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.