Brown algae of the genus Cystoseira s. l. are key species in the Mediterranean basin, including the Black Sea and the Sea of Azov. In addition to the typical attached forms of Cystoseira sensu lato (Ericaria crinita f. bosphorica (Sauvageau) S.S.Sadogurska, J.Neiva et A.Israel, and Gongolaria barbata (Stackhouse) Kuntze), an unattached form was previously described for the Black Sea. The taxonomic status of this taxon remained uncertain. According to the results of a comparative morphological analysis, it is shown that the Black Sea unattached samples belong to the species Gongolaria barbata. But unattached thalli, collected in the Dzharylhach Bay in the seagrass meadows of Zostera marina L., have morphological differences. Thalli are 30–50 cm long, light brown or yellow-brown; the holdfast is absent, the main axis is reduced or, if present, very short and thin (2–4 mm). Lateral branches are thin (2–3 mm), smooth; their length is 15–30 cm. The ultimate branches are filamentous, sometimes with oval-shaped aerocysts, single or arranged in chains. Receptacles are rarely present, spindle-shaped, 8–12 mm in length, without spines, but with a sterile mucron at the end. Therefore, a new nomenclature combination is proposed: Gongolaria barbata f. repens (A.D. Zinova & Kalugina) S.S. Sadogurska comb. nov. It is shown that the nomenclature combination Cystoseira concatenata f. repens A.D.Zinova & Kalugina is invalid. In addition, it is shown that the unattached Gongolaria barbata f. repens is not synonymous with the species Cystoseira aurantia Kützing sensu Orellana et al. (2019), which authors previouslyrearranged to the genus Cystoseira s.s. Analysis of phylogenetic trees from the works by different authors showed that unattached samples from the Mediterranean Sea and the Atlantic Ocean do not belong to the Gongolaria barbata clade and may be unattached forms of other species.
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