Womersleyella Setacea Research Articles

Standardization of ALEX index for the ecological quality assessment under European Framework Directives in Mediterranean sensitive habitats affected by biological invasions

Abstract This paper aims at providing a standardized procedure to assess the ecological quality of Mediterranean marine habitats in relation to biological invasions by macroalgae through the employment of the ALEX index. Available data from previous ALEX applications were used to optimize the sampling design and to propose a standardized procedure. The procedure was applied to the main Mediterranean sensitive habitats, coralligenous reefs and meadows of the endemic seagrass Posidonia oceanica, at two sites in the north‐western Mediterranean Sea. A total of 73 native macroalgal taxa and four introduced taxa (the Chlorophyta Caulerpa cylindracea and the turf forming Rhodophyta Acrothamnion preissii, Womersleyella setacea and Asparagopsis spp.) were identified. Coralligenous reefs exhibited a higher abundance of both C. cylindracea and introduced turf species, while P. oceanica meadows were mostly affected by W. setacea and A. preissii. The application of ALEX for different years on coralligenous reefs highlighted the consistency of the index over time in the absence of further invasions. The use of ALEX index with other ecological quality indices showed similar patterns in both habitats. The concurrent assessment of the ecological quality of ecosystems by generic indices and the level of invasion through ALEX index may be a useful approach for answer to requirements of the European Directives and to plan even more complete and effective conservation programmes.

New records of introduced species in the Mediterranean Sea (November 2023)

This collective article presents new information about 15 introduced taxa belonging to five phyla: one Rhodophyta, one Chlorophyta, one Mollusca, one Annelida, two Arthropoda, and nine Chordata (one Ascidiacea and eight Osteichthyes). The records refer to eight Mediterranean countries and extend from the Alboran Sea to the Levantine Sea as follows: Spain: first record of the African hind Cephalopholis taeniops for the Alboran Sea, and a further record of the Monrovia surgeonfish Acanthurus monroviae, extending its distribution northwards in the Western Mediterranean. Italy: an additional record of the squat lobster Scyllarus subarctus based on its nisto stage, new records of the lionfish Pterois miles in the north-western Ionian Sea, first records of the bivalve Fulvia fragilis for the Italian Adriatic coast, and a record of the amphipod Ptilohyale littoralis in the northern Adriatic Sea, also representing the first report for the Mediterranean Sea. Montenegro: first record of the non indigenous ascidian Ciona robusta. Albania: first record of the red cornetfish Fistularia petimba. Greece: first record of the cryptogenic polychaete Alitta succinea in association with ship fouling, suggesting the possibility of a non-indigenous origin of Mediterranean populations of this species, and first record of the Seychelles dragonet Synchiropus sechellensis for the Saronikos Gulf. Türkiye: first record of the red alga Womersleyella setacea, report of an abundant population of the Indian twospot cardinalfish Cheilodipterus novemstriatus in the Turkish Aegean Sea, and first record of Synodus randalli, also corresponding to the first report for the Mediterranean Sea. Syria: first record of the blenny Istiblennius cf. meleagris. Israel: report of an algal bloom of the green alga Codium parvulum, and first record of Synchiropus sechellensis.

Spring composition of the macroalgal vegetation of a small offshore island in the north-western Mediterranean (Gallinara Island, Ligurian Sea)

Gallinara Island, a small island located 1.5 km off the shore of Liguria (Italy, north-western Mediterranean Sea) was included in a list of proposed Marine Protected Areas (MPA) in the early 90s. Since then, its benthic assemblages have been studied in detail and the main macrophytic communities have been mapped. A detailed assessment of its benthic macroalgal flora, however, has never been made. Gallinara was visited in the course of 5 consecutive years and its macroalgal flora was studied based on collections made by snorkelling and SCUBA diving. Overall, 141 macroalgal taxa were collected and identified (23 Chlorophyta, 94 Rhodophyta, 24 Ochrophyta); 91 of them represent new records for the island. One of the most notable new records is the non-indigenous red alga Womersleyella setacea, previously unreported from the island and widely distributed, particularly on the south-eastern shore. Observations made in the course of the surveys confirm the rarefaction of some large-sized brown algae (particularly Sargassum vulgare) but indicate also that others previously reported as rare (Cystoseira compressa, Dictyopteris polypodioides) are still common on the island.

