BIODIVERSITAS ISSN: 1412-033X Volume 22, Number 9, September 2021 E-ISSN: 2085-4722 Pages: 4011-4018 DOI: 10.13057/biodiv/d220949 Physiological responses of scleractinian corals in marginal habitat FUNTY SEPTIYAWATI POLAPA 1,♥ , SHINTA WERORILANGI 2 , SUHARTO M. ALI 2 , JAMALUDDIN JOMPA 2,♥♥ 1 Program of Marine Science, Faculty of Health Agricultural and Marine Science, Universitas Muhammadiyah Palopo. Jl. Jendral Sudirman Km. 3, Palopo 91922, South Sulawesi, Indonesia. Tel./fax.: +62-471-327429,  email: funtyseptiyawati@umpalopo.ac.id 2 Department of Marine Science, Faculty of Marine Science and Fisheries, Universitas Hasanuddin. Jl. Perintis Kemerdekaan Km. 10, Tamalanrea, Makassar 90245, South Sulawesi, Indonesia.  email: Jamaluddin.jompa@gmail.com Manuscript received: 15 July 2021. Revision accepted: 28 August 2021. Abstract. Polapa FS, Werorilangi S, Ali SM, Jompa J. 2021. Physiological responses of scleractinian corals in marginal habitat. Biodiversitas 22: 4011-4018. This study aims to analyze physiological differences in corals in marginal habitats. Under different conditions, the production/respiration (P/R) ratio and photobiology of various coral genera were compared. Samples were taken from three coral reef zones representing typical reef habitats and from the mangrove ecosystem as a marginal habitat. Surveys revealed two coral genera surviving in extreme conditions (marginal habitat). The P/R ratio measurements indicated that corals living in the mangrove ecosystem tend to be heterotrophic. This was supported by observations of colonies with tentacles extended from the corallites. Furthermore, Porites living in the mangrove habitat consume more O 2 directly than saving it for other purposes, such as growth. The genus Dipsastraea exhibited elevated zooxanthellae density in the mangrove ecosystem, whereas Porites exhibited similar densities in both ecosystems. Keywords: Coral reef, Mangrove, P/R ratio, photobiology, physiology INTRODUCTION Understanding the ability of corals to withstand high environmental pressures is not as advanced as knowledge regarding the important role of coral reefs. Coral reefs provide habitats for many marine organisms, and are a major source of income through subsistence, commercial, cultural and recreational fishing activities as well as dive- based tourism and the recreation industry (Weijerman et al. 2016). Despite their recognized importance, coral reefs are under threat. Several coral species are categorized as Endangered (EN) in the IUCN Red List (IUCN 2021). Coral reefs are directly affected by anthropogenic activities through pollution and overfishing (Hughes et al. 2019) in addition to indirect impacts from increasing carbon dioxide emissions (Masson-Delmotte et al. 2018), rising temperatures (Rooke et al. 2017), and decreasing seawater pH (Chan and Connolly 2013). Recognizing the severity of these threats, much research has been devoted to seeking solutions. Several studies have found corals that can survive in conditions outside the limits considered “normal”, in habitats referred to as “marginal” (Yamano et al. 2011; Beger et al. 2014; Yates et al. 2014; Camp et al. 2016; Camp et al. 2018). Marginal habitats have been proposed as one alternative type of future refugia for coral reefs (Yamano et al. 2011). Marginal reefs can also be categorized as extreme reefs with their own unique ecological aspects (Yamano et al. 2011; Camp et al. 2018), composed of ecologically and functionally distinct reef communities (Soares 2020). In many tropical waters, coral reef ecosystems coexist or are associated with seagrass and mangrove ecosystems. Although mangroves are often considered unsuitable for coral growth due to high sedimentation, lack of substrate for coral attachment and unsuitable water quality for recruitment and coral growth, Yates et al. (2014) found coral species living in a mangrove ecosystem; these corals were able to survive in harsh or marginal habitat conditions including relatively low pH levels with fluctuations in temperature and light. Understanding how corals can withstand high environmental pressures can help to predict how things will be in future coral reef ecosystems (Camp et al. 2016). Furthermore, marginal habitats may provide coral species or strains selected to survive in future conditions, and thus help with efforts to ensure the survival of coral reefs (Morikawa and Palumbi 2019; Schoepf et al. 2019). Hoga Island and Kaledupa Island, located within the Wakatobi Marine National Park, Southeast Sulawesi, Indonesia, have extensive coral reef and mangrove ecosystems, making this area appropriate as a site for studying marginal corals. Therefore, the purpose of this study was to analyze physiological differences between corals found in typical coral reef habitats and in the mangrove ecosystem (a marginal habitat); determine the distribution of corals in different natural conditions; and examine the adaptation processes of corals living in mangrove habitat.