First Report of Red Alga Halymenia sp. Overgrowth of Stony Corals at Shallow Water Coral Reefs of Palk Bay Coast, India

A fleshy red alga of the genus <i>Halymenia</i> C. Agardh rapidly attained alarming levels of benthic coverage, including stony corals, in shallow water reefs of Palk Bay, during March 2024 and posed a serious threat to the survival of reef-building corals and their associated marine biota. This finding is considered as the first report of <i>Halymenia</i> sp. blooms in the coral reefs of Palk Bay regions of Tamil Nadu. The study was conducted at the Vedhalai reef of Palk Bay, which is well known for its live coral cover. During the study, average live coral cover was recorded as 34.5±15.6%. In this case, the frequency of occurrence of <i>Halymenia</i> sp. cover on live and dead forms of benthic communities accounted for 80.19% (<i>n</i>=313). This macroalga invaded live colonies of four coral genera and dead corals. After a thorough investigation of 313 colonies, it was found that <i>Halymenia</i> sp. affected 38.65% of live colonies and 61.35% of dead corals. Among the coral species, <i>Porites</i> (60.94%), <i>Favites</i> (66.67%), and <i>Dipsastraea</i> (71.43%) were highly affected by this foliose gelatinous red alga. Macroalgal dominance in live reefs causes severe harm to stony corals. It reduces the coral species' recovery capability due to overgrowing benthic substrate, prevents fresh recruitment to the reefs, and promotes their microbial infection, as evidenced by previous studies.