The introduction of non-native species is a constant concern around the world since it represents one of the main threats to biodiversity, impacting negatively on native populations, some of them with commercial importance. Hence, monitoring these introductions is fundamental to the management and conservation of the biodiversity of a region. Herein, we report the presence of Moerisia cf. inkermanica in the ballast water of oil tankers loaded at the Cayo Arcas oil terminal. The taxonomy of Moerisia members is uncertain due to the lack of comprehensive morphological descriptions and the few molecular data available. So, we provide a detailed morphological comparison among its congeners. The taxonomic identity of the specimens was determined based on the length of the perradial lobes of the manubrium, the number of tentacles, and the features of their nematocyst rings. Some Moerisids are considered invasive in different localities of the world. However, this genus had not been reported in coastal ecosystems of the Gulf of Mexico over the years until now. Sampled tankers came from different ports of the region, mainly from the northern Gulf of Mexico. Therefore, we encourage systematic monitoring of these ecosystems to recognize the establishment of this species as invasive in the region, know its population dynamics over time, and evaluate the possible ecological impacts that could exert on native populations.

The epibenthic euryhaline hydromedusa Vallentinia gabriellae Vannucci Mendes, 1948 is an olindiid species native to tropical Atlantic waters. Here, we describe the cryptic introduction of this species in an estuary along the coast of Kerala, in southwestern India. This study records the existence of V. gabriellae outside of Atlantic waters for the first time and documents its geographical range expansion. Our identification was based on a combined morphological and DNA barcoding assessment using the COI and 28S markers. Although we noted some morphological differences between our specimens and those from their native range, our findings were conclusive. We suggest that phenotypic plasticity may result from differences such as prey availability between the native and introduced habitats. V. gabriellae medusae are epibenthic and cling to a variety of hard and soft substrates, including bivalves. Our specimens were associated with the invasive fouling mussel Mytella strigata, and we suggest that V. gabriellae could have been transported along with these mussels to the Kerala coast.

Gelatinous zooplankton are an abundant and diverse group of animals in the pelagic environment. However, knowledge of species diversity and spatial distributions, as well as their ecological role, is scarce. We present information of epi- and mesopelagic gelatinous zooplankton recorded by the ‘DeepSee’ submersible between 2006 and 2012 at Isla del Coco (Cocos Island), Costa Rica, an oceanic island in the Eastern Tropical Pacific. Two species of scyphomedusae, three species of hydromedusae, two genera of siphonophores, and two species of ctenophores were observed in the videos, at depths between 50 and 400 m. None of these species had been previously recorded in the waters around the island. Furthermore, except for the jellyfish Pelagia noctiluca and a siphonophore in the genus Praya, all are new records for Costa Rican waters. This study also includes the first record of the cnidarians Modeeria rotunda, Solmissus sp., Halitrephes maasi and Apolemia spp., and the ctenophore Thalassocalyce inconstans in the Eastern Tropical Pacific. We show that surveys in regions with little information about gelatinous zooplankton may broaden our knowledge of their natural history and may result in new records of gelatinous species.

The hydroid and hydromedusa fauna of Russian Arctic seas, totalling 161 species, has been revised taxonomically and biogeographically. Diversity is highest in the Barents Sea, where 133 species are known to occur. Species composition of Hydrozoa throughout Russian Eurasia is decidedly uniform, with marked similarity among all regional faunistic lists. An assemblage of Arctic ubiquitists, a majority of them boreal-Arctic species, comprise the main element of hydrozoans in all Arctic seas. This faunistic main element is responsible for the faunal uniformity observed from one sea to the next across thousands of kilometres along the northern Eurasian coast. Exceptions occur in marginal regions including western parts of the Barents Sea and south-eastern parts of the Chukchi Sea, where species distribution area contours (named as ‘synperates’) come close together. Based on a biogeographic analysis of faunistic data and species distributions of Hydrozoa, all temperate and cold waters of the Eurasian seas and the Central Polar Basin were referred to a single Arctatlantic biogeographic realm. Biogeographic subdivisions within this realm have rather low hierarchical rank, the result of low endemism, high faunal similarity across the northern seas, and predominance of a North Atlantic fauna in Russian northern seas as far as the easternmost Chukchi Sea.

Concern has been expressed over future biogeographical expansion and habitat capitalization by species of the phylum Cnidaria, as this may have negative implications on human activities and ecosystems. There is, however, a paucity of knowledge and understanding of jellyfish ecology, in particular species distribution and seasonality. Recent studies in the UK have principally focused on the Celtic, Irish and North Seas, but all in isolation. In this study we analyse data from a publicly-driven sightings scheme across UK coastal waters (2003–2011; 9 years), with the aim of increasing knowledge on spatial and temporal patterns and trends. We describe inter-annual variability, seasonality and patterns of spatial distribution, and compare these with existing historic literature. Although incidentally-collected data lack quantification of effort, we suggest that with appropriate data management and interpretation, publicly-driven, citizen-science-based, recording schemes can provide for large-scale (spatial and temporal) coverage that would otherwise be logistically and financially unattainable. These schemes may also contribute to baseline data from which future changes in patterns or trends might be identified. We further suggest that findings from such schemes may be strengthened by the inclusion of some element of effort-corrected data collection.