Monitoring non-indigenous macroalgae in a Mediterranean MPA: Lessons from a short-temporal variability of pristine habitats invasion

Since there is no local management for Marine Protected Areas (MPAs) to prevent the establishment of macroalgae NIS, successful efforts to contrast their spread probably should concentrate in the conservation of resistant habitats. The study aimed to evaluate the role of depth and habitat in the spread of macroalgal non-indigenous species (NIS). A multifactorial sampling design was employed to compare the abundance of NIS macroalgae over a one-year period across different substrates and depths within a Mediterranean MPArea. Moreover, the ecological quality of macroalgal assemblages in relation to invasion was assessed through the ALien Biotic IndEX (ALEX). The NIS macroalgae found in the MPA, Caulerpa cylindracea, Acrothamnion preissii, Womersleyella setacea and Falkenbergia sp., were overall quite low in abundance and, accordingly, the ALEX index had generally high values. The substrate, depth and time significantly affected NIS abundance; however, the lack of significant interannual variability (December 2018–December 2019), both in pooled and separated NIS abundance, suggested that the occurrence of these four NISs will no further expand in the MPA at the scale of habitat. The highlighted patterns in NIS abundance suggest the importance of detailed sampling designs of monitoring taking into consideration different habitats and depths and using suitable sampling methods to assess NIS establishment and spread, such as those required by international directives.

ALEX index enables detection of alien macroalgae invasions across habitats within a marine protected area

A modified version of the ALien Biotic IndEX (ALEX) has been recently proposed to evaluate biological invasions in macroalgal assemblages. ALEX was applied in a Marine Protected Area where a recreational-fishing port is present testing the following hypotheses: ALEX increases with the distance from the port, it changes between the two directions off the port and it changes among three different habitats: Cystoseira beds, algal turf and dead matte of the seagrass Posidonia oceanica. A total of 78 native macroalgal taxa and 4 introduced species were found, the Chlorophyta Caulerpa cylindracea and the Rhodophyta Apoglossum gregarium, Acrothamnion preissii and Womersleyella setacea. All study sites were in high quality status highlighting that the assemblages investigated were at an early stage of NIS invasion. However, ALEX detected different values among conditions and habitats within the MPA, suggesting a local dynamics of NIS spread and different resistance to invasion of the investigated habitats.

Introduced marine macroflora of Lebanon and its distribution on the Levantine coast

The marine macroflora introduced into Lebanon was studied through an analysis of historical data and field studies carried out since 1991. A total of 29 introduced species have been listed: 3 Chromobionta, 13 Rhodobionta, 12 Chlorobionta and 1 Streptobionta. Among them, Caulerpa taxifolia var. distichophylla, Spatoglossum variabile and Womersleyella setacea are recorded for the first time from Lebanon. Among them, Asparagopsis taxiformis, C. taxifolia var. distichophylla, Cladophora patentiramea, Codium arabicum, Codium parvulum, Galaxaura rugosa, Halophila stipulacea, Hypnea cornuta, Laurencia cf. chondrioides, Lophocladia lallemandii, Stypopodium schimperi, Ulva lactuca and Womersleyella setacea are potentially invasive species, and several of them already occupy extensive stretches of the Lebanese coast. The distribution of species on the Levantine coast, their dynamics and the current status of their populations are discussed.

An updated overview of the marine alien and cryptogenic species from the Egadi Islands Marine Protected Area (Italy)

An updated overview of the marine alien and cryptogenic species recorded in the Egadi Islands Marine Protected Area (Tyrrhenian Sea, Italy), based on relevant publications, grey literature and unpublished data, is presented and discussed. Altogether, 17 species (14 aliens and 3 cryptogenic) belonging to five taxa are present in the area: Rhodophyta (Antithamnionella elegans, Asparagopsis armata, Asparagopsis taxiformis, Bonnemaisonia hamifera, Botryocladia madagascariensis, Ceramium strobiliforme, Laurencia caduciramulosa, Lophocladia lallemandii, Neosiphonia harveyi, Womersleyella setacea), Chlorophyta (Caulerpa cylindracea and Caulerpa taxifolia), Mollusca (Aplysia dactylomela), Arthropoda (Percnon gibbesi) and Cordata (Fistularia commersonii, Kyphosus vaigiensis and Stephanolepis diaspros). The overall Mediterranean presence of one further taxon recorded from the area, Zygochlamys patagonica (Mollusca), is here confuted since recent findings of living specimens with certain data are missing. One loose valve of “Pinctada imbricata radiata (Mollusca)” was found in 2010 at Favignana, but we did not include it in the list of alien species, pending further finding of living or dead specimens. Unpublished distributional data are reported for some of these species, and their known distribution in the Aegadian Archipelago is mapped. Records of P. gibbesi (a conspicuous number of specimens) constitute the first for the archipelago. All the species included in the present paper were recorded after the Egadi Islands Marine Protected Area was established, with the first record dating back to 2000. This is presumably due to the recent increase in research programs on marine reserves. Finally, we provide the establishment status of all the recorded species updated to 2015, and discuss why unaided spread (whether alien or native) and/or transport-stowaway constitute the more plausible vectors of introduction.

