Modern in situ survey technologies such as crewed submersibles, remotely-operated vehicles (ROVs), towed camera arrays, and visual/video plankton recorders (VPRs) were used to characterize the dominant anthomedusan species off the eastern seaboard of Japan. Notes on the taxonomy, distribution, behaviour and interspecies interactions are presented for the four observed species: Euphysa japonica, E. flammea, Calycopsis nematophora and Pandea rubra. A new generic definition for the genus Calycopsis is proposed. The possibility of run-on, cascading detrimental effects of oceanic acidification on midwater ecosystems was identified from observations made during the present study.

Previous evidence has shown that benthic hydroids capture all kinds of available prey and the only known constraint was prey size. Among the prey captured are phytoplankton cells but it is not known whether they are digested and assimilated. To test the hypothesis that benthic hydroids assimilate phytoplankton cells, a series of feeding experiments was carried out with the Mediterranean species Eudendrium racemosum. Ingestion rates and assimilation efficiency were determined by analysing the 14C incorporated from a labelled population of the diatom species Thalassiosira weissflogii. Eudendrium racemosum fed on T. weissflogii, after a period of starvation, and with the diatoms as the sole food item. In the presence of approximately 15,000 diatoms ml−1, Eudendrium fed at rates ranging from 16 to 55 diatoms polyp−1 hour−1. Accumulation of radioactivity in the hydrocaulus and the polyps of the hydroids were observed. A maximum ingestion of 31.6 diatoms per μgC of polyp (i.e. 175 diatoms per polyp) was observed in the experiments. Most of the diatom 14C ingested would have ended up in the Eudendrium tissue (efficiency 94%), and it was expected that a certain percentage would have been respired by the polyps. These data show that Eudendrium feed on phytoplankton, which can satisfy almost 100% of their energy demand when this type of food is sufficiently abundant.

The inconspicuous hydrozoan Clytia hummelincki, first recorded from the Mediterranean Sea in 1996, is presently widespread throughout the middle Mediterranean Sea and the Adriatic Sea. Two surveys carried out in 2003 and 2004 along the coast of the Salento Peninsula, Italy, showed that this species inhabits a belt between 0.5 and 2 m depth in sea urchin barrens and in areas damaged by date mussel fisheries. The comparison between the two surveys indicates that the frequency of the alien increased from 2003 to 2004. The life cycle of C. hummelincki is described for the first time.

A new location for Protohydra leuckarti is reported near Plymouth at Millbrook Lake (Tamar Estuary). To place this finding in context, notes follow on a familiar habitat of this species in the White Sea, and on the general ecology and distribution of Protohydra and its enigmatic phylogeny.

In the White Sea the colonial hydroid Laomedea flexuosa inhabits a narrow belt of the lower littoral zone. How is so limited a habitat determined? We studied the time of planula release and the behaviour of larvae during the free-swimming stage and settlement in natural and laboratory conditions. Three methods were used to record the tidal-dependent dynamic of planula release: (1) plankton collecting bags around Fucus distichus macrophytes with mature hydroid colonies; (2) active stirring of Fucus with hydroids in a container with water, which is an old way to stimulate planula release; and (3) direct counts of the mature planulae in gonangia. The dynamic intensity of L. flexuosa planula release was investigated according to 3–4 phases of the tidal cycle. A moment of general release of larvae was shown to be correlated with a period of low water. Following the incoming tide keeps planulae in the littoral area. The planulae of L. flexuosa have a short period of swimming (less than 30 minutes), unlike those of Gonothyrae loveni and Dynamena pumila whose swimming period is about 36 hours. Quick settlement and tidal planula release could explain the strong spatial limitation in the distribution of L. flexuosa within the lower level of the intertidal zone. It is hypothesized that distribution over large areas is most likely by drift of colonies attached to detached seaweeds. Our outlook about the biological ways of ecological niche differentiation among marine organisms is broadened by the results of this study.

The biodiversity and distribution patterns of epiphytic hydroids were studied in kelp forests (composed of Laminaria digitata, Saccharina latissima and Alaria esculenta) located in an Arctic glaciated fiord (Hornsund, west Spitsbergen). In total, twenty-eight species were found colonizing algae, stones connected to holdfast, and overgrowing the surface of other animals associated with kelps. The characteristics of the algal host (e.g. algae species, age, rhizoid volume or biomass) did not show any effect upon hydroid species richness or species composition. High hydroid biodiversity was strongly dependent on microsubstrate heterogeneity. The highest biodiversity as well as frequency of hydroid occurrence were noted at a site located furthest from the glacier and characterized by the lowest sediment concentration and sedimentation rate. Sexual reproduction also seemed to be inhibited by glacier-derived disturbance. Of ten fertile species found at the ‘clearest’ site only two were fertile at sites under the strong influence of such perturbations. Potential physical drivers of species occurrence were linked to the activity of tidal glaciers, particularly to high loads of mineral sedimentation and iceberg scouring.

