Morphological and molecular approach uncovers hidden cryptic diversity within <i>Polysiphonia sertularioides</i> species complex (Rhodomelaceae, Rhodophyta)

The use of a combination of molecular and morphological data in biodiversity assessments is increasing the knowledge of turf-forming seaweeds, showing that cryptic diversity appears to be widespread. Polysiphonia sertularioides is a turf-forming filamentous alga with a presumed global distribution based on morphological identifications. Published molecular data, however, suggest that it represents a species complex with at least four lineages. To investigate the extent of the P. sertularioides complex and species distribution, we analyzed molecular and morphological data from 75 specimens from Australia, Brazil, Europe, and South Africa. The rbcL gene sequences were used for phylogenetic inference and species delimitation using Generalized Mixed Yule Coalescent, Poisson Tree Processes, and Assemble Species by Automatic Partitioning methods. Our analyses revealed that the P. sertularioides complex comprises 14-21 species, with 10 of the species recovered in all analyses. The species were recovered in two main, highly supported clades, one composed of European (Mediterranean and Macaronesian) species and the other composed of species from Australia, Brazil, Europe, Panama, and South Africa. Statistical analyses of a morphometric dataset of nine characters revealed that the species represents a morphological continuum, and although species from different regions were distinct, species from a region were morphologically indistinguishable. Most of the recovered species were restricted to a biogeographical region, rarely spanning different continents, with 10 endemic species. Our results revealed a case of true cryptic diversity in the P. sertularioides complex, highlighting the need for studies of small-sized algae (<5 cm).