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Global climate change has aggravated the severity of space competition put up by marine sponges in the tropical coral reef ecosystems. We report here an outbreak of coral-killing sponge Clathria (Microciona) aceratoobtusa (Carter, 1887) over live coral colonies of the genus Turbinaria in an unprotected reef (mainland patch reef) in the Gulf of Mannar in India. An outbreak of this orange-reddish sponge was observed during an explorative dive in April 2020. Live coral cover in the reef was 62.06% (SD±3.36), which was dominated by the genus Turbinaria with 82.31%. Among the total of 549 Turbinaria colonies counted in five transects, 21.86% (n = 120) of colonies were found infested by the sponge. As the mainland patch reef lies outside the boundary of marine protected area, it is exposed to human-induced threats, apart from climate change implications. This report would serve as an evidence to indicate the intensity of the space competition waged by sponges in a disturbed reef ecosystem.

The Gulf of Mannar (GoM) in southeast India is known for its coral reefs and associated biodiversity. Multiple coral bleaching events driven by global climate change and subsequent space competition have damaged the coral reefs of GoM during the past couple of decades. Certain marine sponges are space competitors to corals and can outgrow corals by overgrowing. Spongivore fishes play vital role in regulating sponge abundance in a reef. The present study was aimed to identify spongivore fishes at Pattinamaruthoor patch reef (55.25% coral cover) in Tuticorin coast of GoM. Gut content analysis and underwater assessment were carried out between March and September 2019. Known spongivores such as Apolemichthys xanthurus, Pomacanthus annularis and P. imperator had about 75, 50 and 80% of their diet as sponge. Other fishes such as Sargocentron rubrum (30%), Myripristis murdjan (30%) and Scarus ghobban (10%) were also found to feed on sponges which are not regular sponge feeders. It is presumed that increased availability of sponges as a result of climate change has made these fishes opportunistically feed on sponges. A total of 58 reef fish species including obligate and opportunistic sponge feeders were observed at Pattinamaruthoor patch reef. Protection of spongivore fishes from overexploitation is critical as it could allow corals to recover from climate change implications.

We used a 12-yr data set of benthic cover (2005�2017), spanning two bleaching events, to assess changes in benthic cover and coral community composition along 21 islands within Gulf of Mannar (GoM), southeast India. Overall, between 2005 and 2017 reefs had a simultaneous decrease in relative coral cover (avg. = ? 36%) and increase in algal cover (avg. = + 45%). Changes in benthic cover were not consistent among islands, ranging from ? 34 to + 5% for coral cover and from ? 0.3 to + 50% for algae. There was a spatial gradient in coral mortality, which increased among islands from west to east. However, there was a disconnect between coral loss and subsequent increases in algae. Algal cover increased more on islands in west GoM where coral loss was minimal. Environmental co-factors (coral cover, percent bleaching, degree heating weeks, fish densities, Chl-a, pollution) explained > 50% of the benthic cover responses to successive bleaching. Coral survival was favored on islands with higher fish densities and chlorophyll-a levels, and increases in algal cover were associated with higher measures of pollution from terrestrial runoff. Coral morphotypes differed in their response following successive bleaching resulting in changes in the relative abundance of different coral morphotypes. Existing climate projections (RCP8.5) indicate a 22-yr gap in the onset of annual severe bleaching (ASB) for reefs in the east versus west GoM, and ASB was ameliorated for all reefs under the RCP4.5 projections. There is limited knowledge of the resilience of GoM reefs, and this study identifies coral morphotypes and reefs that are most likely to recover or decline from successive bleaching, in the context of forecasts of the frequency of future bleaching events in GoM. � 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature. Climate projections for GoM were done under the Third NATCOM (National Communication) to the United Nations Framework Convention on Climate Change (UNFCCC) project of Ministry of Environment, Forest and Climate Change (MoEF & CC), Government of India�(GOI). The authors are thankful to the MoEF & CC,�GOI; Tamil Nadu Forest Department (TNFD), Government of Tamil Nadu (GOTN); Department of Environment, GOTN; Gulf of Mannar Biosphere Reserve Trust, GOTN; Coral Reef Degradation in Indian Ocean, Sweden; and V.O.Chidambaranar Port Trust (VOCPT), GOI for funding support. Thanks are also due to Chief Wildlife Warden, TNFD; Wildlife Warden, Gulf of Mannar Marine National Park (GOMMNP), GOTN; and Chairman, VOCPT, GOI, for research permissions to carry out coral reef surveys and monitoring within the Marine National Park and harbour area and to Suganthi Devadason Marine Research Institute for logistical support.�Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Scopus

