Cystoseira sensu lato (s.l.) - encompassing the genera Cystoseira sensu stricto (s.s.), Ericaria and Gongolaria - is a diverse group of forest-forming brown macroalgae endemic to the warm-temperate North-east Atlantic. These algae have immense biogeographic and ecological significance and have been experiencing recent regional declines. Most Cystoseira s.l. display important morphological plasticity and can be confused with similar species. Therefore, species boundaries, geographic ranges and phylogenetic affinities remain imprecise for most. In the face of persistent taxonomic difficulties, several authors underlined the necessity for new molecular-based approaches, but studies so far lacked representativity, resolution and standardization. To fill in these gaps, in this study we sequenced a comprehensive collection of Cystoseira s.l. spanning its entire North-east Atlantic range for a similar to 1200 bp cox1 barcode, and sequenced selected individuals representing major genetic entities for a few additional plastid markers. Phylogeographic, phylogenetic and species delimitation methods revealed 27 Molecular Operational Taxonomic Units, including unaccounted cryptic diversity, and elucidated with unprecedented resolution species compositions and phylogenetic relationships within each genus. Some entities within the lineages Cystoseira compressa/humilis, Ericaria brachycarpa/crinita, E selaginoides and tophulose Gongolaria, as well as among free-living algae, conflicted with a priori taxonomic assignments, and required the redefinition, reinstatement and recognition of new taxa. For some, diagnostic mutations and biogeography were more useful for species identifications than morphological characters or conventional barcoding gaps. A few species showed narrow geographic ranges and others were the sole representatives of their respective lineages. Several sister-species showed Atlantic vs Mediterranean complementary ranges. phylogenetic signal of coxl was nevertheless insufficient to confidently determine patterns of lineage splitting in several lineages and species complexes and did not improve significantly with additional plastid markers. We discuss novel systematics and biogeography insights considering the advantages and shortcomings of the barcoding approach employed, and how this comprehensive baseline study can be expanded to address multiple questions still left unanswered.

The article presents the results of morphological and anatomical analysis of the Cystoseira sensu lato taxa in the flora of Ukraine, taking into account modern views on the nomenclature of the genus. Earlier, based on the morphological and molecular phylogenetic analysis, it was shown that along the Ukrainian shores the polyphyletic genus Cystoseira s. l. is represented by three taxa (Ericaria crinita f. bosphorica (Sauvageau) S.S.Sadogurska, J.Neiva et A.Israel., Gongolaria barbata (Stackhouse) Kuntze f. barbata and Gongolaria barbata f. repens (A.D.Zinova & Kalugina) S.S.Sadogurska). As the result of the present study, for these taxa, a set of morphological features of vegetative and generative organs was analysed (general thallus appearance, structure of the apex and branches, presence/absence of appendages, aerocysts, shape and size of receptacles, presence/absence of iridescence, conceptacles, shape and structure of gametangia), as well as an analysis of the anatomical structure of the thallus (cortex and meristoderm cells on a cross section) was carried out. The analysis showed that these taxa have a wide range of morphological variability, which makes their identification difficult. At the same time, some characteristics are diagnostic and allow reliably distinguish and identify the studied taxa of the genus Cystoseira s. l. Based on the analysis, we provided a description of the main morphological and anatomical c haracteristics of the taxa and proposed identification keys for taxa of the genus Cystoseira s. l. of the flora of Ukraine. The results of the study of morphological features and the range of macroalgae morphological variability are important for further taxonomic studies, as well as clarification of keys for their identification.

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.

Brown algae of the genus Cystoseira sensu lato form the most diverse and productive marine ecosystems throughout the Mediterranean Sea and have equal roles also in the Black Sea where they have been decreasing in the recent years. The taxonomy of Cystoseira s.l. taxa from the Black Sea is still not well understood, and questions arise when related taxa have to be delimited. In addition to morphological descriptions, this study provides for the first time molecular data of the Black Sea Cystoseira s.l. distinct morphologies as an additional tool to clarify their identities and phylogenetic affinities. The analysis of two mitochondrial markers (cytochrome oxidase subunit 1—COI, and 23S-tRNAVal intergenic spacer—mt-spacer) showed that Cystoseira s.l. specimens from the Black Sea belong to two recently resurrected genera, namely Gongolaria and Ericaria. Molecular data confirm the morphological identification of G. barbata, which is characterized by high morphological plasticity in the Black Sea. The morphological data presented in this study support the transition of G. barbata to the genus Gongolaria, which was previously proposed based solely on genetic data. For the Black Sea endemic taxon C. bosphorica, sequence divergence suggests conspecificity with Mediterranean Sea species E. crinita and E. barbatula. However, considering original morphological characteristics of the taxon, its geographical isolation, and endemism, the new combination Ericaria crinita f. bosphorica comb. nov. is proposed.