‘Phalanx’ and ‘guerrilla’ phenotypes have been characterized as distinct, adaptive growth strategies exhibited by marine encrusting taxa, as well as a variety of other colonial taxa, that differ in patterns of colonial growth and areal expansion. Phalanx morphs exhibit compact growth, expanding outward concentrically and generating radially symmetric colony shapes, whereas guerrillas exhibit diffuse growth and typically elongate, asymmetric colony shapes. Several species in the colonial hydroid genus Hydractinia display inter-genotypic morphological variation in early developmental growth, although it is unclear if and how this growth form variation is tied to colony symmetry. Here I show that the phalanx versus guerrilla distinction does not adequately characterize genetic variation in Hydractinia growth form. Genotypes characterized by extreme mat and stoloniferous growth exhibited high levels of symmetry while genotypes generating growth forms intermediate between these two extremes were more asymmetric. Asymmetric growth is tied to reduced field fitness as a result of slower growth, reduced investment in future reproduction, and increased susceptibility to abiotic environmental stress. Asymmetric, guerrilla-like growth may be the morphological symptom of maladaptive growth early in colony development. This notion contrasts greatly with the traditional view of guerrilla growth as an adaptive strategy. Several hypotheses are proposed to address why asymmetric, guerrilla-like growth may be maladaptive in this and similar systems.

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 zooxanthellate Aglaophenia 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.

At the head of Tanabe Bay, on the Pacific coast of Japan, dense populations of Proboscidactyla ornata medusae were observed at 24°C, 34 psu from July 2001 to September 2002 (maximum 472.4 individuals m−3). In the laboratory, 288 immature medusae of P. ornata collected from Tanabe Bay in May–June 2006 were cultured individually and fed with asphyxiated Artemia nauplii under 12 experimental conditions (four different temperatures (17°C, 20°C, 23°C and 26°C) combined with three different salinities (28 psu, 31 psu and 34 psu)). For 30 days, 94% of the medusae survived at all conditions. The maximum number of clones produced was 0.32 clones medusa−1 day−1 at 20°C, 34 psu; the number of medusae increased 10.5 times in 30 days. The maturation percentage at 23°C (54.2–58.3%) was higher than at 17°C (12.5–25.0%) and was positively correlated with the number of nauplii ingested. Temperature-specificity of energy usage was observed: 20°C for asexual budding and 23°C for sexual reproduction. The direct relationship between budding and temperature is demonstrated among hydromedusae for the first time in this study. Additionally, the influence of salinity on budding suggests a hypothesis on an advantage of budding at offshore and high saline waters.

At the end of the breeding season in autumn, under natural conditions, mature medusae of Eugymnanthea japonica are released from its host Mytilus galloprovincialis at night-time. In laboratory experiments, mature medusae of the congeneric species E. inquilina are also released at night-time in autumn. At that time of the year, sunset is earlier and the water temperature is lower than in summer, when, under natural conditions, medusa release of E. japonica takes place at sunset instead. The release thus takes place at the same hours of the day in summer as well as in autumn. The circadial timing of medusa release of E. japonica is likely constant throughout the whole period in the breeding season and not correlated with the decrease of light intensity at sunset.

Several hydroid species have an epibiontic lifestyle, living associated with organisms of many different phyla. On the other hand, hydroids can also host dense assemblages of microflora and microfauna, mainly composed of protists and bacteria. Among protists, diatoms are the most abundant and diversified group, followed by foraminifera and sessile ciliata such as Vorticella and suctorians. Regarding the spatial distribution of epibionts, hydroid colonies represent a mosaic of different microhabitats: in some species, each colony portion (base of the stem, branches, pedicels, inner space between the polyp and the theca) hosts different diatom species. Moreover, three foram species have been shown to occupy different positions according to the plasticity of their shell. A host specificity has been also observed: some epibionts are typical of only one or a group of species, such as Vorticella living on the teeth of the Aglaophenia thecae or coralline algae that cover mainly Aglaophenia and Sertularella colonies. The microassemblage associated to Eudendrium racemosum showed a typical seasonal cycle and a vertical distribution which reflects the selective advantage of the different life forms. Experiments with plastic structures miming hydroid colonies demonstrated that the living hydroid affects the assemblage structure. Probably, the perisarc composition and secondary metabolites play a crucial role in the relationships between hydroids and their microassemblage.

