The Hydrozoan Society was founded in 1985 with the aim of promoting all aspects of the study of hydrozoans, from taxonomy, ecology and physiology to molecular biology. Instead of holding meetings with a formal agenda of talks and lectures, the Society has carried out its objectives by organizing workshops, which encourage internal work groups and new collaboration. Conference sessions and round tables are nevertheless an important activity of the Society and are where the most recent research in the field of hydrozoan biology is presented. The 6th Workshop of the Hydrozoan Society was held in June 2007 at Plymouth in collaboration with the Marine Biological Association of the United Kingdom. It followed the same philosophy as before and as usual the working atmosphere was very open and enthusiastic, making it easy for young scientists to collaborate with more experienced researchers. This propitious environment has always been a key factor in the Society's activities (Boero, Reference Boero2007) and it has distilled into one of its best legacies. The Society does not have an official consolidated structure, since it is the work of researchers during and between the workshops that gives it direction and consistency. In addition, although the structure is not very rigid, this actually promotes collaboration and the participation of specialists in joint projects and other activities.
Like previous meetings, the Plymouth Workshop hosted a wide range of themes related to the biology of hydrozoans (Mills et al., Reference Mills, Boero, Migotto and Gili2000). The diversity of the work presented, however, was even broader than hitherto. The scientific community has evolved to such an extent that, although basic research on taxonomy and ecology continues hydrozoans also offer a resource increasingly used for studies related to genetics and molecular biology. Papers from this meeting report a bright spectrum of discoveries but do not pretend to include all strands of contemporary research. Many of the authors work at only one remove from live material in the field. Indeed, the Society's meetings are held within sight of the sea (and where freshwater forms such as hydra are rare).
A group of papers on ecology and distribution is full of surprises. The exploration of new habitats, such as the mid-water ecosystems, has led not only to the discovery of new species but also to an understanding of the functions of mid-water habitats and of new aspects of the biology and ecology of their pelagic fauna (Lindsay et al., Reference Lindsay, Pagès, Corbera, Miyake, Hunt, Ichikawa, Segawa and Yoshida2008). The capture of large amounts of phytoplankton and its digestion confirm the omnivorous and also herbivorous feeding of hydroids and hence their significant role in transferring energy from pelagic to benthic ecosystems (Gili et al., Reference Gili, Duró, Garcia-Valero, Gasol and Rossi2008). Detailed studies even of highly surveyed areas, such as the Mediterranean, provide unexpected observations concerning unknown species and their life cycles, which demonstrate that even in the best-known areas there is still much to discover (Gravili et al., Reference Gravili, D'Ambrosio, Di Camillo, Renna, Bouillon and Boero2008). This has been the case in the rediscovery of Protohydra leuckarti near Plymouth (Kilvington et al., Reference Kilvington, Collins, Kosevich, Pyataeva and Robson2008). The study of colonization processes in species with a patchy distribution has also given unforeseen results, such as asexual reproduction processes not being those which favour aggregation, whereas patchiness in some species is explained by the poor dispersion capacity of larvae with a sexual origin (Marfenin & Belorustseva, Reference Marfenin and Belorustseva2008). In epiphytic hydrozoan communities, the pattern of biodiversity is related to microhabitat characteristics associated with the algal host's morphology, on a scale very different from that of environmental processes such as the physical drivers of glacial activities in Arctic communities (Ronowicz et al., Reference Ronowicz, Wlodarska-Kowalczuk and Kuklinski2008). A study of the morphological characteristics of different ecological strategies such as phalanx and guerilla phenotypes in Hydractinia suggests that the traditional view of guerilla growth as an adaptive strategy is no longer correct, and that in several species guerilla-like growth is better seen as poor adaptation (Ferrell, Reference Ferrell2008). In populations of tropical hydroids, studies of seasonal variation show that seasonal trends are linked to rainfall and hence to abundant food availability (Di Camillo et al., Reference Di Camillo, Bavestrello, Valisano and Puce2008). Environmental differences in temperature and salinity influence the timing of asexual budding in a hydromedusa (Proboscidactyla) (Kawamura & Kubota, Reference Kawamura and Kubota2008). In bivalve-inhabiting hydroids, the liberation of medusae occurs regularly, and although they are all released at sunset this is not related to a decrease in light intensity (Kubota, Reference Kubota2008a). Hydrozoans which form large colonies, such as the genus Eudendrium, host rich epibiontic organisms that show a highly heterogeneous distribution over the colony as well as some unexpected seasonal patterns (Bavestrello et al., Reference Bavestrello, Cerrano, Di Camillo, Puce, Romagnoli, Tazioli and Totti2008).
