Genetic divergence between French Atlantic populations of the red alga Gelidium pulchellum: evidence of a cryptic species complex and an undescribed distinct species

Recent interest in the cultivation of agar-producing seaweeds has renewed focus on species delineation within the genus Gelidium . Gelidium pulchellum , a species with potential for aquaculture, has undergone several taxonomic reassessments in recent decades. Its extensive distribution along the French coastline made it a compelling subject for further investigation. In this study, we aimed to reassess the distribution and genetic diversity of G. pulchellum along the Atlantic coast of France using a DNA barcoding approach targeting two organellar markers: mitochondrial cytochrome oxidase subunit I (COI-5P) and chloroplast ribulose-1,5-bisphosphate carboxylase/oxygenase ( rbcL ). Phylogenetic analysis placed the examined specimens into four independent clades, representing taxonomic entities displaying strong genetic divergence. Our results reveal that the current taxonomic treatment of G. pulchellum underestimates its genetic diversity. At least three distinct genetic lineages were identified under the name G. pulchellum with Lineages A and B, co-occurring in Brittany and Lineage D ( rbcL only) occurring in Spain. We also observed that morphologically similar specimens restricted to the coast of the Basque Country were found to be an undescribed sister species of another Gelidium species, G. spinosum .