Cystoseira sensu lato (Ochrophyta) forests are important habitat formers in the Mediterranean Sea, but they have mostly been studied in the western basin where many species are under decline. In the eastern basin, where fewer species occur, Cystoseira rayssiae Ramon was described in the year 2000 as an endemic species based on morphological characteristics from herbaria samples collected on the Israeli coast. No further investigations have been conducted on this peculiar species since, but recently it has been recorded in contiguous Lebanon and outside the Mediterranean. Our work was aimed at confirming the taxonomic validity and endemic nature of this species, including its position among the recently split Cystoseira sensu stricto, Carpodesmia Greville and Treptacantha Kutzing genera, by sequencing the mitochondrial COI gene and by examining morphological characteristics in samples from three different sites in northern Israel. Notwithstanding considerable morphological plasticity, molecular analyses revealed a single unique COI sequence. Phylogenetic analyses show that Cystoseira rayssiae belongs to the resurrected genus Treptacantha and hence, the new combination Treptacantha rayssiae (Ramon) M.Mulas, J.Neiva & A.Israel, comb. nov., is proposed. Unique sequences and a restricted range support its Levantine-endemic status. Intriguing extra-Mediterranean reports from the Red Sea and the Persian Gulf are probably misidentifications rather than reflecting a disjunct distribution or recent invasion.

The information on distribution of the Cystoseira s. l. species in the Sea of Azov (SA) is presented based on the results of our own research and literature data. It is shown that the first record was made 100 years ago at Cape Khroni and at the Kazantyp Peninsula (Crimean coast). Currently, 22 reports from 18 sites have been registered along the Southern coast of the SA (17 along the Crimean coast and one at the Taman Peninsula). Treptacantha barbata (Stackh.) Orellana et Sansón (= Cystoseira barbata (Stackh.) C.Agardh) is the most widespread of the two species (17 sites). It forms dense communities with a biomass of 1.5–3.5 kg/m2. Carpodesmia crinita (Duby) Orellana et Sansón (= Cystoseira crinita Duby) was found in a small number only at two locations. It was found that the modern boundaries of the Azov fragment of the Cystoseira s. l. distribution range coincide with the boundaries of the Pre-Strait area of the SA, where salinity significantly higher, than in other areas. The distribution range has a linear configuration: it covers the Crimean coast from Cape Krasny Kut in the west to Cape Khroni in the east; is interrupted by the open water area of the Kerch Strait, and continues further on the Taman coast from Cape Achilleion to Cape Pekly. Outside the specified area, Cystoseira s. l. do not occur even if hard substrates are present. This shows, that salinity is limiting factor for the distribution of T. barbata and C. crinita in SA. In the past the system of marginal seas of the Mediterranean basin (to which SA also belongs) went through a series of transgressive and regressive phases. As a result, the boundaries, ecological conditions, and composition of biota have been changed several times in the entire basin and in its individual parts. The assumption was made that in the past Cystoseira s. l. species several times settled in the modern boundaries of the SA. The last invasion into the Pre-Strait area of SA (and inhabiting it up to the present time) should have happened in the Late Holocene 3.4–3.1 thousand years ago. Further alternation of regressions and transgressions, certainly, was accompanied by fluctuations of the distribution range boundaries, which either receded from the SA to the Kerch Strait, or again came back to its southern shores. At present, they can also fluctuate to a limited extent, following the salinity fluctuations. It is mostly relevant to the Crimean coast, where hard substrates are widespread. Considering the continuing salinization of the SA due to the climatically caused decrease in river runoff, the spread of Cystoseira s. l. species is possible on the Akmonay coast up to the top of the Arabat Bay. All this does not allow us to classify them as alien species in SA. However, in SA under modern conditions, the sustainable vegetation of the Cystoseira s. l. species with the formation of dense communities at a considerable distance from the Pre-Strait area is impossible. This must be taken into account when artificial reefs are installed and conservation strategies are developed.