Stefania Puce

The problem of seasonality of benthic hydroids in temperate waters

A wealth of data are available on the seasonal variation of plankton communities, but few studies take into consideration the circannual cycles of benthic organisms. In fact, the macrobenthos is generally considered as composed by slow-growing organisms showing variations mainly in relation to substrate competition. On the contrary, hydroids are an important group of macrobenthos that, in temperate but also in tropical waters, undergo sharp seasonal cycles. For example, on the Portofino Promontory (Ligurian Sea), about half of the recorded species thrive in winter and 30% in the summer, the rest being constant in presence. Here, we revise the available data about the environmental factors switching the passage from quiescent to active phases. The available data suggest that a mosaic of physical factors, biotic interactions, and internal clues triggers the hydroid cyclic behaviour.

Hydroidomedusae (Cnidaria: Hydrozoa) symbiotic radiation

Hydroids can establish symbiotic relationships with most marine phyla. Almost entire genera or even families are associated with specific groups (e.g. Hydractiniidae and Cytaeididae with gastropods and hermit crabs, Zancleidae with bryozoans, Dipurena with sponges, Ralpharia with octocorals, Eugymnanthea with bivalves, Proboscidactyla and Teissiera with serpulids, Bythotiara with tunicates). Generally, the symbiotic groups belong to the Anthomedusae that, due to the absence of theca, are more plastic in establishing trophic relationships with the hosts. Nevertheless a number of scattered species, mainly Leptomedusae, are strictly associated to algae or sea grasses: in these cases no evident morphological or behavioural adaptations were observed. In animal symbiosis several unrelated symbiotic species show polymorphic colonies or a strong reduction in number and/or size of the tentacles, which are sometimes completely lost. Moreover, these symbiotic species may lack perisarc even in the hydrorhiza. In this paper we summarize the morphological and behavioural adaptations of symbiotic species suggesting that the described aptitude of hydroids to establish relationships with other organisms is not only the result but also the source of the evolutionary radiation of this group.

Contribution to the understanding of seasonal cycle of Aurelia aurita (Cnidaria: Scyphozoa) scyphopolyps in the northern Adriatic Sea

Natural populations of Aurelia aurita scyphopolyps, settled on a wreck in the northern Adriatic Sea, were monitored for 22 months in order to study their seasonal cycle. The trend of Aurelia polyps is strongly dependent on water temperature with a peak of abundance in summer and low densities in winter. The strobilation process occurs only once a year, during the cold season, and the estimated number of released ephyrae is 780,000–2,600,000/m 2 . The species has been recorded only on the iron wreck while it is completely lacking from the close rocky cliff of Conero Promontory, suggesting that the increase of underwater structures of anthropogenic origin could be related to the increases of jellyfish biomass.

Hydroids (Cnidaria: Hydrozoa) from the Levant Sea (mainly Lebanon), with emphasis on alien species

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 ofcircum-(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.

Spatial and temporal distribution in a tropical hydroid assemblage

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.

Hydroids symbiotic with octocorals from the Sulawesi Sea, Indonesia

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.

Relationships between benthic diatoms and hydrozoans (Cnidaria)

Some examples of relationships between hydroids and epibionthic diatoms from the Mediterranean Sea are described, verifying the kind of interaction existing between the two partners. The athecate Eudendrium racemosum hosts an extremely rich diatom assemblage, mainly comprising Licmophora spp., Amphora spp. and Cocconeis spp. On the contrary, only adnate growth forms ( Cocconeis pseudonotata , C. dirupta ) were observed in diatom communities growing on the external side of thecate species Campanularia hincksii , Clytia linearis and Synthecium evansi . Some diatom species ( Cocconeis notata , Cylindrotheca sp. and Navicula sp.) are able to survive in the intrathecal microenvironment. They live in the narrow space between hydrotheca and polyp, receiving protection and probably using the nutrients produced by hydroid metabolism. Sunlight can penetrate through transparent thecae and reach the diatom layer, making photosynthesis possible.

Foraminifers epibiontic on Eudendrium (Cnidaria: Hydrozoa) from the Mediterranean Sea

Four species of foraminifers Rosalina globularis, Lobatula lobatula, Acervulina inhaerens and Miniacina miniacea were observed as epibionts on some species of the athecate hydroids of the genus Eudendrium—E. armatum, E. glomeratum, E. moulouyensis and E. racemosum —from the Mediterranean Sea. The abundance and the distribution of the protozoans were determined for each hydroid and considered in relation to the shape and size of the Eudendrium branches. Results revealed differences in the composition of the epibiontic assemblage among different species of Eudendrium which were dependent on several factors, but were mainly related to the habitat and to the morphology of the host. The interactions between Eudendrium and its epibionts were considered to emphasize the role of the Eudendrium colonies as a hotspot not only of poorly known species but also of co-evolutionary relationships and life histories.

Biological Cycle of Podocoryna Exigua (Cnidaria: Hydrozoa) from a Sandy Bottom of the Ligurian Sea

The genus Podocoryna is characterized by species living on shells usually inhabited by hermit crabs; but the mutual benefits of this association are not well understood. This study, besides describing the biological cycle of the hydroid Podocoryna exigua , brings new information on the relationship between hydroid and the hermit crab Diogenes pugilator , especially concerning their trophic strategies. The hydroid shows a seasonal cycle with a minimum density during summer and an increase during autumn and winter, reaching the highest value in spring. The production of medusae is almost constant during all seasons. The dwelling behaviour of the hermit crab affects a particular sedimentivorous strategy of the polyps while a clepto-commensalistic strategy is adopted by the hermit crab towards the hydroid. Laboratory experiments show that the presence/absence of the hermit crab in the shell and also the seawater temperature strongly influence the number of spines, particularly structures developing from polyps. In addition, free-living polyps in the sediments may represent a strategy to colonize free shells.

Water movement activating fragmentation: a new dispersal strategy for hydractiniid hydroids

The fragmentation process in Hydractinia pruvoti (Motz-Kossovska, 1905) which live on shells inhabited by hermit crabs is activated by the intensity of water movement that induces a rapid and abundant production of propagules by transversal fission. Being produced by all kinds of polyps in the polymorphic colony, fragments are very different from each other both in shape and size. After their liberation, fragments first settle on the bottom, then dedifferentiate and produce a network of hydrorhizae from which new polyps arise.