Population dynamics of Eudendrium glomeratum (Cnidaria: Anthomedusae) on the Portofino Promontory (Ligurian Sea)

Eudendrium glomeratum Picard, in the Ligurian Sea, is one of the major components of hard-bottom sessile zoobenthos in the cold season. It settles mainly between 10 and 40 m depth, forming a seasonal facies. The presence of E. glomeratum has been evaluated by measuring in situ the height of the colonies present within a standard surface of 1 m². Observations were carried out every other week for a whole year. The height of the colonies was correlated with their biomass (wet weight) and trophic functionality (number of polyps). The quantitative evaluation of the population shows that the situation is stable, with almost identical values at the extremes of the observed yearly time series. The species is present from October to April–May, and it is absent in the summer. The formation and disappearance of the E. glomeratum population are sudden. The colonies are sexually mature from their appearance in October to December–January. Different categories of colonies are recognizable; and are essentially referable to two groups: (1) colonies deriving from regeneration of colonies present in the previous year (45%); (2) colonies deriving from planula settlement (43%). A third group (12%) is of uncertain origin and may belong to either of the abovementioned categories. Mortality is 60% in the first group, and 82% in the second one. By dead colonies is meant those which do not reappear after a yearly cycle.