Updated review of marine alien species and other ‘newcomers’ recorded from the Maltese Islands (Central Mediterranean)

An updated review of marine alien species and other ‘newcomers’ recorded from the Maltese Islands is presented on account of new records and amendments to a previous review in 2007. Species were classified according to their establishment status (‘Questionable’, ‘Casual’, ‘Established’, ‘Invasive’) and origin (‘Alien’, ‘Range expansion’, ‘Cryptogenic’). A total of 31 species were added to the inventory, while 6 species have been removed, bringing the total number of species to 73. Of these, 66 are considered to be aliens (or putative aliens but with uncertain origin) with the remaining 7 resulting from range expansion. Six records are considered to be questionable and hence unverified. For verified records, the dominant taxonomic groups are Mollusca (represented by 21 species) and Actinopterygii (15 species), followed by Crustacea (8 species) and Rhodophyta (7 species). Eight of these species (aliens: Caulerpa cylindracea, Lophocladia lallemandi, Womersleyella setacea, Brachidontes pharaonis, Percnon gibbesi, Fistularia commersonii, Siganus luridus; range extender: Sphoeroides pachygaster) are considered to be invasive. The introduction pathway for 30 species is unknown. Amongst the alien species, ‘Shipping’ is the most common introduction pathway, followed by ‘Secondary dispersal’ from elsewhere in the Mediterranean Sea. An increasing trend in the number of alien marine species reported from the Maltese Islands is evident, with a peak of 22 species recorded during the last decade (2001–2010). A discussion on the rationale for including range-expanding species in national inventories of recent arrivals, and in the analysis of trends in records from the Maltese Islands, is included. In particular, the general warming trend of Mediterranean surface waters appears to be facilitating the westward spread of thermophilic alien species from the Eastern to the Central Mediterranean, and the eastward range expansion of tropical and subtropical Eastern Atlantic species.

Review of alien marine macrophytes in Tunisia

In the present study, the list of alien marine macrophytes introduced into Tunisia was updated in the light of available data and new observations. A total of 27 alien marine macrophytes have been recorded so far from Tunisia: 18 Rhodophyta, 3 Ochrophyta, 5 Chlorophyta and 1 Magnoliophyta. For each species, the locality (-ies), the year (or) period and the source of the first observation in Tunisia are given. The distribution and the status (casual, cryptogenic, established or questionable) of species in Tunisia were evaluated and, where appropriate, discussed. Among them, Hypnea cornuta is reported for the first time from Tunisia. Fourteen alien marine macrophytes are established, whereas seven cryptogenic and two casual species require further investigation. Eleven species are considered as invasive or potentially invasive in the Mediterranean Sea: Acrothamnion preissii, Asparagopsis armata, A. taxiformis Indo-Pacific lineage, Hypnea cornuta, Lophocladia lallemandii, Womersleyella setacea, Caulerpa chemnitzia, C. cylindracea, C. taxifolia, Codium fragile subsp. fragile and Halophila stipulacea. Finally, the case of four questionable species is also discussed.

Effects of non-native turf-forming Rhodophyta on mobile macro-invertebrate assemblages in the north-western Mediterranean Sea

The study compared mobile macro-invertebrate assemblages invaded by the two turf-forming invasive macroalgae Womersleyella setacea and Lophocladia lallemandii with non-invaded assemblages. The study was carried out at Pianosa Island, in the Tuscan Archipelago National Park, within the depth ranges where the two invasive species showed high abundance: 35 m depth for Womersleyella setacea and 10 m depth for Lophocladia lallemandii. Results showed significant differences between invaded sites and non-invaded ones for the two turf-forming Rhodophyta. Invaded assemblages showed a lower species richness and a reduction of decapods and molluscs and an increase of polychaetes compared with non-invaded ones. Amphipods showed a drastic reduction in abundance in sites invaded by W. setacea, while they appeared less influenced by the presence of L. lallemandii. Womersleyella setacea showed more severe effects than L. lallemandii, causing deep changes in the structure of assemblages and a reduction of beta diversity of the system.