The branching structure of the coral colonies from the genus Millepora provides substrate, shelter, and food resources for several reef organisms, such as fish and invertebrates. The present work analysed the association and behaviour of reef fish on the fire coral Millepora alcicornis and M. brasiliensis from September 2010 to February 2011, on the coastal reefs of Tamandaré, north-east Brazil. A total of 473 reef fish individuals of 27 species were sighted associated with the coral colonies, mainly composed of juveniles (65%). The most frequently observed behaviours were sheltered/stationary and swimming close to the coral; however, individuals were also observed foraging and breeding among Millepora spp. branching colonies. The agonistic behaviour performed by Stegastes fuscus individuals was highlighted in the present study, especially against Chaetodon striatus, Diodon holacanthus and Ophioblennius trinitatis individuals. Millepora spp. hydrocorals are ecologically important for juvenile and adult reef fish belonging to several trophic guilds. The complex structure of the coral colonies provides a wide variety of ecological functions on coral reefs (e.g. shelter, reproduction area, food and even territory). Nevertheless, the abundance of branching fire coral in Brazilian reefs have been declining over the past years; therefore, efforts must be made to protect this key habitat.

Six new records of hydromedusae are reported as secondary hosts of Opechona sp. (Lepocreadiidae) for the Brazilian coast. Among the 392 hydromedusae sampled, 43 (~11%) were harbouring metacercaria. Prevalence (P) and intensity of infection (I) of Opechona sp. varied considerably among the different host species. Gossea brachymera (Limnomedusae) was highly parasitized (P = 30%; I = 1–7), while the other five hydromedusae species were not (P = 1.5–10; I = 1–2). The high parasitism in G. brachymera suggests the importance of this species in the transmission of Opechona sp. to fish, the definitive hosts, and highlights the hydromedusae as a probable noteworthy food item for zooplanktivorous fish in the area.

The new species of Zanclea sango sp. nov. is described from Okinawajima Island, Ryukyu Archipelago, south-western Japan. The new hydrozoan species is associated with at least three scleractinian corals (Pavona divaricata, P. venosa and Psammocora contigua). Zanclea sango sp. nov. is a polymorphic hydroid and the hydrorhiza grows between the coral skeleton and calicoblastic ectoderm. The hydrocaulus and hydrorhiza are surrounded by perisarc. Newly released medusae are almost spherical, with four perradial exumbrellar nematocyst pouches including stenoteles, and two long marginal tentacles with cnidophores containing macrobasic euryteles. Zanclea sango sp. nov. is allied to Zanclea gilii Boero et al., 2000 and Zanclea margaritae Pantos & Bythell, 2010, but it is distinguished by its cnidome, the presence of a perisarc around hydrorhiza, and lower host-specificity. According to a hypothetical Zanclea phylogeny, the ancestral species of Zanclea had an opportunistic association with some benthic organisms, such as algae or bivalves, and its hydrorhiza was covered by a perisarc. Later, some species established specific associations with benthic animals, after which the hydrorhiza lost the perisarc and became directly covered with host tissue. Among Zanclea inhabiting corals, the present species, with multiple coral host species and a perisarc around the hydrorhiza, seems to retain more ancestral character states than Z. gilii and Z. margaritae, which have specific coral host species and no perisarc around the hydrorhiza.

Several hydroid species live associated with many organisms, including bacteria. Hydroid–bacteria associations were searched for in twenty Hydrozoa species that were collected in the northern Ionian Sea and observed under blue light excitation. Of these, six showed high fluorescence on the outer perisarc, five appeared medium fluorescent, four were slightly fluorescent, and five did not show any fluorescence. Luminous bacteria were isolated and counted from the surface of the fluorescent hydroids. Their association with hydrozoan species could be explained by their feeding activity on the chitinous structures of the perisarc, as previous research on the hydroid Aglaophenia octodonta showed. Moreover, microalgae were always recovered together with luminous bacteria in the strongly, medium and slightly fluorescent hydroids. Further studies will be undertaken to characterize the luminous bacteria isolated from the surface of the examined hydrozoans as well as to better understand whether their interaction with hydroids is only related to chitin utilization or if their coexistence with microalgae in hydrozoans has an ecological meaning.

Hydroids in Kongsfjorden, Svalbard, were studied on five hard-bottom transects along gradients of environmental conditions from the glacier at the fjord's head to the fjord's mouth at depth-range 0–30 m. Hydrozoa colonies are widely distributed on rock and gravel substrata in Kongsfjorden. In general, however, hydroids are not dominant or subdominant in zoobenthic communities. The exception is Symplectoscyphus tricuspidatus var. acuminatus, colonies of which were sometimes abundant enough to determine the community structure and characteristics of benthic diversity. Of the 23 species recorded in this study, Laomedea flexuosa, Phialella quadrata and Halecium tenellum as well as representatives of family Stylasteridae were recorded from Svalbard waters for the first time. Hydroid diversity was highest in the zoobenthic community structured by branched bryozoans. The abundance and distribution of hydroids were reduced, to some extent, in the inner part of the fjord compared to the outer fjord. Species richness of hydroids was high at shallow depths, decreased at around 15 m and then increased again to 30 m depth. Species with Low-boreal–Arctic and Panoceanic distributional ranges were the most frequently occurring species. Depth, type of background substratum and amount of silt on its surface were the main factors influencing hydroid distribution. Description of all recorded taxa distributions together with data on their habitat and associations in zoobenthic communities are presented in an Appendix.