Ocean warming is increasing the incidence, scale, and severity of global-scale coral bleaching and mortality, culminating in the third global coral bleaching event that occurred during record marine heatwaves of 2014-2017. While local effects of these events have been widely reported, the global implications remain unknown. Analysis of 15,066 reef surveys during 2014-2017 revealed that 80% of surveyed reefs experienced significant coral bleaching and 35% experienced significant coral mortality. The global extent of significant coral bleaching and mortality was assessed by extrapolating results from reef surveys using comprehensive remote-sensing data of regional heat stress. This model predicted that 51% of the world’s coral reefs suffered significant bleaching and 15% significant mortality, surpassing damage from any prior global bleaching event. These observations demonstrate that global warming’s widespread damage to coral reefs is accelerating and underscores the threat anthropogenic climate change poses for the irreversible transformation of these essential ecosystems.

A study was undertaken to assess the morphological variations during the period 1969–2018 in the twenty-one islands in Gulf of Mannar, southeast India, using Survey of India Toposheet, Google Earth images and field survey techniques. The results show that two of the islands have eroded and submerged, fifteen islands area cover has reduced and four have their area expanded during the last 49 years. Overall, the Tuticorin group of islands has experienced the highest percentage of land cover reduction (78.55%), followed by Keelakarai (43.49%), Vembar (36.21%) and Mandapam (21.84%) groups. The exceptions are the four islands of the Mandapam group viz. Hare, Manoli, Krusadai and Shingle, whose area covers have increased (16.44%). A small change in the area of an island induces a greater change in its morphology. From the Island Change Ratio plot, it is observed that islands with an area cover of 10–30 ha have suffered loss of more area cover and undergone mobility from the core area. The constant changes undergone and the mobility exhibited by these islands clearly indicate that they are dynamic. The main factors that are responsible for the current changes in their extent and position are coral mining and sea level rise. Thus, it is clear that the low-lying reef islands of Gulf of Mannar are very dynamic, and that their expansion, loss in area cover and mobility are mainly controlled by coral mining and climate change. Immediate conservation action is necessary to prevent the islands from submergence.

Gulf of Mannar (GoM) in the southeast coast of India is known for its coral reefs and reef-associated biodiversity. Corals in GoM were affected to a significant extent by climate change-driven coral bleaching in 2016, and are currently recovering. After the bleaching mortality that corals suffered, the competition for space between corals and sponges is obvious in GoM. Rhabdastrella globostellata is a common marine sponge found overgrowing live coral colonies of the patch reefs in GoM at Pattinamaruthoor in March 2019. Underwater assessment of the reef revealed that 60.06% live coral cover was dominated by Acropora corals (81.91%). Among the acroporans 8.23% of colonies were found overgrown by R. globostellata. During the night dives the tiger cowrie Cypraea tigris was observed to feed on R. globostellata. From this observation the present study infers that C. tigris helps the corals fight these sponges, and concludes that tiger cowries should be protected and promoted to tackle climate change implications.

Tropical marine ecosystems are biologically diverse and economically invaluable. However, they are severely threatened from impacts associated with climate change coupled with localized and regional stressors, such as pollution and overfishing. Non-native species (sometimes referred to as ‘alien’ species) are another major threat facing these ecosystems, although rarely discussed and overshadowed by the other stressors mentioned above. NNS can be introduced accidentally (for example via shipping activities) and/or sometimes intentionally (for aquaculture or by hobbyists). Understanding the extent of the impacts NNS have on native flora and fauna often remains challenging, along with ascertaining when the species in question actually became ‘invasive’. Here we review the status of this threat across key tropical marine ecosystems such as coral reefs, algae meadows, mangroves, and seagrass beds. We aim to provide a baseline of where invasive NNS can be found, when they are thought to have been introduced and what impact they are thought to be having on the native ecosystems they now inhabit. In the appended material we provide a comprehensive list of NNS covering key groups such as macroalgae, sponges, seagrasses and mangroves, anthozoans, bryozoans, ascidians, fishes, and crustaceans.

