Multi-Gene Analysis, Morphology, and Species Delimitation Methods Reveal a New Species of <em>Melanothamnus, M.</em> Coxsbazarensis sp. nov. (Rhodomelaceae, Ceramiales) for the Marine Red Algal Flora from Bangladesh

Turf-forming Polysiphonia-looking algae were collected from a small (&lt; 1.0 km2 area) Agarophyton tenuistipitata (Gracilariaceae, Gracilariales) farm on the East coast of the Bay of Bengal. DNA was extracted from silica gel-preserved specimens, and plastid-encoded rbcL, nuclear-encoded small subunit SSU, large subunit LSU, and universal plastid amplicon (UPA) were amplified and sequenced. Maximum likelihood (ML) and Bayesian inference were performed for the phylogenetic analysis. Four single-locus species delimitation methods (SDMs), namely the Generalized Mixed Yule-Coalescent (GMYC) method, a Poisson Tree Processes (PTP) model, the Automatic Barcode Gap Discovery (ABGD), and the Assemble Species by Automatic Partitioning (ASAP) method, were performed to segregate the putative species from other taxa in the Polysiphonia sensu lato clades. Our results revealed that rbcL had 1.4% interspecific genetic divergence, whereas LSU, UPA, and SSU had 1.6%, 2.5%, and 5.4% genetic divergence, respectively, from the nearest neighbors. Both comparative genetic and distinct morphological data revealed that the collected Bay of Bengal specimens comprise a species new to science. In addition, the above-mentioned SDMs supported the genetic data and segregated our specimens as Melanothamnus coxsbazarensis sp. nov. as a distinct species.