Historical data reveal power‐law dispersal patterns of invasive aquatic species

Understanding how invasive species spread is of particular concern in the current era of globalisation and rapid environmental change. The occurrence of super‐diffusive movements within the context of Lévy flights has been discussed with respect to particle physics, human movements, microzooplankton, disease spread in global epidemiology and animal foraging behaviour. Super‐diffusive movements provide a theoretical explanation for the rapid spread of organisms and disease, but their applicability to empirical data on the historic spread of organisms has rarely been tested. This study focuses on the role of long‐distance dispersal in the invasion dynamics of aquatic invasive species across three contrasting areas and spatial scales: open ocean (north‐east Atlantic), enclosed sea (Mediterranean) and an island environment (Ireland). Study species included five freshwater plant species, Azolla filiculoides, Elodea canadensis, Lagarosiphon major, Elodea nuttallii and Lemna minuta; and ten species of marine algae, Asparagopsis armata, Antithamnionella elegans, Antithamnionella ternifolia, Codium fragile, Colpomenia peregrina, Caulerpa taxifolia, Dasysiphonia sp., Sargassum muticum, Undaria pinnatifida and Womersleyella setacea. A simulation model is constructed to show the validity of using historical data to reconstruct dispersal kernels. Lévy movement patterns similar to those previously observed in humans and wild animals are evident in the re‐constructed dispersal pattern of invasive aquatic species. Such patterns may be widespread among invasive species and could be exacerbated by further development of trade networks, human travel and environmental change. These findings have implications for our ability to predict and manage future invasions, and improve our understanding of the potential for spread of organisms including infectious diseases, plant pests and genetically modified organisms.

Impact of an invasive alga (Womersleyella setacea) on sponge assemblages: compromising the viability of future populations

The effects of invasive species on native fauna are understudied, even though their consequences should be taken into consideration for the proper conservation and management of marine systems. Furthermore, bioinvasions may have greater consequences if they affect key structural species with slow dynamics such as marine sponges. We propose that reproductive output could be used as a potential early warning signal to detect possible future changes in population trends of long-lived species (i.e. sponges) as a result of biological invasions. The aim of this study was to investigate the effects of invasive algal (Womersleyella setacea) overgrowth on sponge reproduction by comparing the presence of reproductive elements (spermatic cysts, oocytes, embryos, and larvae) in sponges covered by a thick carpet of the invasive algae and in sponges dwelling in the same habitat but without the invasive algae. Three variables were calculated to assess the impact of the invasive alga on sponge reproduction: the reproductive effort, the proportion of individuals in reproduction, and the size of the reproductive structures. We studied eight sponge species representing the main components of the deep rocky reefs of the area. Our results showed that W. setacea had a strong negative effect on sponge reproduction in six out of eight sponge species studied, with lower and even nil reproductive structures on the sponges subjected to the algal overgrowth. Thus, considering that sexual reproduction is necessary for the persistence of most sponge populations, a significant and constant reduction of the reproductive effort may compromise their viability and affect future trends in these benthic systems.

Marine Invasion in the Mediterranean Sea: The Role of Abiotic Factors When There Is No Biological Resistance