Reduction in the impact of human-induced factors is capable of enhancing the environmental health. In view of COVID-19 pandemic, lockdowns were imposed in India. Travel, fishing, tourism and religious activities were halted, while domestic and industrial activities were restricted. Comparison of the pre- and post-lockdown data shows that water parameters such as turbidity, nutrient concentration and microbial levels have come down from pre- to post-lockdown period, and parameters such as dissolved oxygen levels, phytoplankton and fish densities have improved. The concentration of macroplastics has also dropped from the range of 138±4.12 and 616±12.48 items/100m2 to 63±3.92 and 347±8.06 items/100m2. Fish density in the reef areas has increased from 406 no. 250 m-2 to 510 no.250 m-2. The study allows an insight into the benefits of effective enforcement of various eco-protection regulations and proper management of the marine ecosystems to revive their health for biodiversity conservation and sustainable utilization., Graphical abstract Unlabelled Image

Additional file 2. Reference list for Additional file 1, plus additional table and figures.

The abundance and frequency of coral diseases have increased significantly during recent decades. The Gulf of Mannar in India has well-established coral reefs that have been affected by various climatic and non-climatic issues. Coral diseases, predominantly growth anomalies and white syndromes, have been reported on these reefs, but here we report the first observations of the ciliate disease brown band (BrB) on reefs of the Gulf of Mannar. Colonies with BrB lesions were found at 2 of the 21 islands surveyed, namely Poovarasanpatti and Puluvinichalli. Microscopic analysis of coral lesions confirmed the presence of zooxanthellae within ciliates indicating active feeding of coral tissues. The overall prevalence of BrB disease was 0.8% in Poovarasanpatti and 0.3% in Puluvinichalli. We only found Porites and Platygyra colonies infected with BrB ciliates even though Acropora, which is usually targeted by BrB, was the most abundant coral in our transects. This study extends the geographic range of BrB to the Gulf of Mannar and the host range to include Platygyra spp. The authors thank the Tamil Nadu Forest Department for permission to undertake this research and the Suganthi Devadason Marine Research Institute for providing research facilities. Scopus

Coral reefs around the world are undergoing severe decline in the past few decades. Mass coral mortalities have predominantly been reported to be caused by coral bleaching or disease outbreaks. Temporary hypoxic conditions caused by algal blooms can trigger mass coral mortalities though are reported rarely. In this study in Gulf of Mannar (GoM), southeast India, we report a significant coral mortality caused by a bloom of the ciguatoxic dinoflagellate Noctiluca scintillans during September–October 2019. Dissolved oxygen levels declined below 2 mg l−1 during the bloom causing temporary hypoxia and mortality (up to 71.23%) in the fast growing coral genera Acropora, Montipora and Pocillopora. Due to global climate change, more frequent and larger algal blooms are likely in the future. Hence, it is likely that shallow water coral reefs will be affected more frequently by episodic hypoxic conditions driven by algal blooms. More studies are, however, required to understand the mechanism of coral mortality due to algal blooms, impacts on community composition and the potential for subsequent recovery.

The study reports the colonization and ecological succession of epibenthic communities on perforated trapezoidal Artificial Reef (AR) modules deployed in Vaan Island of Gulf of Mannar, southeast India. These AR communities assist in building climate resilience among the coastal ecosystems, which have faced severe climate change impacts during the last decade. Based on wave dynamics modeling studies, 10,600 modules were deployed in two layers to protect the Island from beach erosion and to enhance biodiversity. Randomly selected modules were subject to continuous monitoring from 2015 to 2020 to understand the ecological succession of the epibenthic communities. Hard corals turned out to be the dominant epibenthic category with 76.01 no.module−1(SE ± 2.16) by 2020. A total of 37 coral species belonging to 16 genera were found inhabiting the modules at the end of five years. The other major epibenthic categories were molluscs, hydroids, sponges, ascidians, octocorals and echinoderms with a total density of 40.44 no.module−1(SE ± 1.41) by 2020. A couple of years after the deployment, when hard corals started to flourish on the AR modules, the density of other epibenthic categories began to decline. A comparison of the numbers of epibenthic organisms on the inner and outer layers of AR modules reveals that the inner layer has a larger epibenthic community. After five years, the AR modules have started to act as coral refugia and are expected to create reef ecosystem analogous to natural reefs. The modules also provide the much needed topographic complexity for the fish to move in. This planned deployment not only helped in reef recovery and enhancement of associated biodiversity in Gulf of Mannar, which is severely impacted by global climate change, but also would serve to enhance the climate resilience of the coral ecosystems by increasing the coral biomass and thereby restoring the ecological services, which help to sustain the livelihood of the dependent small-scale fisher folk.