Study of living colonies of Salacia tetracythara Lamouroux, 1816, led to an accurate description of the hydranths and adjoining tissues. A mantle is reported for the first time for this species, lining stem, hydrocladia and hydrotheca. An adcauline nematocyst clump located in the median part of the column, composed of large microbasic mastigophores, is also reported as a new structure for sertulariids. The hypothetical modified tentacles noted by Billard (1925) are confirmed and re-described. Among the 22–28 tentacles of the hydranth in a circlet, two exhibit nematocyst-bearing organs connected at base. These organs are massive, racket-shaped, refringent, and composed of large microbasic mastigophores. They do not affect the tentacles in respect of their general shape and behaviour. One nematocyst-bearing organ can sometimes be linked to more than one tentacle depending on its size. The abcauline position of the two modified tentacles imparts a strong bilateral symmetry to the hydranth. This location explains why only one modified tentacle was previously described from preserved material, the second being hidden by the first when hydranths are retracted inside the hydrotheca. The modified tentacles of S. tetracythara are compared with the nematodactyls of the genus Nemalecium, with the ligula found in several sertulariid species, and with nematophores described in other thecate families. Beside remarks on species taxonomy and distribution, incitation is given to the study, with emphasis especially on hydranths.

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.

Despite the hydroid fauna of the Mediterranean Sea being considered one of the best known in the world, the eastern basin of the Mediterranean Sea remains nearly unexplored. This paper reports on 38 species collected along the Levant Sea coast (mainly Lebanon), of which three are new records for the Mediterranean and nine for the Levant Sea. Six alien species, i.e. Eudendrium carneum, Sertularia marginata, Sertularia techocarpa, Macrorhynchia philippina, Diphasia digitalis and Dynamena quadridentata, are described in detail and illustrated on the basis of Levant Sea material. The last four species are considered as immigrants from the Red Sea. The synonymy of Sertularia stechowi, described from Japan, with S. techocarpa is established. Taken as a whole, the collection comprised a majority of circum-(sub)tropical species, and a reduced proportion of Atlantic–Mediterranean elements and Mediterranean endemics. The ecology (seasonality, depth distribution and habitat preference) of the indigenous species resulted similar to what is known for the more studied western Mediterranean, with some exceptions. Adding the present species inventory to the scanty published information, the total of hydroid species known from the Levant Sea rises to 70, indicating the need for future investigation in this sector of the Mediterranean Sea.

A creeping movement, which often results in clockwise or counterclockwise bodily rotation, is described for ordinary zooids of two species of bivalve-inhabiting hydrozoans, Eutima japonica Uchida and Eugymnanthea japonica Kubota, attached to artificial hard substrata (polystyrene and glass) in the laboratory. Time-lapse video recordings furthermore showed that the non-transparent half of the flat, ellipsoidal pedal disc always faces forward during this motion, with no lateral or backward creeping during either day or night. Because an ordinary zooid generally crept in a circle of small radius, the zooid's body rotated as the creeping proceeded. One full rotation of a zooid of either species took about 5–6 hours on average at 21–25°C. The direction of rotation of an individual zooid was nearly constant irrespective of developmental state, this being true for zooids with or without a medusa bud on the hydrocaulus or a daughter zooid on the pedal disc, those with a normal or a double hydranth, or those consisting of a bare hydrocaulus. Asexually produced tiny daughter zooids quickly became well separated from their mother zooids. The attachment site of the latter did not shift much as they rotated, but daughter zooids and other tiny free zooids creep along a nearly straight line, thereby markedly shifting their position. This may contribute to the prevention of crowding of zooids in the restricted space between soft body parts in the host bivalve's mantle cavity.

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.

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.

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.

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.

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.

We report the first record of the hydroid Bedotella armata (Pictet & Bedot, 1900) from a bathyal region of the Porcupine Seabight, southwest Ireland. Bedotella armata was thought to be endemic to the southern Lusitanian province and the Mediterranean.Yet specimens were common in box cores on and off living coral mounds formed by the scleractinian Lophelia pertusa. Irish colonies were slightly larger than those from the Strait of Gibraltar and the Bay of Biscay.We hypothesize that the association of B. armata with L. pertusa could explain morphological differences, as hydroids on reef structures may benefit from better feeding opportunities in enhanced flow conditions. We also hypothesize that post-glacial hydroid colonization of these mounds during the Holocene was partly due to species emigration from the warm saline Mediterranean Outflow Water (MOW) mass. Our findings support the hypothesis that MOW could play a significant role in determining the distribution and recent origins of mound fauna in the Porcupine Seabight.The current findings become the deepest, northernmost record of B. armata.