The tropical red alga Womersleyella setacea (Rhodomelaceae, Rhodophyta) is causing increasing concern in the Mediterranean Sea because of its invasive behavior. After its introduction it has colonized most Mediterranean areas, but the mechanism underlying its acclimatization and invasion process remains unknown. To understand this process, we decided i) to assess in situ the seasonal biomass and phenological patterns of populations inhabiting the Mediterranean Sea in relation to the main environmental factors, and ii) to experimentally determine if the tolerance of W. setacea to different light and temperature conditions can explain its colonization success, as well as its bathymetric distribution range. The bathymetric distribution, biomass, and phenology of W. setacea were studied at two localities, and related to irradiance and temperature values recorded in situ. Laboratory experiments were set up to study survival, growth and reproduction under contrasting light and temperature conditions in the short, mid, and long term.Results showed that, in the studied area, the bathymetric distribution of W. setacea is restricted to a depth belt between 25 and 40 m deep, reaching maximum biomass values (126 g dw m−2) at 30 m depth. In concordance, although in the short term W. setacea survived and grew in a large range of environmental conditions, its life requirements for the mid and long term were dim light levels and low temperatures. Biomass of Womersleyella setacea did not show any clear seasonal pattern, though minimum values were reported in spring. Reproductive structures were always absent. Bearing in mind that no herbivores feed on Womersleyella setacea and that its thermal preferences are more characteristic of temperate than of tropical seaweeds, low light (50 µmol photon m−2 s−1) and low temperature (12°C) levels are critical for W. setacea survival and growth, thus probably determining its spread and bathymetric distribution across the Mediterranean Sea.

Differential herbivory of invasive algae by native fish in the Mediterranean Sea

The potential role of generalist herbivores to serve as a source of biotic resistance against algal invasion in marine ecosystems has been poorly examined. The present study investigates the capacity of Mediterranean herbivorous fishes to consume three of the most invasive seaweeds of the Western Mediterranean ( Caulerpa racemosa, Lophocladia lallemandii and Womersleyella setacea) and examines vertical and temporal variations of such consumption. Our results show that although fish feed throughout the depth gradient examined (5–35 m), they concentrate in shallow waters, and can consume high amounts of C. racemosa. Such high ingestion of C. racemosa does not appear to be random, since this alga is consistently chosen when offered in pairs with several native species. Conversely, L. lallemandii and W. setacea are barely eaten by fish even though they can be very abundant in the field throughout the year. Our results suggest that fish could be an important controlling agent that has been overlooked in temperate marine invasions, and they may be able to provide certain resistance to C. racemosa invasion. In contrast, they are unlikely to exert any important control effects on L. lallemandii or W. setacea.

Deep-water stands of Cystoseira zosteroides C. Agardh (Fucales, Ochrophyta) in the Northwestern Mediterranean: Insights into assemblage structure and population dynamics

Populations dominated by Cystoseira zosteroides, an endemic and threatened Mediterranean seaweed, colonize deep-water rocky habitats down to more than 50 m depth. Assemblages dominated by this species display high algal and invertebrate species richness. Algal biomass averages 1134 g dw m −2. Erect and turf algae account for only 25% of total algal dry weight, while encrusting corallines are responsible for the remaining 75%. Sponges, bryozoans and ascidians constitute the dominant sessile macrofauna. Cystoseira zosteroides is the dominant erect algae, with a mean biomass of 60.6 g dw m −2, and densities ranging from 4 to 7 plants m −2. The alien turf alga Womersleyella setacea has a biomass of 104.2 g dw m −2 and covers most of the understory substrate. The size-frequency distribution of C. zosteroides populations shows differences over time. Mean annual growth of the main axis is around 0.5 cm and mean annual mortality rate is lower than 2%. Recruitment was almost nil during the studied period of time (10 years). Processes structuring these deep-water Cystoseira stands must be driven by episodic disturbances, after-disturbance recruitment pulses, and long periods of steady growth that last at least 10 years. However, it is also possible that recruitment is irreversibly inhibited by the alien alga W. setacea in which case these old-growth stands are faced with extinction. The highly diversified assemblages and the low growth and low mortality rates of C. zosteroides indicate high vulnerability to natural and anthropogenic disturbances, and call for effective measures to ensure their conservation.

Macroalgal assemblages of disturbed coastal detritic bottoms subject to invasive species

Characteristic flora and fauna that are highly sensitive to disturbances colonize coastal detritic bottoms in the Mediterranean Sea. In the present study, a comparison of the assemblage composition and colonization by invasive macroalgae was made between two coastal detritic macrophyte assemblages, one dominated by rhodoliths (free-living non-geniculate Corallinales) and the other dominated by fleshy algae, in an area that has been exposed to important levels of anthropogenic disturbance, mainly pollution (including changed sedimentation regimes) in the recent past (bay of Marseilles, France). In comparison with less strongly impacted Mediterranean regions, the macrophyte assemblages in the bay of Marseilles were characteristic in terms of species identity and richness of coastal detritic macrophyte assemblages. However, extremely low species abundance (cover) was observed. As far as invasive species were concerned, Caulerpa racemosa var. cylindracea was only abundant in the rhodolith assemblage whereas the two invasive Rhodophyta Asparagopsis armata and Womersleyella setacea were mainly found in the fleshy algae assemblage. The seasonality observed in the Rhodolith assemblage seemed to be related to the development of C. racemosa var. cylindracea and did not follow the typical pattern of other Mediterranean assemblages. This study represents the first study of coastal detritic assemblages invaded by C. racemosa var. cylindracea.

