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.

The occurrence, growth, external morphology and reproductive phenology of a Gelidium adriaticum settlement from the midlittoral zone in St. Simon Bay (Slovenian coast, Gulf of Trieste, northern Adriatic Sea) were investigated. The study was based on monthly sampling conducted for one year, from January to December 2023. The studied settlement was well developed in shaded rock crevices, forming dense turfs with a very patchy and irregular distribution. It was characterized by a slight predominance of tetrasporic thalli, with a ratio between the three reproductive stages (tetrasporic:sterile: cystocarpic) of 1.19:1.04:1.00. The mean values of the morphological characteristics studied (height and width of the thallus, width of the widest part of the thallus axis, width of the apical, middle and basal parts of the erect axes) varied between seasons and reproductive stages. In general, these values were lower in the cystocarpic thalli than those in the tetrasporic and sterile stages in all seasons.

Fucus virsoides is a glacial relict-species endemic to the Adriatic that was widespread from northern Italy to southern Albania. In recent decades, however, it has suffered an alarming decline. In this study, all available records were reviewed to reconstruct its historical and current occurrence and to investigate the possible causes of its decline. Comprehensive mapping revealed a continuing decline with a significant shift, leaving only about 20 fragmented populations. While the species is already classified as critically endangered in Albania, F. virsoides could be considered functionally extinct in Istria (Croatia), critically threatened with extinction in Italy and Montenegro and locally extinct in Slovenia. The status of the species in the rest of Croatia is still unclear. The remaining populations are located in areas characterised by low temperatures, low salinity and high nutrient concentrations. Long-term analyses of these key abiotic factors revealed profound changes that suggest a link to the species' decline. This study provides a thorough assessment of the overall status of F. virsoides and argues for its inclusion on the IUCN Red List. Immediate conservation measures are needed for the long-term survival of this species.

This study reports the occurrence of the non-native red alga Caulacanthus okamurae, previously unknown on the Slovenian coast (Gulf of Trieste, northern Adriatic Sea). The distribution of the species, its abundance, morphological characteristics and reproductive phenology are described. The study is based on seasonal sampling conducted during one year, from winter to autumn 2021, on two different substrates (limestone and sandstone) of the midlittoral zone, and showed a moderate variation in the biomass of C. okamurae populations between the winter-spring and summer-autumn periods. The native algal assemblages of the two substrates impacted by C. okamurae were characterized by generally limited species diversity and consisted mainly of small-sized species with simple morphology. During the study, in addition to non-fertile thalli of C. okamurae, tetrasporophytic thalli with tetrasporocysts and female gametophytes with cystocarps were found.

The results of a study on morphological features and reproductive phenology of the brown alga Cystoseira foeniculacea f. tenuiramosa (Ercegović) A. Gómez Garreta, M.C. Barceló, M.A. Ribera & J. Rull Lluch from the marine lagoon of Strunjan (Gulf of Trieste), are reported. Based on monthly samplings carried out during one year period, it was observed that this species shows considerable seasonal morphological variations. The Strunjan marine lagoon appears to be the only location of the Slovenian coast where population of this taxon occur. Based on the high associated biodiversity, we propose the inclusion of this population in the conservation management plans in the area.

This study reports the occurrence of the non-native red alga Caulacanthus okamurae, previously unknown on the Slovenian coast (Gulf of Trieste, northern Adriatic Sea). The distribution of the species, its abundance, morphological characteristics and reproductive phenology are described. The study is based on seasonal sampling conducted during one year, from winter to autumn 2021, on two different substrates (limestone and sandstone) of the midlittoral zone, and showed a moderate variation in the biomass of C. okamurae populations between the winter-spring and summer-autumn periods. The native algal assemblages of the two substrates impacted by C. okamurae were characterized by generally limited species diversity and consisted mainly of small-sized species with simple morphology. During the study, in addition to non-fertile thalli of C. okamurae, tetrasporophytic thalli with tetrasporocysts and female gametophytes with cystocarps were found.

The occurrence of unattached spherical (aegagropilous) populations of the green alga Cladophora prolifera in the marine lagoon of Strunjan (Gulf of Trieste) is reported. Species identification was based on molecular and morphological data, and the distribution, ecology and morphological features of the aegagropilous populations are described. The ball-shaped form of this species differs from the typical upright attached form found on open shores by the radial arrangement of the branches and the absence of cells and rhizoids with clear annular constrictions. The formation of the aegagropilous form of C. prolifera likely results from specific hydrodynamic conditions in the human-mediated environment of the marine lagoon, in combination with morphological development of the species. The Strunjan marine lagoon appears to be the only location in the Mediterranean Sea, or even worldwide, where aegagropilous forms of C. prolifera occur in large abundance. Based on the high associated biodiversity, we propose the inclusion of these populations to be considered in conservation management plans in the area.

