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Although the biometric investigation of the cnidome represents an essential diagnostic feature to successfully identify cnidarian species, further efforts are needed to fully characterize eventual differences. Important model actiniarians, such Anemonia viridis and Aiptasia spp., are commonly used for scientific purposes; however, their identification is often difficult, and the description of their cnidome is, therefore, essential. To contribute to this necessity, specimens of Anemonia viridis and Aiptasia couchii from the North Adriatic Sea were collected to acquire data (i.e. length, width, size range, length/width ratio) about cnidae from different tissues (i.e. column, tentacles, actinopharynx, mesenteries, and acrorhagi/acontia). Four types of cnidae were observed in each species. A. viridis shows microbasic b-mastigophores, microbasic p-mastigophores, holotrichous isorhizas, and spirocysts, whereas A. couchii harbored basitrichs, microbasic p-mastigophore, microbasic p-amastigophore and spirocysts. Molecular and phylogenetic analyses were also performed using three mitochondrial markers to confirm the correct classification of the specimens here analysed.

Mesophotic habitats, hosting benthic assemblages totally unknown and unexpected in the early 2000s, are attracting an increasing interest from scientists. Realizing that many long living and habitat forming species are playing a unique pocket of biodiversity in the frame of climate crisis, it is important to recognize the potential of the mesophotic zone in conservation strategies. The mesophotic zone is characterized by environmental factors generally more stable than the shallow counterpart; therefore, it represents a refuge habitat to preserve species that, in shallow waters, may be involved in bleaching events, massive mortalities or other pathologies generally triggered by thermal anomalies. The definition of the mesophotic zone in the temperate seas is confused, creating possible misunderstandings and problems not only from a scientific point of view but also in communication and outreach strategies. Here, we review the scientific literature to build the best definition possible based on the maximum consensus coming from papers analysis. Moreover, the key roles of mesophotic habitats, including ecosystem engineers as animal forests, have been schematized to enhance the perception of the potential risks we are running in case of damage or loss of these habitats.

The new species Cladocarpus salix and three species of Lytocarpia (L. brevirostris, L. delicatula and L. phyteuma) were recorded from the Bunaken National Marine Park (North Sulawesi, Indonesia). Cladocarpus salix forms large colonies reminiscent of willow trees and differs from the closely similar species C. keiensis in the shape and the size of the intrathecal septum, and in the phylactocarp structure. Moreover, the presence of grooves on the external wall of theca is unique feature of this species. The differences of the three Lytocarpia species are discussed using microscopic imaging, comparison of measurements, and macroscopic features of living specimens. The bathymetric distribution of these species, studied through vertical transect, showed that C. salix and L. delicatula are deep species occurring below 20 m and increasing their abundance from 30 to 50 m depth; L. phyteuma is more abundant from 5 to 30 m and L. brevirostris is more abundant in the shallowest and deepest part of the reef and rarer between 10 and 30 m depth. A series of surveys conducted in the same locality during a year indicated that these species are generally more abundant from November to May (wet season), except for L. delicatula which is active in July.

Five species of Macrorhynchia from the Bunaken National Marine Park (North Sulawesi, Indonesia) are described. Three species, M. philippina, M. spectabilis and M. phoenicea were already known for the Indo‐Pacific area, M. asymmetrica and M. fulva are described as new species. Macrorhynchia asymmetrica n. sp. is easily distinguished from the other species of the genus due to the hypertrophic lateral nematotheca that is bent downwards. Macrorhynchia fulva n. sp. is distinguishable from its congeners especially through the lateral nematothecae pointing in opposite directions, the large marginal tooth, and the presence of the well‐marked adcauline teeth. The shape and the colour of living colonies are described, thereby allowing the recognition of the species directly in situ. Generally, the observed species live on the deeper portions of the reef, but M. spectabilis occurs at more shallow depths. Almost all the recorded species have an annual cycle characterised by higher abundances during the wet season whi...