Underwater survey was conducted to assess the accumulation and impact of marine debris in the reef areas of Gulf of Mannar in southeast India. A combination of roving diver technique and belt transect method was applied for the assessment, which was conducted during the period between February 2018 and March 2019. An estimated total reef area of 1152 m2 has been affected by marine debris. Abandoned fishing nets were found to constitute the major portion of 43.17 ± 5.48% of the marine debris. Live corals were found to be dominant substrates for marine debris with 39.11%. The average prevalence of coral colonies in contact with marine debris was 3.28 ± 0.27%. Prevalence of corals in contact with debris was very high in genus Acropora with 8.23 ± 1.29% followed by Montipora with 4.63 ± 1.29% due to their complex growth form. Of the corals in contact with debris, 47.56% were fragmented and 34% were found with tissue loss.

Seagrass beds are important marine ecosystems that provide significant ecological services. The global decline of seagrass beds is becoming severe due to the increasing pressure of human-induced factors and changing climatic conditions. Restoration of seagrasses is an evolving science that started in 1939. In this study, we report a remarkably successful restoration activity carried out in the Gulf of Mannar (GoM), Southeast India. This is the first wide-scale effort in Indian waters. After the initial experimentation, manual transplantation of seagrass sprigs was carried out near Vaan and Koswari islands in GoM. Transplantation was performed with PVC quadrats and jute twines in areas of 800 m2 in both the islands during February to May 2014. An increase from 16.4 ± 0.3 to 32.3 ± 0.6% in Vaan and from 15.1 ± 0.2 to 35.1 ± 0.9% in Koswari was observed in seagrass percentage cover during the period from June 2014 to May 2016. Area cover, shoot density, macrofaunal density and fish density increased at the restoration sites after the transplantation. Bottom trawling was found to be the most serious threat to the seagrass beds in these islands. This method of transplantation can be replicated in other areas of degraded seagrass in India to carry out wide-scale restoration of seagrasses.

We report the occurrence of coral recruits on bivalve shells of Modiolus phillipinarum, which served as an effective substrate for coral larval settlement within the reef area Gulf of Mannar, India.

Acropora white syndrome (AWS) and Terpios sponge overgrowth (TSO) are serious threats to coral communities in various regions; however, information on these 2 lesions in the Indian Ocean is much more limited than in the Indo-Pacific. The present study revealed the impact of these lesions on the Palk Bay reef, India, and covered an area of 7 km2. In total, 1930 colonies were permanently monitored to assess incidences of AWS and TSO and consequent mortality for a period of 1 yr. TSO affected 5 coral genera and caused 20.7% mortality; overall prevalence increased from 1.3% (n = 25) to 25.5% (n = 492). In contrast, AWS only affected Acropora colonies and caused a mortality of 8%; overall prevalence increased from 0.9% (n = 17) to 12.9% (n = 249). Year-round monitoring revealed an increasing trend of both AWS and TSO, followed by temperature rise. These results add to the known geographic distribution of these coral diseases and reveal the impacts of AWS and TSO on coral reefs in the Indian Ocean.

Terpios hoshinota is an encrusting cyanobacteriosponge which grows aggressively over live coral colonies and has been reported to undergo outbreaks which kill corals. In an underwater survey conducted on the reefs of Gulf of Mannar, an outbreak of this coral-invading sponge was witnessed for the first time. It was found invading approximately 5% of the Montipora divaricata colonies (n = 383) at 1 m depth in Vaan Island. The affected site had a high coral cover (85.13%) dominated by montiporids (79.97%). T. hoshinota was confirmed via underwater morphology and microscopic observation of distinctive lobed tylostylespicules. This sponge is reported to have caused significant damage to many reefs around the world and hence immediate steps are needed to protect the reefs of Gulf of Mannar. Further studies are needed to assess the extent of T. hoshinota invasion in Gulf of Mannar and progression rate over live coral colonies.

