No abstract

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.

Intensive underwater surveys have been conducted to assess the extent of coral bleaching and subsequent mortality in the Gulf of Mannar between March and October 2016. The extent of bleaching was 23.92%  10.55% during the period between March and June 2016, and the live coral cover was drastically reduced to 22.69  9.07% during October 2016 with a mortality of 16.17  8.46%. Fast-growing coral forms, including the genera Acropora, Montipora and Pocillopora were most affected, not only by bleaching but also by severe mortality. Boulders, including the genera Porites, Favia and Favites were found to be resistant to bleaching. During the bleaching period, water temperature was between 31.2C and 32.6C. The current bleaching is in alignment with the third global coral bleaching event which occurred between 2014 and 2017. Management interventions, including protection and rehabilitation using the native resistant coral species will not only help in the recovery process, but also increase the live coral cover.

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.

Sponges play an important role in biogenic coral-reef degradation, and it is acknowledged that elevated levels of sponge erosion commonly indicate poor health of coral-reef environments. An increase in the abundance of coral-excavating sponge has been reported from several locations, a development that may move coral-reef carbonate budgets increasingly towards net erosion. The role of coral-excavating sponges on Indian reefs has not been studied in as much detail as elsewhere. The present paper describes the observation of a coral-excavating sponge from the family Clionaidae. This brown, endolithic sponge formed a coherent thin layer over the surface of the substratum and had a spicule complement of tylostyles and spirasters. Therefore it belongs to the Cliona viridis species complex, which, as a group, is widely distributed and commonly displays high bioerosion rates. Accurate identification will require molecular studies and is presently deferred. The sponge was found excavating only Turbinaria mesenterina colonies of Gulf of Mannar. Within the surveyed area of 60m2, 38.58% of T. mesenterina colonies were found to be invaded by the sponge. Targeted long-term studies across a larger spatial scale are warranted to assess the role of this sponge in more detail, and whether its abundance changes over time.