Eudendrium moulouyensis is a zooxanthellate hydroid originally described from the Chafarinas Islands (Alboran Sea, south-western Mediterranean) in summer 1991. According to the original description, this species can be identified due to the occurrence of symbiotic zooxanthellae in the entire endodermal layer of the colony (gastrodermis and tentacle endodermis), a unique feature among the Mediterranean Eudendrium species. However, several aspects of its life cycle and the extent of its phenotypical variability are still unknown. Since winter 2004, colonies of E. moulouyensis were recorded throughout the year from 0.5 m to 30 m depth from the southern Adriatic Sea (Otranto Channel) and the Gibraltar Strait (Alboran Sea). Additional specimens were collected from the northern Adriatic (Vis, Croatia), Sicily Channel (Pantelleria and Lampedusa Islands), and western Sardinia (Costa Paradiso). These findings offered the opportunity to describe for the first time the full life cycle and to elucidate several biological aspects related to phenotypical variation of colony morphology, vertical zonation, seasonality, zooxanthellae–polyp relationship, and cnidome morphology and distribution. The number and morphology of male gonophores per reproductive polyp is described here for the first time, providing a useful taxonomic character to easily discriminate Myrionema amboinense from E. moulouyensis. From the available information, the occurrence of M. amboinense in the Mediterranean Sea should be regarded as doubtful, if they are not accompanied by observations of cnidome, male gonophores or distinctly separate tentacles whorls.

Kinds and distribution of nematocysts of the cubomedusa Carybdea marsupialis were described by light and scanning electron microscopy. Three kinds of nematocysts were found in tentacles: heterotrichous microbasic euryteles, flattened holotrichous isorhizas and subspherical holotrichous isorhizas. The microbasic euryteles of the tentacles show a characteristic detachable multilayered lancet, already described in the euryteles of Carybdea alata. A possible explanation of the function of the lancet is proposed. The pedalia and the umbrella are armoured by holotrichous isorhizas together with an unidentified kind of small, drop-shaped nematocysts. Gastric cirri, characterized by a rough and ciliated surface, contain heterotrichous microbasic euryteles.

Some examples of relationships between hydroids and epibionthic diatoms from the Mediterranean Sea are described, verifying the kind of interaction existing between the two partners. The athecate Eudendrium racemosum hosts an extremely rich diatom assemblage, mainly comprising Licmophora spp., Amphora spp. and Cocconeis spp. On the contrary, only adnate growth forms (Cocconeis pseudonotata, C. dirupta) were observed in diatom communities growing on the external side of thecate species Campanularia hincksii, Clytia linearis and Synthecium evansi. Some diatom species (Cocconeis notata, Cylindrotheca sp. and Navicula sp.) are able to survive in the intrathecal microenvironment. They live in the narrow space between hydrotheca and polyp, receiving protection and probably using the nutrients produced by hydroid metabolism. Sunlight can penetrate through transparent thecae and reach the diatom layer, making photosynthesis possible.

The patchy distribution pattern of benthic hydrozoans reflects high sensitivity of colonies to local and micro-scale environmental factors, which may affect the outcome of biodiversity inventories based on different sampling methodologies. We compared three quantitative sampling methods for benthic hydrozoans, differing for strategy (scraping standardized surfaces or picking visually located colonies) and tactics (extension of examined surface). Each of the three methods consisted of eight replicate samplings to compare relative and absolute method efficiencies, also in terms of minimum sampling area representative of the hydroid diversity in the study site. A single horizontal belt transect on a vertical rocky cliff was chosen at 15 m depth near Otranto (South Adriatic). A total of 37 species were identified by the three methods and their relative abundances were evaluated. The hydroid assemblage was dominated by generalist species with high polyp numbers able to colonize different substrata. We compared the outcomes of each of the three sampling methods, and by a cost-benefit analysis we identified the optimal sampling methods to be adopted according to the different targets of the investigations. On data standardized to the same total sampled areas, the three sampling methods did not originate significant differences in the number of polyps of each recorded species (PDA, Polyp Detection Ability), whereas the visually-oriented collection (VIS) and the scrapings over 20 × 20 cm standard surfaces (S20) were more effective in the detected number of species (SDA, Species Detection Ability) than over 10 × 10 cm squares (S10). For ecological purposes, samplings based on five S20 replicates can be considered an appropriate experimental design to get a statistically robust representation of the studied hydroid assemblage at Punta Palascia. However, variation in depth, exposure and geographical locations may require different sampling approaches, and pre-surveys should be always carried out before addressing taxonomical surveys of sessile organisms.