One of the most complex tasks in the taxonomic study of a phylum such as the Cnidaria is not only to reorganize existing information and situate species correctly according to their morphological characteristics but also to deal with their evolutionary and geographical characters. This is the case of the genus Gymnogonos, where the discovery of a species in the Pacific has led to the theory that it might have arisen by neotony from the tropical Corymorphidae during the period of glacial cooling (Stepanjants & Svoboda, Reference Stepanjants and Svoboda2008). The exploration of areas that have only recently become accessible has led to the discovery of very interesting new species in the Aleutian Islands, Alaska (Brinckmann-Voss & Linder, Reference Brinckmann-Voss and Lindner2008; and front cover), and the very surprising case of hydrozoans that live inside Arctic marine ice (Piraino et al., Reference Piraino, Bluhm, Gradinger and Boero2008). Close associations of hydrozoans with other species can lead to extreme examples, such as a sharing in both partners of morphological modifications that reinforce mutual or symbiotic relationships (Puce et al., Reference Puce, Cerrano, Di Camillo and Bavestrello2008a). The study of unknown life cycles may reveal aspects of hydroid life histories that cast doubt on the original descriptions of these species or apparently established features of their biology (Di Vito et al., Reference De Vito, Boero, Di Camillo, Megina and Piraino2008).
Phylogenetic studies of Hydrozoa bear on wider perspectives of the Cnidaria, and the use of molecular methods may suggest evolutionary sequences. Current studies confirm the monophyletic character of Leptothecata and Siphonophorae but cast doubt on the situation of Anthothecata. The two groups considered until now, Filifera and Capitata, could be subdivided into four separate filiferan clades (Cartwright et al., Reference Cartwright, Evans, Dunn, Marques, Miglietta, Schuchert and Collins2008). A consideration of molecular data has shown that Trachymedusae is diphyletic, as well as suggesting that the polyp stage has been lost independently at least twice during trachyline evolution (Collins et al., Reference Collins, Bentlage, Lindner, Lindsay, Haddock, Jarms, Norenburg, Jankowski and Cartwright2008).
The significance of new data from morphological and behavioural studies, especially those using living material, should not be underestimated. In Leptomedusae the discovery and description of rare morphotypes leads to an understanding and reconstruction of the morphogenetic evolutionary scenario (Kosevich, Reference Kosevich2008). The cytomorphological study of Polypodium, a unique cnidarian parasite of fish eggs, suggests that this species is not aberrant as previously thought but is a relic of a major hydrozoan group (Raikova, Reference Raikova2008). Study of the organization of soft tissue in hydroids has shown that their structural complexity, comprising something more than two epithelial layers, does not contravene limits to the ground plan of this group of Hydrozoa (Pyataeva & Kosevich, Reference Pyataeva and Kosevich2008). A revision of the symbiotic relationships of hydrozoans with other phyla demonstrates that together with ecological processes, symbioses are a source of evolutionary diversity (Puce et al., Reference Puce, Di Camillo and Bavestrello2008b). In live specimens of well-known hydrozoan species, measurement of their morphological characteristics has led to the discovery of new species and of group structures (Gravier-Bonnet, Reference Gravier-Bonnet2008a): examples include colonial polymorphism (Gravier-Bonnet, Reference Gravier-Bonnet2008b), colonization processes in extreme situations (Kubota, Reference Kubota2008b) and aspects of bioluminescence of as yet unknown biological significance (Kubota et al., Reference Kubota, Pagliara and Gravili2008).
These contributions on the biology of Hydrozoa confirm their unusual morphological, ecological and physiological plasticity and a remarkable degree of adaptability to environmental conditions during their evolution. New advances are supported on one hand by more classical studies of taxonomy and morphology, and on the other by new genetic and molecular investigations. The hydrozoans are a zoological group whose wide distribution throughout marine environments is due in part to diversification of phases in the life cycle. In seeking to understand their biology, faunistic data and current developments in ecology and physiology show that whether in respect of their environment, geographical area or species, we still have a long way to go. The detailed papers which follow are full of new questions and contribute to the mainspring of zoological research across much broader horizons.
