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Scleractinian corals provide habitats for a broad variety of cryptofauna, which in turn may contribute to the overall functioning of coral symbiomes. Among these invertebrates, hydrozoans belonging to the genus Zanclea represent an increasingly known and ecologically important group of coral symbionts. In this study, we analysed 321 Zanclea colonies associated with 31 coral genera collected from 11 localities across the Indo-Pacific and Caribbean regions, and used a multi-disciplinary approach to shed light on the evolution and biogeography of the group. Overall, we found high genetic diversity of hydrozoans that spans nine clades corresponding to cryptic or pseudo-cryptic species. All but two clades are associated with one or two coral genera belonging to the Complex clade, whereas the remaining ones are generalists associated with both Complex and Robust corals. Despite the observed specificity patterns, no congruence between Zanclea and coral phylogenies was observed, suggesting a lack of coevolutionary events. Most Zanclea clades have a wide distribution across the Indo-Pacific, including a generalist group extending also into the Caribbean, while two host-specific clades are possibly found exclusively in the Red Sea, confirming the importance of this peripheral region as an endemicity hotspot. Ancestral state reconstruction suggests that the most recent common ancestor of all extant coral-associated Zanclea was a specialist species with a perisarc, occurring in what is now known as the Indo-Pacific. Ultimately, a mixture of geography- and host-related diversification processes is likely responsible for the observed enigmatic phylogenetic structure of coral-associated Zanclea.

Nudibranchs have a mostly carnivorous diet, and they prey on a wide variety of other animal taxa. Many species, mainly belonging to the Cladobranchia suborder, feed on cnidarians, including member of the class Hydrozoa. Several hydrozoan species display a symbiotic lifestyle, being associated with other benthic invertebrates, including for instance bryozoans, corals, octocorals, and sponges. In our knowledge, no record of nudibranch predation on symbiotic hydrozoans has been reported so far, possibly thanks to the protective action by the host towards its symbiotic hydrozoan. Here, we show the unexpected case of a nudibranch belonging to the recently described species Sakuraeolis marhe (Fernández-Simón and Moles, 2023) feeding on Zanclea sp. 2, a hydrozoan associated with the cheilostome bryozoan Celleporaria sp. This trophic association is confirmed by the presence and storage of the nematocysts into the nudibranch cnidosacs. Moreover, the nudibranch appears to selectively store mostly a single type of nematocyst, that is large size stenotele. The observation here reported represents the first well-documented record of a nudibranch feeding on a symbiotic hydrozoan and the first confirmed case of predation on Zanclea polyps. Moreover, we provide additional genetic information and the first description of the internal anatomy of S. marhe.

Scleractinian reef corals have recently been acknowledged as the most numerous host group found in association with hydroids belonging to the Zanclea genus. However, knowledge of the molecular phylogenetic relationships among Zanclea species associated with scleractinians is just beginning. This study, using the nuclear 28S rDNA region and the fast-evolving mitochondrial 16S rRNA and COI genes, provides the most comprehensive phylogenetic reconstruction of the genus Zanclea with a particular focus on the genetic diversity among Zanclea specimens associated with 13 scleractinian genera. The monophyly of Zanclea associated with scleractinians was strongly supported in all nuclear and mitochondrial phylogenetic reconstructions. Furthermore, a combined mitochondrial 16S and COI phylogenetic tree revealed a multitude of hidden molecular lineages within this group (Clades I, II, III, V, VI, VII, and VIII), suggesting the existence of both host-generalist and genus-specific lineages of Zanclea associated with scleractinians. In addition to Z. gallii living in association with the genus Acropora, we discovered four well-supported lineages (Clades I, II, III, and VII), each one forming a strict association with a single scleractinian genus, including sequences of Zanclea associated with Montipora from two geographically separated areas (Maldives and Taiwan). Two host-generalist Zanclea lineages were also observed, and one of them was formed by Zanclea specimens symbiotic with seven scleractinian genera (Clade VIII). We also found that the COI gene allows the recognition of separated hidden lineages in agreement with the commonly recommended mitochondrial 16S as a DNA barcoding gene for Hydrozoa and shows reasonable potential for phylogenetic and evolutionary analyses in the genus Zanclea. Finally, as no DNA sequences are available for the majority of the nominal Zanclea species known, we note that they will be necessary to elucidate the diversity of the Zanclea-scleractinian association.