Most works concerning growth and reproduction of Mediterranean sponges have been performed in the oligotrophic western Mediterranean while little is known about sponge dynamics in the North-western Adriatic Sea, a basin characterized by low winter temperature and eutrophy. In order to deepen our understanding of sponges in the North Adriatic Sea and verify how its peculiar trophic and physical conditions affect sponge life cycles, temporal trend of sponge cover (%) and reproductive timing of Chondrosia reniformis and Tedania (Tedania) anhelans were studied over a 1-year period looking for a possible relation with variations of temperature or food availability. In C. reniformis, although little variations of sponge cover were evidenced around the year, the number of individuals and their size increase during spring. Asexual reproduction, via drop-like propagules, mainly occurs in spring and summer, while sexual reproduction is characterized by a maximum number of oocytes in August. T. anhelans progressively grows from spring to summer and develops propagules on its surface that reach their maximum size in July. In autumn, the sponge undergoes a process of progressive shrinkage and almost disappears in winter when temperature reaches 7–8°C. Larvae occur during summer. In the North Adriatic Sea sponges have larger sizes, higher density and a wider period of oocytes production compared with the same species from the Mediterranean Sea, suggesting these differences could be due to high food availability characterizing the eutrophic Adriatic basin. On the contrary, the sharp water temperature variations and the very low winter temperature, 5–6°C lower than what has been reported for the Mediterranean Sea, regulate temporal variations in abundance and cause the disappearance of thermophile species during winter.

A solitary hydroid of the genus Ectopleura was described associated with several Mediterranean anthozoans: the black coral Antipathella subpinnata and three gorgonian species, Eunicella cavolinii, Paramuricea clavata and Paramuricea macrospina. This find represents the first record of a hydroid species epibiont of an antipatharian coral and also the first record of such association with Mediterranean gorgonians. Hydroids arise from the branches of the hosting corals and are enveloped by a thin sheet of their skeleton and by a layer of living coenenchyme up to the neck of the hydranths. The relationship causes no apparent damage to the host, while the epibiotic habitus allows the hydroid to avoid siltation and to gain in defence and support. It is hypothesised that actinulae of Ectopleura sp. are able to settle on the corals (both skeleton or tissue) and, during their growth, to be enveloped by the skeletons of their host.

The growth rate of two species of whip black corals belonging to the genera Cirrhipathes and Stichopathes has been evaluated in different experiments conducted in the Marine Park of Bunaken (Indonesia) for over one year. Unbranched, unpinnulated black corals result among the fastest growing colonial organisms, with maximal growth rates of 13.25 cm month−1 for Cirrhipathes cfr anguina and 1.3 cm month−1 for Stichopathes cfr maldivensis. Several observations made both on transplants and on natural colonies add new information on the growth strategy of black corals. The easy handling, the high rate of survivorship and the fast growth of transplants indicate that black corals can be successfully employed in the recovery projects of harvested reefs.

Four species of foraminifers Rosalina globularis, Lobatula lobatula, Acervulina inhaerens and Miniacina miniacea were observed as epibionts on some species of the athecate hydroids of the genus Eudendrium—E. armatum, E. glomeratum, E. moulouyensis and E. racemosum—from the Mediterranean Sea. The abundance and the distribution of the protozoans were determined for each hydroid and considered in relation to the shape and size of the Eudendrium branches. Results revealed differences in the composition of the epibiontic assemblage among different species of Eudendrium which were dependent on several factors, but were mainly related to the habitat and to the morphology of the host.The interactions between Eudendrium and its epibionts were considered to emphasize the role of the Eudendrium colonies as a hotspot not only of poorly known species but also of co-evolutionary relationships and life histories.

