REPORT Gametogenesis and fecundity of Acropora tenella (Brook 1892) in a mesophotic coral ecosystem in Okinawa, Japan Rian Prasetia 1 • Frederic Sinniger 1,2 • Saki Harii 1 Received: 1 February 2015 / Accepted: 3 September 2015 / Published online: 12 September 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract Mesophotic coral ecosystems (below 30–40 m depth) host a large diversity of zooxanthellate coral com- munities and may play an important role in the ecology and conservation of coral reefs. Investigating the reproductive biology of mesophotic corals is important to understand their life history traits. Despite an increase in research on mesophotic corals in the last decade, their reproductive biology is still poorly understood. Here, gametogenesis and fecundity of the Indo-Pacific mesophotic coral, Acropora tenella, were examined in an upper mesophotic reef (40 m depth) in Okinawa, Japan for the first time. Acropora tenella is a hermaphrodite with a single annual gameto- genic cycle, and both oogenesis and spermatogenesis occurring for 11–12 and 5–6 months, respectively. Timing of spawning of this species was similar to other shallow Acropora spp. in the region. However, colonies had longer gametogenic cycles and less synchronous gamete matura- tion compared to shallow acroporids with spawning extended over consecutive months. Both the polyp fecun- dity (number of eggs per polyp) and gonad index (defined as the number of eggs per square centimeter) of A. tenella were lower than most acroporids. Our findings contribute to understanding of the life history of corals on mesophotic reefs and suggest that the reproductive biology of upper mesophotic corals is similar to that of shallow-water corals. Keywords Upper mesophotic coral ecosystem Á Sexual reproduction Á Acropora tenella Introduction Mesophotic coral ecosystems (MCEs) occur at depths of 30–40 m to over 150 m, depending on the region, and are characterized by limited light intensity (i.e., between 0.3 and 9.3 % of surface irradiance) and the presence of zooxanthellate corals (Kahng and Kelley 2007; Lesser et al. 2009; Kahng et al. 2010). Over the last decade, knowledge of biogeography and biodiversity in these ecosystems has improved rapidly (Armstrong et al. 2006; Kahng and Kelley 2007; Rooney et al. 2010; reviewed in Kahng et al. 2014). Although MCEs experience much lower levels of irradiance than shallow tropical reefs, zooxanthellate corals are often abundant, especially in upper parts (30–60 m depth) (Armstrong et al. 2006; Kahng and Kelley 2007). Recently, a higher diversity of corals was found in Indo- Pacific MCEs than previously thought (i.e., 84 species reported below 50 m depth in Bridge et al. 2012 versus 30 species in Carpenter et al. 2008). They form topographi- cally complex ecosystems often highly adapted to the specific environmental conditions encountered at meso- photic depths (Bongaerts et al. 2010). For example, mesophotic corals depend more on heterotrophy (Lesser et al. 2010) and have higher capacity to absorb light through skeletal morphological adaptations (Mass et al. Communicated by Biology Editor Dr. Anastazia Banaszak Electronic supplementary material The online version of this article (doi:10.1007/s00338-015-1348-1) contains supplementary material, which is available to authorized users. & Saki Harii sharii@lab.u-ryukyu.ac.jp 1 Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 902-0227, Japan 2 Environmental Impact Assessment Research Group, R&D Center for Submarine Resources, Japan Agency for Marine Science and Technology (JAMSTEC), 2-15 Natsushima, Yokosuka, Kanagawa 237-0061, Japan 123 Coral Reefs (2016) 35:53–62 DOI 10.1007/s00338-015-1348-1