Sutami 36A, Surakarta 57126, Central Java, Indonesia 4 Department of Aquaculture, Faculty of Fisheries and Marine Science, Universitas Jenderal Soedirman. Jl. Dr. Soeparno, Purwokerto Utara, Banyumas 53122, Central Java, Indonesia 5 Faculty of Resource Science and Technology, Universiti Malaysia Sarawak. Jl. Datuk Mohammad Musa, 94300 Kota Samarahan, Sarawak, Malaysia 6 Biodiversity Research Group, Universitas Sebelas Maret. Jl. Ir. Sutami 36A, Surakarta 57126, Central Java, Indonesia Manuscript received: 26 December 2022. Revision accepted: 19 January 2023. Abstract. Fadilah RN, Yulia IT, Alfitra ZS, Armadhan WS, Widyaningtyas R, Rahmayani D, Permatasari DP, Igustita, Kusuma D, Prambudi SA, Berlin GE, Triyanto A, Sutarno, Indrawan M, Dadiono MS, Rahim KABA, Setyawan AD. 2023. Population density and distribution patterns of Austruca annulipes (Ring-legged fiddler crab) in the mangrove of Bogowonto Estuary, Kulon Progo, Yogyakarta, Indonesia. Intl J Bonorowo Wetlands 13: 1-8. The mangrove forest ecosystem is a forest with coastal plant characteristics that are influenced by tides and tolerate high salt levels. Mangrove forests in Indonesia contain about 89 species of flora, and around 18% of their fauna are keystone species. Keystone species are species that play an important role in maintaining the balance of the ecosystem. Austruca annulipes (H. Milne-Edwards, 1837), or Ring-legged fiddler crab is one of the fauna species in the mangrove ecosystem, which has an important role in maintaining the balance of the food chain and nitrogen cycle, so the study of this species is very important for the sustainability of the ecosystem in mangrove area. This study aimed to determine the population density and distribution patterns of A. annulipes in the Mangrove Bogowonto Estuary Area, Kulon Progo, Yogyakarta, Indonesia. The research method was purposive random sampling using a 10m x 10m square plot. Furthermore, the identification of morphology and documentation of the activities of A. annulipes was carried out, and data were examined using calculations of crab density. Based on the research finding, the population density of A. annulipes is 0.9975 individuals/m 2 . Calculating the Morisita Index of A. annulipes obtained a value of 1.38 with the category of clumped distribution patterns. Various factors influence this distribution, including abiotic factors such as soil moisture, temperature, air, salinity, and pH. In addition, the presence of vegetation has also affected the density of A. annulipes because it is used as a shelter and foraging among its roots. Keywords: Behavior, coastal ecosystem, Crustacea, Morisita Index, morphology INTRODUCTION Indonesia is one of the countries with the highest levels of biodiversity in the world and ranks second after Brazil (Dahlia and Mukti 2021). However, biodiversity in Indonesia continues to experience declining significantly. The increase in population will be linear with increasing anthropogenic activities, which also increase human needs leading to the emergence of exploitation of various species, which could cause decreased biodiversity (Mujiono et al. 2021). The loss of biodiversity is a crucial problem because it has implications for the survival of other living things (Suryana and Antara 2021). Therefore efforts to protect and preserve biodiversity are important things to do in the future. The mangrove forest ecosystem is one type of forest in the tidal area. Mangrove forests are a meeting place for two groups of fauna: terrestrial and aquatic (Hasan et al. 2020). Mangroves are described as the characteristics of plants that live in river mouths or on the coast, which are influenced by sea tides and tolerating on high salt levels (Ramadhan et al. 2022). Mangroves play an important role in absorbing carbon in the atmosphere, which positively impacts global climate stability. Meanwhile, mangroves have important value in terms of ecological aspects as habitats for various fauna, breeding grounds for various marine biota, shelters, nutritional sources for various living things, and preventing coastal abrasion (Zeng et al. 2021). The existence of mangroves in Indonesia is recorded at the remaining 1.2 million hectares (Irma et al. 2020). Mangroves can be found between land and sea as well as