Sara D'AMbros Burchio

Cystoseira sensu lato (s.l.) (Fucales, Phaeophyceae) form structurally complex marine forests along the warm-temperate coasts of the Mediterranean Sea and the eastern Atlantic Ocean. They are experiencing widespread decline, yet their conservation and restoration are hindered by taxonomic uncertainty. Within Cystoseira sensu stricto (s.s.), species boundaries among taxa traditionally referred to as C. compressa, C. foeniculacea, C. humilis (including the C. canariensis morphotype), and C. pustulata have long been debated due to the high plasticity and partial overlap of diagnostic morphological traits. Here we reassess species boundaries within the Cystoseira s.s. complex using an integrative approach combining morphology, ecology, and multilocus genetic data. We collected specimens from 20 sites spanning the Mediterranean–Atlantic distribution. Genomic DNA was extracted from 220 specimens and Sanger sequencing generated 243 new sequences (ITS2: n = 99; cox1: n = 88; rbcL–rbcS: n = 56). We inferred single-locus and concatenated phylogenies, reconstructed haplotype networks, and applied multiple single-locus species delimitation approaches to test the robustness of inferred boundaries. We consistently recovered two primary lineages corresponding to C. foeniculacea and a C. compressa complex, the latter including specimens historically identified as C. pustulata and the C. humilis with the C. canariensis morphotype. Within an integrative framework that prioritises cross-locus concordance and diagnosability, we recognise two species-level lineages in Cystoseira s.s. and treat the morpho-ecologically coherent entities within C. compressa at the variety rank. Accordingly, we propose the new status and combination Cystoseira compressa var. pustulata (Ercegović) D’Ambros Burchio & Falace, comb. et stat. nov.

Gongolaria barbata (Sargassaceae, Fucales) is a widespread species for which several infraspecific taxa have been described, indicating its polymorphism. This study contributes to the understanding of the molecular, nomenclatural, morphological and ecological aspects of G. barbata in the Mediterranean and sheds light on the infraspecific diversity and its implications for the taxonomy of this species. Molecular analyses were performed using sequencing of the mitochondrial cox1 gene on both haptophytic and pleustophytic forms from different sites in the Adriatic and Tyrrhenian Seas. Vegetative and reproductive morphology was studied on thalli samples from the Adriatic. Our results showed that there are different morphotypes within G. barbata populations related to specific environmental conditions, suggesting infraspecific variation. In contrast, molecular analyses showed no differences between samples, regardless of whether individuals are growing “attached” to a substrate or “unattached”. We also discussed the taxonomic status and nomenclatural issues related to certain infraspecific taxa previously proposed for G. barbata. In particular, the confusion surrounding Cystoseira aurantia is clarified.