Scleractinian reef corals have recently been acknowledged as the most numerous host group found in association with hydroids belonging to the Zanclea genus. However, knowledge of the molecular phylogenetic relationships among Zanclea species associated with scleractinians is just beginning. This study, using the nuclear 28S rDNA region and the fast-evolving mitochondrial 16S rRNA and COI genes, provides the most comprehensive phylogenetic reconstruction of the genus Zanclea with a particular focus on the genetic diversity among Zanclea specimens associated with 13 scleractinian genera. The monophyly of Zanclea associated with scleractinians was strongly supported in all nuclear and mitochondrial phylogenetic reconstructions. Furthermore, a combined mitochondrial 16S and COI phylogenetic tree revealed a multitude of hidden molecular lineages within this group (Clades I, II, III, V, VI, VII, and VIII), suggesting the existence of both host-generalist and genus-specific lineages of Zanclea associated with scleractinians. In addition to Z. gallii living in association with the genus Acropora, we discovered four well-supported lineages (Clades I, II, III, and VII), each one forming a strict association with a single scleractinian genus, including sequences of Zanclea associated with Montipora from two geographically separated areas (Maldives and Taiwan). Two host-generalist Zanclea lineages were also observed, and one of them was formed by Zanclea specimens symbiotic with seven scleractinian genera (Clade VIII). We also found that the COI gene allows the recognition of separated hidden lineages in agreement with the commonly recommended mitochondrial 16S as a DNA barcoding gene for Hydrozoa and shows reasonable potential for phylogenetic and evolutionary analyses in the genus Zanclea. Finally, as no DNA sequences are available for the majority of the nominal Zanclea species known, we note that they will be necessary to elucidate the diversity of the Zanclea-scleractinian association.

The life cycle of the anthoathecate hydroid Zanclea sessilis (Gosse, 1853) (Cnidaria: Hydrozoa: Zancleidae), symbiotic with colonies of the cheilostomate bryozoan Turbicellepora magnicostata (Barroso, 1919) (Bryozoa: Gymnolaemata), was investigated in the laboratory using material collected in the Basque Country (Spain, southeast of the Bay of Biscay), and its medusae were reared to maturity. The gastrozooids, dactylozooids and the newly released and adult medusae are described and depicted, including their cnidome. The feeding behaviour of the medusae in the cultures is described. The colonies have polymorphic hydranths, and gastrozooids and dactylozooids can be distinguished. The latter are described for the first time in this hydroid, and have been known in the genus Zanclea Gegenbaur, 1857 only in species from the Indo-Pacific. Dactylozooids were observed only once in colonies in the field, although they developed occasionally in colonies kept in aquaria on their original substrate. The interaction between the hydroid and the bryozoan is considered a facultative mutualistic cooperation, as both species can live without forming an association. This interaction is common in the study area, as, in some shallow water environments, up to 65 % of the colonies of T. magnicostata host Z. sessilis .