The crab Geryon longipes lives in muddy bottoms of bathyal environments. Its epibiontic assemblage was studied in a population from the Southern Adriatic Sea. Four species of invertebrates were obs...

The hydroid assemblage of the Siladen Island coral reef (North Sulawesi, Indonesia) is made up of 107 species belonging to 51 genera and 28 families. The study of their spatial distribution has identified three different zones which are well-characterized: (1) the reef flat, where hydroids are mainly represented by cryptic species living in the shady crevices of the corals, epibiontic on sponges or sea grasses; (2) the edge of the reef characterized by hydroids growing mainly on the shady side of the corals, with the exception of the tufts of the large colonies of the stinging zooxanthellateAglaophenia cupressina; and (3) the vertical reef, where hydroids reach their maximum diversity and abundance and are often involved in symbiotic relationships.The number of hydroid species shows a seasonal trend with a summer minimum (July–September) and a winter maximum (November–February). The strong correlation between these variations and the abundance of rainfall suggests that food availability, strictly related to the seasonal amount of rain, represents the main abiotic factor triggering the hydroid species richness and the abundance of this tropical assemblage.

Living diatoms are commonly found within Antarctic Porifera, and generally interpreted as additional food source, or as mutualists or parasites of sponge tissues. However, no data are available about temporal variations of the abundance of diatoms inside sponges especially during the winter period. In this paper we analysed the amount of diatom frustules and chlorophaeopigment concentration in six species of Antarctic sponges (Dendrilla antarctica, Homaxinella flagelliformis, Kirkpatrickia variolosa, Suberites montiniger, Haliclona dancoi, Haliclona penicillata) sampled weekly from November 2001 - before the ice melting - to February 2002. Frustule concentration in the sponge tissues was very low at the beginning of November in all the sponge species, and increased between 9 and 22 January, to reach maximum values between 29 January and 7 February. Diatom abundances were significantly higher in H. dancoi tissues, comparing to the other sponge species, reaching values up to 1217′178 cells *10 g - 1 dw at the end of January. Chlorophaeopigments, very low at the beginning of November, increased between 5 and 19 December, before the peak of diatoms. Maximum chlphaeo values (650.5 ′ 5.9 μg g - 1 dw) were observed in D. antarctica. The planktonic Fragilariopsis curta was the most common diatom species recorded inside sponges. These data clearly indicate that diatom concentration inside the sponge tissues is related to the summer phytoplankton bloom. The shift between the pigment and frustule peaks strongly suggests that diatoms are used as a food source by sponges and that their frustules are accumulated inside the sponge body. The lack of frustules at the beginning of summer indicates that diatom frustules are expelled or dissolved during the cold season.

Intense blooms of the benthic dinoflagellate Ostreopsis cf. ovata have occurred in the northern Adriatic Sea since 2006. These blooms are associated with noxious effects on human health and with the mortality of benthic organisms because of the production of palytoxin-like compounds. The O. cf. ovata bloom and its relationships with nutrient concentrations at two stations on the Conero Riviera (northern Adriatic Sea) were investigated in the summer of 2009. O. cf. ovata developed from August to November, with the highest abundances in September (1.3 × 106 cells g−1 fw corresponding to 63.8 × 103 cells cm−2). The presence of the single O. cf. ovata genotype was confirmed by a PCR assay. Bloom developed when the seawater temperature was decreasing. Nutrient concentrations did not seem to affect bloom dynamics. Toxin analysis performed by high resolution liquid chromatography–mass spectrometry revealed a high total toxin content (up to 75 pg cell−1), including putative palytoxin and all the ovatoxins known so far.