Coral killing Sponges have the potential to overgrow live Corals, eventually killing the Coral polyps, and thus leading to an epidemic. The Cyanobacteria-symbiotic Sponge, Terpios hoshinota, also known as the Black Disease, was first reported from Guam in 1973, and later described from the coral reefs of the Ryukyu archipelago (Japan). It is identified by its gray to blackish encrustations. Since its first occurrence, it has been observed in several coral reef localities around the globe, viz., the Great Barrier Reef, Papua New Guinea, Taiwan, Philippines, Indonesia, South China Sea, Thailand, Palk Bay and the Gulf of Mannar on the southern tip of India, Maldives, and Mauritius.
In a paper published in the Journal of Threatened Taxa on 26 October 2020, Rocktim Ramen Das of the National Centre for Sustainable Coastal Management Forest and Climate Change in Chennai, Tamil Nadu, and the Graduate School of Engineering and Science at the University of the Ryukyus, and Chemmencheri Ramakrishnan Sreeraj, Gopi Mohan, Kottarathil Rajendran Abhilash, Vijay Kumar Deepak Samuel, Purvaja Ramachandran, and Ramesh Ramachandran, also of the National Centre for Sustainable Coastal Management, confirm that the species has further extended its habitat into the pristine atolls of the Lakshadweep Archipelago in the Arabian Sea, and requires urgent attention.
During the coral reef surveys conducted at Lakshadweep in November 2016, Terpios hoshinota was observed overgrowing on several colonies of Acropora muricata, Isopora palifera, Cyphastrea sp., Dipsastraea lizardensis and Porites lutea in the atoll encircling Bangaram and Thinnakara Islands. Out of 34 sites surveyed, six exhibited the presence of Terpios hoshinota. The Coral colonies on the atoll were patchy and the depth of the atoll varied between 2 and 12 meters. As depth increased, (i.e. deeper than 5 m) large boulder Corals were observed whereas the shallow regions (shallower than 5 m) had greater Coral diversity. Certain areas consisting of large Acropora beds, rocks, rubbles, and dead reef were also observed. The affected Corals displayed grayish/blackish encrustations of Terpios hoshinota forming a mat-like layer on live Corals taking the shape of the Coral in all cases. The osculum in the Sponge, a primary character with a radiating network of canals, was clearly visible and the thickness of the mat was less than 1mm. It was observed that the encrusting Sponges were propagating laterally and infecting the other live Coral colonies. Other associated communities such as Ascidians and Clams remain unaffected; calcareous Serpulid tubes were overgrown by the Terpios, although the Animal was unharmed. Furthermore, in some colonies along with Terpios hoshinota, Algal presence was noted, but the Sponge was absent in the colonies which were completely covered with Turf Algae. Environmental parameters assessed with a multiparameter water quality probe revealed that the area was unpolluted with an optimum level of dissolved oxygen (5.04-8.21 mg per litre), and low turbidity (0.3 to 0.8 Nephelometric Turbidity Units). Sea surface temperature during the survey was 28.2°-30.1°C. It is important to note that, Bangaram and Thinnakara is one of the few atolls in Lakshadweep where tourism is permitted, as a result, limited amounts of diving and other water-related recreational activities can be seen in the area.
Previous studies suspected that the outbreak of Terpios hoshinota is related to increased water turbidity or due to high anthropogenic stress/pollution its close proximity to mainland, as reported in the south eastern reefs of India (about 800 km from Lakshadweep), Guam, and in Green Island, Taiwan. A similar conclusion, however, cannot be applied in the case of Lakshadweep because of its isolated geography and with comparatively less anthropogenic activities. As a result, Das et al.'s observation contradicts this hypothesis and is more in line with the findings of Qi Shi, Gou Hui Liu, Hong Qiang Yan, and Hui Ling Zhang, who observed Terpios hoshinota outbreak in unpolluted areas of Yongxing Island (South China Sea), highlighting the difficulty in establishing a negative co-relationship between water quality and Black Disease outbreak. In terms of host selectivity, the Killer Sponge has affected several Coral species in different parts of the world and in the reefs of Palk Bay, it has affected all genera surveyed. In Vaan Island, Gulf of Mannar, the dominant genus Montipora was the most susceptible. Das et al.'s observation though could not reveal any specific host coral selectivity, Das et al. speculate that the dense branching Acropora Coral beds in site 3, 5 and 6 were more easily overgrown because the Killer Sponge prefers branching Corals as reported from Mauritius. Das et al. further conclude that the Coral composition in any specific location may play an important role in determining its host.
Terpios hoshinota is a belligerent contender for space and is known to overgrow corals from its base where it interacts with Turf Algae. Branching Acropora beds in site 3, 5 and 6 consisted both Algae (e.g. Dictyota sp.) and the Killer Sponge. Additionally, a massive Turf Algae which covered area of about 0.35 km in the Terpios hoshinota occurrence site highlights a complex ecological scenario. Such complex interactions between Sponges, Corals and Algae can be only understood through long term monitoring. Manuel González-Rivero, Laith Yakob, and Peter Mumby stated that Sponges can act as a potential group that can facilitate and influence Coral-Algal shifts by acting as a 'third antagonist' as observed in Glover’s atoll (Belize).
Based on their knowledge of the life history of Terpios hoshinota Das et al. hypothesize site 5/6 scenario as follows: (1) Terpios hoshinota invades and overgrows the Acropora beds (2) The Coral dies which is followed by the death of the Killer Sponge (3) Turf Algae takes over. Moreover, reports of Turf Algae being a dominant component in the atolls might indicate a faster transition. Globally Elevated sea surface temperature is a major threat to Coral Reefs, and the reefs of India, including the atolls, are no different. With reports indicating that elevated sea surface temperature has already depleted the Coral ecosystem of Lakshadweep, which was evident during 1998, 2010, and 2016, mass bleaching events, it can provide an opportunity for Sponges to invade. The dynamics of waterflow may also play a crucial role in this regard.
Das et al.'s findings confirm that the infestation of Terpios hoshinota on the coral colonies of Lakshadweep is currently limited to only Bangaram and Thinnakara as it was not observed in the other atolls surveyed. Although there is a possibility that the Killer Sponge could invade nearby atolls as seen in other regions, large-scale damage cannot be concluded at this stage. This is in fact the first documentation of Terpios hoshinota on the reefs of Lakshadweep and can be regarded as a baseline for subsequent studies. Further, to protect the reefs of Lakshadweep, a long term Coral health monitoring program is required which will allow us to understand the nature of occurrence, distribution, the impact and the causative factors of the Killer Sponge and to understand it’s larger threat to the reefs. Black Disease along with other Coral associated diseases needs enlarged emphasis according to which various Coral Reef management plans can be initiated.
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