Malvan Marine Sanctuary (MMS), Maharashtra, India is rich in coral reefs and the associated resources, which provide livelihood for the people involved in fishing and tourism. The elevated sea-surface temperature triggered by climate change had caused the coral reefs around the world to undergo severe bleaching during 2014–2016. Scientists have declared this as the third global coral bleaching event. Two underwater surveys during December 2015 and May 2016 were conducted in MMS to assess the intensity and trend of coral bleaching. A high prevalence of coral bleaching, i.e. 70.93% (SD = 4.53) was recorded inside MMS during December 2015, with a mortality of about 8.38% (SD = 0.91). After a lapse of six months, corals were found to recover. This is borne out by the reduction in the bleaching prevalence to 6.77 ± 0.12% during May 2016. Climate change being a global issue, reduction in the local stressors such as fishing and tourism is highly recommended in order to allow the corals to recover and enable sustainable utilization of coral reef resources around MMS.

Fishing pressure on marine ecosystems has increased drastically all over the world, especially in developing countries and particularly in India. Shore seine, a traditional fishing method, involves bottom trawling which makes huge impact on the ecosystems and consequently on fishing yield. It was observed that important habitats such as seagrasses and coral reefs are affected severely by this shore seine operation. Most of the catch was observed to be juvenile in nature or very small in size. Immediate measures are needed to check this operation by creating awareness among the fishermen coupled with providing alternative livelihood options.

Information on the progression of coral diseases and transmission to live corals is scarce despite the fact that coral disease poses one of the most lethal threats to the survival of coral reefs. In this study, in situ progression rates of lesions similar to black band disease (BBD) and white band disease (WBD) were measured in different species of corals from the Gulf of Mannar (GoM) and Palk Bay, southeastern India, during the period between January and December of 2009. Maximum progression rates of 3 and 1.6 cm mo �1 for BBD and WBD, respectively, were observed during May, when the temperature exceeded 30°C. The annual progression rate was 10.9 and 4.9 cm yr �1 for BBD at GoM and Palk Bay, respectively. Significant variation in the pro- gression rate (p < 0.001) was observed between months in all the examined species. Significant correlation between temperature and disease progression rates for BBD (R 2 = 0.875, p ≤ 0.001) and WBD (R 2 = 0.776, p ≤ 0.001) was recorded. Rates of disease progression were higher in Palk Bay than in GoM. This could be attributed to the higher temperature coupled with higher anthro- pogenic activities in Palk Bay. Severe mortality was observed due to both BBD and WBD. No sign of recovery was noticed in the disease-affected colonies at either study site. Anthropogenic activ- ities should be checked, and further research on both the transmission and progression rate and role of the diseases in reef dynamics should be carried out to understand the causal factors in reef degradation and generate a plan to manage the reef properly.

The present study reports coral mortality, driven primarily by coral diseases, around Shingle Island, Gulf of Mannar (GOM), Indian Ocean. In total, 2910 colonies were permanently monitored to assess the incidence of coral diseases and consequent mortality for 2 yr. Four types of lesions consistent with white band disease (WBD), black disease (BD), white plaque disease (WPD), and pink spot disease (PSD) were recorded from 4 coral genera: Montipora, Pocillopora, Acropora, and Porites. Porites were affected by 2 disease types, while the other 3 genera were affected by only 1 disease type. Overall disease prevalence increased from 8% (n = 233 colonies) to 41.9% (n = 1219) over the 2 yr study period. BD caused an unprecedented 100% mortality in Pocillopora, followed by 20.4 and 13.1% mortality from WBD in Montipora and Acropora, respectively. Mean disease progression rates of 0.8 +/- 1.0 and 0.6 +/- 0.5 cm mo(-1) over live coral colonies were observed for BD and WBD. Significant correlations between temperature and disease progression were observed for BD (r = 0.86, R-2 = 0.75, p < 0.001) and WBD (R-2 = 0.76, p < 0.001). This study revealed the increasing trend of disease prevalence and progression of disease over live coral in a relatively limited study area; further study should investigate the status of the entire coral reef in the GOM and the role of diseases in reef dynamics.