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In spite of being one of the most relevant components of the biosphere, the plankton-benthos network is still poorly studied as such. This is partly due to the irregular occurrence of driving phenomena such as gelatinous plankton pulses in this realm. Gelatinous plankters rely on their life cycles and histories to exploit temporarily abundant resources with an undeniable, but often overlooked, impact on marine food webs. Dramatic increases of gelatinous filter-feeders and/or carnivores (both native and nonindigenous species) are frequently observed, and explanations of these blooms alternatively invoke ecosystem variability, climate change, unspecified anthropogenic perturbation or removal of top predators from trophic networks. Gelatinous plankters, however, are not anomalies in plankton dynamics: the recognition of the ecological importance of their pulses, based on their life cycle patterns (often involving benthic stages), is a critical breakthrough to understand the cycling diversity of plankton in space and time. The current study focuses on the many neglected aspects of the ecology and biology of gelatinous zooplankton, describes how life cycle patterns are central in marine ecology, as are the pulses of gelatinous organisms, and highlights how such a dramatic lack of knowledge can affect our understanding of the marine ecosystem as a whole.

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Abstract. .The distribution in space and time of a hydroid community on a vertical rocky substratum is studied from the surface to 20 m depth. The role of the major environmental factors (light, water movement, temperature) in determining the zonation is evaluated. The results are compared with the main zonation models of Mediterranean benthos. The autecology of the collected species is briefly described.

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The genera Zanclea, Halocoryne and Zanclella are surveyed, with description of all known species, including eight new ones of Zanclea (Z. bomala, Z, divergens, Z. fanella, Z. giancarloi, Z. medusopolypata, Z. retraduis, Z. gilii, Z. hirohitoi), two new species of Halocoryne (H. frasca, H. pirainoid), and two new ones of Zanclella (Z. diabolica, Z. glomboides). For most species, the description covers the whole life cycle. The hydroids of this group can be symbiotic with either bryozoans, bivalves, or corals and only few species are not substrate-specialist. Symbiotic relationships led to polymorphism and colony integration with the hosts. The morphologies of hydroid and medusa stages are often not consistent, so that species with very derived hydroids have nonderived medusae (e.g., Halocoryne pirainoid), or species with very derived medusae have non-derived hydroids (e.g., Zanclella diabolica). The architecture of the newly released medusae of Zanclella is exceptional due to a sharp compression of the exumbrella and of the presence of just two radial canals. A phylogenetic scenario is suggested, in spite of the inconsistencies in the rates of change in the two main stages of the cycle.

1. Taxonomic sufficiency concerns the use of higher-taxon diversity as a surrogate for species diversity.\ud It represents a fast and cost-effective method to assess community responses to natural and\ud anthropogenic environmental drivers. In spite of the potential applications of using higher taxa as\ud surrogates for species, little research has been carried out to determine the underlying reasons that\ud might make taxonomic surrogacy effective for detecting diversity changes.\ud 2. Here, we determine whether the effectiveness of higher taxa as species surrogates relies mostly on\ud taxonomic relatedness of species (i.e. the relative closeness of species in the Linnaean taxonomic\ud hierarchy) or depends simply on the numerical ratio between species and higher taxa (i.e. the degree\ud of species aggregation). We reviewed the current literature on taxonomic sufficiency to check for\ud any correlation between the effectiveness of higher taxa and the degree of species aggregation across\ud different types of organisms. Tests based on random simulations from diverse marine mollusc\ud assemblages were also carried out to ascertain whether the ability of higher taxa to detect variation\ud in the multivariate structure of assemblages depended on the degree of species aggregation.\ud 3. Mollusc data showed that information loss and the ensuing decrease in statistical power to detect\ud natural or human-driven changes in assemblages at higher taxonomic levels depend on the degree\ud of species aggregation, rather than on the taxonomic resolution employed. Analyses of the literature\ud suggested that such outcomes could be generalizable to a wide range of organisms and environmental\ud settings.\ud 4. Our findings do not support the idea of a direct relationship between taxonomic relatedness and\ud ecological similarity among species. This indicates that taxonomic ranks higher than species may\ud not provide ecologically meaningful information, because higher taxa can behave as random groups\ud of species unlikely to convey consistent responses to natural or human-driven environmental\ud changes.\ud 5. Synthesis and applications. Surrogates of species-level information can be based on the ‘highest\ud practicable aggregation’ of species, irrespective of their taxonomic relatedness. Our findings cast\ud doubt on static taxonomical groupings, legitimizing the use of alternative ways to aggregate species\ud to maximize the use of species surrogacy.\ud Key-words: biodiversity, conservation, higher-taxon approach, impact assessment, marine\ud molluscs, natural environmental variations, phylogenetic relatedness, taxonomic surrogates,\ud taxonom

Abstract. The history of the study of hydroid ecology is briefly outlined, pointing out the major methodological innovations which have contributed to the development of ecological research in the last thirty years. The influence of the major ecological factors on hydroid ecology and biology is synthesized, taking into account: substratum, water movement, light, salinity, sedimentation, exposure to air, temperature, food availability and pollution. Besides affecting the species composition of the hydroid community, these factors also influence the morphology and general biology of the individual species. The adaptations and reactions of hydroids to different intensities of the various environmental factors and to their combinations are reported.