INDO PAC J OCEAN LIFE P-ISSN: 2775-1961 Volume 7, Number 1, June 2023 E-ISSN: 2775-1953 Pages: 16-26 DOI: 10.13057/oceanlife/o070102 Diplosoma simile (Ascidiacea: Didemnidae) distribution and its photosynthetic thermal stress responses from Mauritius: Implications for invasive or opportunistic behavior ANGELA LIU YEW FAI 1 , DEEPEEKA KAULLYSING 1,2 , SRUTI JEETUN 1 , MOUNESHWAR SOONDUR 1,2 , RANJEET BHAGOOLI 1,2,3,♥ 1 Department of Biosciences and Ocean studies, Faculty of Science and Pole of Research Excellence in Sustainable Marine Biodiversity, University of Mauritius. Réduit 80837, Mauritius. Tel.: +230-4037916,  email: r.bhagooli@uom.ac.mu 2 The Biodiversity and Environment Institute. Réduit, Republic of Mauritius 3 The Society of Biology. Réduit, Republic of Mauritius Manuscript received: 9 September 2022. Revision accepted: 17 October 2022. Abstract. Fai ALY, Kaullysing D, Jeetun S, Soondur M, Bhagooli R. 2023. Diplosoma simile (Ascidiacea: Didemnidae) distribution and its photosynthetic thermal stress responses from Mauritius: Implications for invasive or opportunistic behavior. Indo Pac J Ocean Life 7: 16-26. This study aimed to investigate the morphological identity of the ascidian Diplosoma, its distribution, experimental thermal stress responses, and its characteristics of being potentially invasive or opportunistic in Mauritian waters. The Diplosoma species were anesthetized in menthol, fixed, and preserved in a formalin-seawater solution. Isolated zooids and embryos were examined under a microscope for distinct morphological characteristics, which revealed their identity as D. simile (Sluiter, 1909). The D. simile percentage cover studied at seven sites in 2019 around Mauritius tended to be high at the near-reef zone, with a high percentage of dead coral cover per 0.5 m 2 . At Flic en Flac, long-term observations in 2008, 2010, 2016, and 2019 indicated a significant decrease and increased from 61.25±3.31 to 6.15±0.60 % and 0.08±0.05 to 2.50±090% for the percentage covers of live coral and D. simile, respectively. The D. simile was recorded on five coral species, and its percentage occurrence increased from 2008 to 2019 in Acropora muricata, A. cytherea, Pocillopora damicornis, and Montipora aequituberculata with the highest levels occurring in Acropora while Porites lutea was not found to be covered by D. simile during the study. Visual observations from the thermal exposures at 27ºC, 30ºC, and 33ºC during 19 hrs revealed that D. simile was more affected by increasing temperature and exposure time. Using four observed conditions of normal, bleached/paled, tissue sloughing, and mortality, D. simile suffered from only bleaching/paling at 33ºC at 19 hrs exposure. Variable visual responses were noted for corals, with P. lutea appearing normal at all trials while A. cytherea was suffering from mortality both at 30ºC and 33ºC at 19 hrs exposure. The effective quantum yield (ΦPSII), measured using a Diving-PAM, declined significantly at 33ºC treatments at 6 and 19 hrs of exposure. D. simile generally tended to be more thermally tolerant than corals like P. lutea, P. damicornis, M. aequituberculata, A. muricata, A, cytherea, though the ΦPSII thermal responses were variable among the tested corals. When considering the reported behavioral characteristics of D. simile, namely the lack of broad dispersal range and wide environmental tolerance, it is tempting to deduce that the species is potentially invasive. However, in this study, D. simile’s significant distribution on dead corals, its presence in zones of high anthropogenic activities, and its relatively more robust thermal stress responses than corals colonized suggest an opportunistic behavior. Keywords: Ascidian, corals, Diplosoma simile, distribution, diving-PAM, Mauritius, opportunistic behavior, thermal tolerance INTRODUCTION Ascidians emerge from the kingdom Animalia and belong to the class Ascidiacea, which is considered the largest class of the Subphylum Tunicata (Phylum Chordata) (Hirose and Hirose 2013). These organisms have been subject to many studies regarding their bioactive compounds and broad physico-chemical tolerance, which provides for their complex behavioral patterns (Hirose and Hirose 2013; Hirose and Nozawa 2010). Furthermore, ascidians have further been reported to display characteristics that significantly increase risks of invasion outbreak, that is, their globally widespread distributions and broad environmental tolerances (Hirose and Hirose 2013; Akram et al. 2015; Koplovitz et al. 2015; Villalobos et al. 2017). However, knowing that these organisms have extensive dispersal ranges and are easily overlooked, it would not be surprising that many occurring species are left uncharted (Goodbody 2000; Hirose et al. 2012; Villalobos et al. 2017). Around Mauritius Island, for instance, only 32 species have been reported around the Island (Table 1) with 2 species being endemic, namely: Pseudodistoma mauritiana and Polycarpa nigricans. However, the discovery of another ascidian species with high morphological similarity to the genus Diplosoma around Mauritius Island has led to considering the high possibility of a documentation gap regarding ascidians. With increasing evidence of coral vulnerability worldwide, it is crucial to understand and address any further potential threats posed by ascidians (Carilli et al. 2010; Pandolfi et al. 2011; Bhagooli and Taleb-Hossenkhan 2012). Coral bleaching and reef degradation rates are escalating worldwide and locally, mostly due to increasing global temperatures and exposure to anthropogenic-related stressors (Mattan-Moorgawa et al. 2012; Louis et al. 2016; 2020; Hughes et al. 2017; Bhagooli and Kaullysing 2019; Bhagooli