Four valid species of the genus Gymnogonos are known at this point of investigations: G. crassicornis (northern Atlantic and the Arctic Seas); G. obvolutus (northern Atlantic and the Arctic Seas); G. ameriensis (circumantarctic distribution) and the new species G. pacificus (northern Pacific). Other species, such as Corymorpha antarctica (Pfeffer, 1889) and Tubularia cingulata (Vanhöffen, 1910) from the southern hemisphere are recognized as ‘species inquirendae’. New descriptions and taxonomic discussions of the known Gymnogonos species have been proposed. This genus has a typical bipolar distribution in the classical meaning of this term. This genus origin may rise from tropical Corymorphidae by neoteny in the period of glacial cooling.

Monocoryne colonialis sp. nov. is described from the Aleutian Islands, Alaska. The new species is unusual among candelabrid hydroids in having a colonial growth form, differing from its congeners in the shape and size of hydranths, in having stolons that anastomose, and by having tentacles not fused or only partly fused into bract-like structures.

A new interstitial hydroid, Sympagohydra tuuli, has been collected from the three-dimensional brine channel system of the coastal fast ice off Barrow, Alaska (71°N 156ºW) in 2003 to 2006. This is the first cnidarian species described from the sea ice interstitial habitat. A morphological and systematic account of the new species is provided here. The hydroid stage is represented by solitary naked polyps, which become almost spherical under contraction. Body length can vary from 200 µm to >1 mm according to the degree of relaxation. The aboral side is extensible into a tubular foot-like projection made by epidermal tissue only, a typical feature of representatives of Protohydridae, order Capitata. However, in contrast to the other two known Protohydridae species, S. tuuli is characterized by three to four solid filiform tentacles, armed with microbasic mastigophore and desmoneme cnidocysts, and located at the base of a short hypostome bearing stenoteles. A small number of mastigophores are also scattered along the body column. Due to its unique combination of features, this taxon is designated here as a new species, and the new genus Sympagohydra is established as a new member of the class Hydroidomedusa, subclass Anthomedusae, order Capitata, family Protohydridae, to accommodate S. tuuli.

Hydroids symbiotic with octocorals in the Bunaken Marine Park and in the Lembeh Strait (North Sulawesi, Indonesia) were studied. Four species, Hydrichthella epigorgia, Ralpharia neira, Pteroclava krempfi and Zanclea timida sp. nov. were recorded and are described. The new species Z. timida is the only one of the genus associated with an octocoral. It is characterized by a naked hydrorhiza producing nematocyst knobs and by polyps which are able to retract into their own rigid, cup-shaped, basal region. The relationship between epibiontic hydroids and their octocoral hosts affects the morphology of both partners. The hydrorhiza of Hydrichthella epigorgia is naked when associated with Anthoplexaura dimorpha, but perisarc-covered when growing on other gorgonian host species. Vice versa, the hydroid is also able to affect the host morphology: Ralpharia neira induces Ellisella sp. to develop new branches, with the inner skeleton enveloping the hydroid stem.

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.

Hydroidolina is a group of hydrozoans that includes Anthoathecata, Leptothecata and Siphonophorae. Previous phylogenetic analyses show strong support for Hydroidolina monophyly, but the relationships between and within its subgroups remain uncertain. In an effort to further clarify hydroidolinan relationships, we performed phylogenetic analyses on 97 hydroidolinan taxa, using DNA sequences from partial mitochondrial 16S rDNA, nearly complete nuclear 18S rDNA and nearly complete nuclear 28S rDNA. Our findings are consistent with previous analyses that support monophyly of Siphonophorae and Leptothecata and do not support monophyly of Anthoathecata nor its component subgroups, Filifera and Capitata. Instead, within Anthoathecata, we find support for four separate filiferan clades and two separate capitate clades (Aplanulata and Capitata sensu stricto). Our data however, lack any substantive support for discerning relationships between these eight distinct hydroidolinan clades.

Some of the most interesting and enigmatic cnidarians are classified within the hydrozoan subclass Trachylina. Despite being relatively depauperate in species richness, the clade contains four taxa typically accorded ordinal status: Actinulida, Limnomedusae, Narcomedusae and Trachymedusae. We bring molecular data (mitochondrial 16S and nuclear small and large subunit ribosomal genes) to bear on the question of phylogenetic relationships within Trachylina. Surprisingly, we find that a diminutive polyp form, Microhydrula limopsicola (classified within Limnomedusae) is actually a previously unknown life stage of a species of Stauromedusae. Our data confirm that the interstitial form Halammohydra sp. (Actinulida) is derived from holopelagic direct developing ancestors, likely within the trachymedusan family Rhopalonematidae. Trachymedusae is shown to be diphyletic, suggesting that the polyp stage has been lost independently at least two times within trachyline evolution. Narcomedusae is supported as a monophyletic group likely also arising from trachymedusan ancestors. Finally, some data, albeit limited, suggest that some trachyline species names refer to cryptic species that have yet to be sorted taxonomically.