Multi-Locus DNA Barcoding and Ultrastructural Analyses Reveal the First Record of Sporolithon (Sporolithales, Rhodophyta) from the Northeastern Bay of Bengal

Background/Objectives: Non-geniculate coralline red algae of the order Sporolithales are ecologically important calcium carbonate producers in tropical and subtropical seas globally; however, molecular surveys of this group in the Bay of Bengal (BoB) are entirely lacking, and no species of Sporolithon Heydrich has been documented from this embayment. Here we report the first occurrence of Sporolithon from Saint Martin Island, the sole coral island of Bangladesh, at the northernmost margin of the BoB. Methods: Seven specimens encompassing free-living rhodoliths to encrusting growth forms were collected from intertidal and shallow subtidal habitats in March 2024. Species identification was assessed by combining scanning electron microscopy (SEM) images with a six-locus molecular dataset comprising three plastid-encoded markers (rbcL, psbA, and UPA), the mitochondrial cytochrome c oxidase subunit I 5′ region (COI-5P), and two nuclear-encoded ribosomal markers (LSU and SSU). Results: Both maximum likelihood and Bayesian phylogenetic analyses resolved all BoB specimens within a well-supported clade of Sporolithon indopacificum Maneveldt, Gabrielson & Kangwe, with 0.00–0.20% concatenated rbcL + psbA sequence divergence from type specimens from Tanzania and previously published sequences from Fiji, China, and Guam (USA). SEM confirmed morpho-anatomical features concordant with the original species description, including a free-living rhodolith habit, monomerous construction, and tetrasporangial compartments with apical pore plugs. Conclusions: This record extends the confirmed range of S. indopacificum into the northeastern Indian Ocean, represents the first documented Sporolithon occurrence in Bangladeshi waters, and highlights the value of combining SEM with multi-locus barcoding for coralline algal surveys in undersampled tropical regions.