The extensive development of the turf-forming red alga Womersleyella setacea (Hollenberg) R. E. Norris (Rhodophyta, Ceramiales) in the Bay of Boka Kotorska, Montenegro (southern Adriatic Sea)

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.

Invasion of alien macroalgae in different Mediterranean habitats

The study evaluated different macroalgal invasions in the main Mediterranean coastal habitats on hard bottom. Biodiversity, species composition and structure of macroalgal assemblages were compared among non-invaded areas and areas invaded by the Chlorophyta Caulerpa racemosa var. cylindracea and by the turf-forming Rhodophyta Womersleyella setacea in three different habitats: shallow rocky bottom, deep rocky bottom and dead matte of the seagrass Posidonia oceanica. Results showed that alien macroalgae constituted a relevant component of benthic assemblages in invaded areas of the Mediterranean Sea. Assemblages invaded by Womersleyella setacea and Caulerpa racemosa showed lower values of diversity and large differences in the structure and species composition related to non-nvaded assemblages. The species that mostly suffered from invasion were erect species reproducing sexually; moreover, the dominance of W. setacea led to low abundance of native filamentous algae, while C. racemosa colonization seemed particularly threatening for encrusting algae. All the studied habitats appeared highly invasible by alien macroalgae, even if W. setacea appeared more invasive in deeper habitats, while colonization of C. racemosa seemed more serious in shallower habitats; the dead matte of P. oceanica represented a suitable substrate for the spread of both species. Differences among assemblages in different habitats were reduced in invaded areas.

A critical review of records of alien marine species from the Maltese Islands and surrounding waters (Central Mediterranean)

An updated list of alien marine species recorded from the Maltese Islands and surrounding waters, compiled from scientific and ‘grey’ literature and from authenticated unpublished reports to the authors, is presented. The listed species are classified in one of four categories as regards establishment status: established, casual, invasive and questionable. Doubtful records are listed as ‘?’. A total of 48 species, including nine dubious ones, are included in the list. Of the accepted records, 64% are established, of which 15.4% are invasive, 18% are casual and 18% are questionable. The most represented groups are molluscs (14 species), fish (13 species) and macrophytes (10 species). Six species are classified as invasive in Maltese waters: Lophocladia lallemandii, Womersleyella setacea, Caulerpa racemosa var. cylindracea, Percnon gibbesi, Fistularia commersonii and Sphoeroides pachygaster; impacts of some of these species on local ecosystems are discussed. Since the early 1900s, there has been an increasing trend in the number of alien marine species reported from the Maltese Islands. Transportation via shipping and in connection with aquaculture, as well as the range expansion of Lessepsian immigrants, appear to be the most common vectors for entry, accounting for 20%, 11% and 32% respectively of the alien species included in this review. The general warming trend of Mediterranean waters and increasing marine traffic may be facilitating the spread of warm-water Atlantic and Indo-Pacific species to the central Mediterranean, including the Maltese Islands.

Zoobenthos associated with the invasive red alga Womersleyella setacea (Rhodomelacea) in the northern Aegean Sea

The spatial and temporal structure of the zoobenthos associated with the invasive red alga Womersleyella setacea was studied in the northern Aegean Sea, Greece. Five replicate quadrats (20×20 cm) were collected, seasonally from July 1997 to August 1998, by SCUBA diving at four sites and two depth levels (15 and 30 m) in the Chalkidiki peninsula. We collected 23,090 specimens representing 278 animal species. Multivariate analysis showed mainly spatial differences in community structure, while the temporal ones were minimal. The ordination of sites showed a clear zonation pattern according to the different algal forms that cover the rocky substrate, with inclination the only environmental factor involved. The epifauna associated with the invasive alga was differently structured compared with three native seaweeds, showing increased species richness and abundance. The filamentous alga W. setacea was dominant in all seasons and depth levels, creating a stable habitat that contributed to the loss of seasonality in zoobenthic community structure.