This study deals with the density and patterns of head-orientation of the midlittoral limpet Patella caerulea L., along the Bay of Koper (Gulf of Trieste, North Adriatic). The aims were to determine the relationship in patterns of head-orientation and density of limpets on vertical, horizontal, wave exposed, and sheltered substrata. The results revealed that the density of limpets on vertical and exposed substrata was greater than those on horizontal and sheltered surfaces; the proportion of downward head-orientated limpets was higher in comparison with upward headoriented limpets; also the proportion of downward-facing limpets was higher on vertical exposed surfaces than on the others.

The authors report the first record of the red alga Polysiphonia carettia (Hollenberg, 1971) in the Adriatic Sea, collected from two loggerhead sea turtles Caretta caretta captured along the coast of the Savudrija peninsula (Croatia, Northern Adriatic Sea). The identification of the samples is based on both vegetative and reproductive tetrasporangial features. The morphological characters of the specimens studied are in agreement with previous reports of this species from other parts of the Mediterranean Sea.

The zonation of midlittoral mainly based on macroalgal communities was studied at six sites of the Bay of Koper (Slovenian south-east part of the Gulf of Trieste) in spring (April) and autumn (November) 2010. The study, using the random selection method, involved different types of substrate (limestone, marl and sandstone). It was surveyed from the upper midlittoral to the infralittoral fringe (from the mean higher high water to the mean lower low water), the upper limit of the occurrence of the brown macroalgae Cystoseira compressa. The zonation of the midlittoral zone was divided into three horizons (upper, middle and lower) by biotic (leader species) and abiotic (sea level) criteria. By using multivariate technique the main dissimilarity was found between the macroalgal communities of the upper and lower horizon. The species that most contributed to this difference was Blidingia minima. The main differences in species composition in different substrates were found between limestone and marl. The species Chaetomorpha linum mainly characterized such differences, while the species Fucus virsoides was the main contributor to dissimilarity among the middle horizons on limestone and sandstone.

In August 2003, dense turfs formed by the filamentous red alga Womersleyella setacea (Hollenberg) R. E. Norris were found in the open part of the Bay of Boka Kotorska, on the shore of Montenegro (southern Adriatic Sea). The habitat and morphology of the alga are described and the impact of the massive development of this species on the structure of the algal communities is discussed.

The algal assemblages of the northern Adriatic Sea have been studied intensively, but few quantitative data on their spatial and temporal dynamics are available. The intertidal assemblages of two coasts on the Gulf of Trieste were studied for a year, and the patterns of spatiotemporal variability were examined at several scales in space and time. Patterns for the whole assemblage and individual species were analysed by multivariate and univariate analyses. The assemblages of the two shores were relatively poor in species and characterised by different compositions. A moderate temporal variation was found, mostly related to differences between the periods winter-spring and summer-autumn. Patterns for individual species were mainly characterised by differences between the two shores (Polysiphonia opaca), interactions among shores and sampling dates (Fucus virsoides, Ulva compressa, Gelidium pusillum, Lophosiphonia obscura) and differences on the spatial scale of tens of metres (Ceramium species in aggregate, Cladophora albida, Ectocarpus siliculosus, Ulva laetevirens). The dissimilarity between the assemblages of the two shores was the emergent pattern, and differences in the texture and composition of the rocky substratum (marl and limestone, respectively) are considered perhaps the main causes of these differences. Future studies in this region should give more attention to this factor.

The Catenella caespitosa (Withering) L.M. Irvine community at two stations on the Slovenian coast of the northern Adriatic Sea was sampled monthly during 2002. The flora consisted of 18 species: seven Chlorophyceae, five Rhodophyceae, two Fucophyceae, and four Cyanobacteria. The diversity index indicated low community complexity at both stations (0.8 at station A and 0.5 at station B). The study also confirms the identification of the zonation of this community. A new sub-association of the upper mid-littoral zone Catenelletosum caespitosae, included in the Fucetum virsoidis Pignatti 1962 association, is described by the synecological method.

In the supralittoral and mediolittoral of the Slovene coast, three species of chthamalids were recorded. The article discusses the species occurrence, vertical distribution patterns on two different substrata (sandstone and limestone) and presents a table for identification of recorded species. IZVLEČEK V supralitoralu in mediolitoralu slovenskega obrežja so bile zabeležene tri vrste vitičnjakov. Članek obravnava pojavljanje vrst ter vzorce vertikalne razširjenosti na dveh različnih podlagah (peščenjaku in apnencu) ter podaja tabelo za identifikacijo zabeleženih vrst.