On October 1, 2009, a small area along the Sicilian coast between the villages of Scaletta Zanclea and Giampilieri was struck by intense and concentrated rainfall causing countless small landslides widespread over the catchment area of steep subaerial creeks, locally known as Fiumara . Dense and quick debris flows were channelized within the Fiumara and destructively hit the villages and entered the sea, where they likely transformed into hyperpycnal flows. The availability of pre- and post-flood high-resolution bathymetry allows us to recognize the main features and the most significant morphological variations related to the impact of the flows on the seafloor. The passage of hyperpycnal flows on the seafloor possibly produced a suite of mass-wasting events, encompassing sheet landslides (i.e. erosive scours), retrogressive slope failure on loose sediment at the canyon headwall and rock-falls on the conglomeratic bedrock along canyon sidewall. The possible causes of these events are discussed on the basis of available morphological evidence and geotechnical considerations. Finally, the widespread occurrence of mass-wasting features (i.e. submarine landslide scars) morphologically similar to those generated by the 2009 flash flood allows us to hypothesize, from one side, a strong correlation between this catastrophic event and the evolution of submarine canyons, and from the other side, the possible use of these features for the assessment of flash-flood occurrence.

To date, four nominal species and several other unidentified species of Zanclea hydrozoans are known to live symbiotically with scleractinians, and recent surveys reported this association also in the Red Sea. Previous molecular studies showed that each coral genus involved in this association hosts only one species or molecular clade of Zanclea , with the only exception being the genus Acropora , which hosts at least two Zanclea species. Moreover, some of the detected genetic lineages were morphologically undistinguishable in the polyp stage, suggesting the presence of cryptic species. In this study, we investigated the morphology and genetic diversity of Acropora -associated Zanclea specimens collected in previous studies in Egypt and Israel, as well as new samples collected in Saudi Arabia. Based on the current data, all the analysed samples were morphologically identical to Zanclea gallii , a species associated with Acropora corals from the Maldives. However, molecular analyses separated the samples collected in the Red Sea from all other coral-associated hydroids. Therefore, phylogenetic reconstructions, haplotype networks, genetic distance analyses and distribution data allowed us to identify a previously unknown cryptic species of Acropora -associated hydroid, here named Zanclea gallii IIa, following a recently proposed molecular nomenclature.

A novel symbiosis between scleractinians and hydroids (Zanclea spp.) was recently discovered using taxonomic approaches for hydroid species identification. In this study, we address the question whether this is a species-specific symbiosis or a cosmopolitan association between Zanclea and its coral hosts. Three molecular markers, including mitochondrial 16S and nuclear 28S ribosomal genes, and internal transcribed spacer (ITS), were utilized to examine the existence of Zanclea species from 14 Acropora species and 4 other Acroporidae genera including 142 coral samples collected from reefs in Kenting and the Penghu Islands, Taiwan, Togian Island, Indonesia, and Osprey Reef and Orpheus Island on the Great Barrier Reef, Australia. Molecular phylogenetic analyses of the 16S and 28S genes showed that Acropora-associated Zanclea was monophyletic, but the genus Zanclea was not. Analysis of the ITS, and 16S and 28S genes showed either identical or extremely low genetic diversity (with mean pairwise distances of 0.009 and 0.006 base substitutions per site for the 16S and 28S genes, respectively) among Zanclea spp. collected from diverse Acropora hosts in different geographic locations, suggesting that a cosmopolitan and probably genus-specific association occurs between Zanclea hydroids and their coral hosts.

Scleractinian corals are the host group with the greatest number of species associated with hydroids in the genus Zanclea. There are 34 nominal species and 31 genera of scleractinians reported as Zanclea hosts from a few localities across the Red Sea, Indo-Pacific, and Caribbean. However, little is known about how consistent and cosmopolitan scleractinian- Zanclea relationships are. To test their geographical consistency, we surveyed Fijian corals for Zanclea presence using both field-based surveys and analysis of in situ coral photographs. Surveys targeted 16 species and seven genera of reported Zanclea hosts and five other corals commonly found at the sites. We observed Zanclea hydroids associated with 22 species in seven genera of scleractinian corals with 17 (77%) of the species being newly reported hosts. Our findings suggest that a broad diversity of scleractinian corals hosting Zanclea remains to be documented, and although Zanclea -scleractinian associations are likely cosmopolitan in their distribution across localities, associations with specific coral species may not be.