The ecology of protists epibiontic on marine hydroids

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.