Unraveling the Genetic and Morphological Boundaries of the Kelps Eisenia cokeri and E. gracilis (Laminariales, Phaeophyceae) from Peru, and Their Phylogenetic Relationship with Eisenia from the Desventuradas Islands (Chile)

The kelp genus Eisenia Areschoug is represented by two species in the Southeast Pacific: Eisenia cokeri, distributed in Peru and Chile, and E. gracilis, endemic to Peru. However, the taxonomic distinction between these species has long been questioned, and it remains unclear whether E. cokeri is conspecific with the Northeast Pacific species E. arborea. To address these issues, we conducted an integrative taxonomic assessment combining morphological analyses, molecular phylogenetics, and molecular species delimitation approaches. Sampling for morphological and molecular analyses was carried out in subtidal zones at five localities along the Peruvian coast, and an additional locality in Chile was included for molecular analyses where E. cokeri was documented. Peruvian Eisenia species exhibited consistent morphological differentiation, including differences in thallus size, holdfast diameter, stipe modifications (e.g., longitudinal division and pseudostipe formation), frond division, frond margins, and surface roughness, as supported by morphometric analyses. Phylogenetic reconstructions based on mitochondrial and chloroplast markers, together with concordant results from multiple species delimitation methods (ABGD, GMYC, and bPTP), consistently recovered three independent evolutionary lineages corresponding to E. cokeri, E. gracilis, and a distinct lineage of Eisenia from the Desventuradas Islands (Chile). Our results confirm the taxonomic distinctiveness of E. cokeri and E. gracilis, demonstrate that E. cokeri is genetically distinct from E. arborea, and reveal the presence of a previously unrecognized lineage of Eisenia in the Southeast Pacific. These findings refine species boundaries within the genus and provide a robust framework for the management and conservation of these endemic, foundation, habitat-forming species.