Incursion of the killer sponge Terpios hoshinota Rützler & Muzik , 1993 on the coral reefs of the Lakshadweep archipelago , Arabian Sea

Our study documents the outbreak of a coral-killing sponge Terpios hoshinota in the coral reefs of Lakshadweep archipelago and highlights that it has further extended its territory into the isolated atolls of Arabian Sea and maybe a growing threat to the existing coral reefs in the region.

and our present observation, confirms that the species has further extended its habitat into the pristine atolls of Lakshadweep (Image 1) (Arabian Sea) and requires urgent attention.
During the coral reef surveys conducted at Lakshadweep in November 2016, T. hoshinota was observed overgrowing on several colonies of Acropora muricata, Isopora palifera, Cyphastrea sp., Dipsastraea lizardensis and Porites lutea (Image 2 and 3) in the atoll encircling Bangaram and Thinnakara Islands. Out of 34 sites surveyed, six exhibited the presence of T. hoshinota (Image 1). The coral colonies in atoll were patchy and the depth of the atoll varied between 2 and 12 meters. As depth increased, (i.e., >5m) large boulder corals were observed whereas the shallow regions (<5m) 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 T. 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 Image 1. Bangaram & Thinnakara atoll (Inset -red star) (QGIS 3.6).
radiating network of canals, was clearly visible and the thickness of the mat was less than 1mm (Image 2). 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 but interestingly the calcareous serpulid tubes, though overgrown by the Terpios, the animal was unharmed (Elliott et al. 2016) (Image 2d). Further, in some colonies along with T. hoshinota, algal presence was noted (Image 3a) but the sponge was absent in the colonies which were completely covered with turf algae (Image 3b). Environmental parameters assessed with a multiparameter water quality probe (YSI optic probe no. 15K100034) revealed that the area was unpolluted with an optimum level of dissolved oxygen (5.04~8.21 mg/l), and low turbidity (0.3 to 0.8 NTU). Sea surface temperature (SST) 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.

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Previous studies (Rützler & Muzik 1993;Thinesh et al. 2015) suspected that the outbreak of T. 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 Our observation though could not reveal any specific host coral selectivity, we can speculate that the dense branching Acropora coral beds (ACB) in site 3, 5 and 6 were more easily overgrown because the killer sponge prefers branching corals as reported from Mauritius (Elliott et al. 2016). We would further conclude that the coral composition in any specific location may play an important role in determining its host.
T. hoshinota is a belligerent contender for space (Plucer-Rosario 1987) and is known to overgrow corals from its base where it interacts with turf algae (Elliott et al. 2016). Branching Acropora beds in site 3, 5 and 6 (Image 3a) consisted both algae (e.g., Dictyota sp.) Our findings confirm that the infestation of T. 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 (Bryan 1973;Reimer et al. 2011), large-scale damage cannot be concluded at this stage. This is in fact the first documentation of T. 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.  www.threatenedtaxa.org

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