15 pages, 6 figures, 4 tables Population dynamics, morphology, density, lipid contents and prey capture rates were studied in a patchy population of the tree-like passive suspension feeder Eudendrium racemosum (Cnidaria: Hydrozoa) during seasonal cycles. Density and size of colonies oscillated due to intraspecific competition (following the self-thinning rule) and predation (nudibranchs) showing size distributions in which alternatively large-and small-sized colonies were more abundant. Increase in the trophic impact on the water column was correlated with colony size (30.7 mg C m -2 d -1 in late spring-early summer compared with 10.2 mg C m -2 d -1 in the rest of the year). Lipid accumulation in the hydrocauli followed the same trend, being higher during the reproductive period (140 μg lipids mg -1 organic matter [OM]) compared with the rest of the year (70 μg lipids mg -1 OM). The short time cyclic variations reproduce, in a short time scale, the variations studied in other benthic suspension feeders, suggesting E. racemosum as a model organism to better understand the importance of large colonies in long-lived species in benthic-pelagic coupling processes. The accordance with the self-thinning rule found for this animal species strengthens the animal forest concept (a 3-dimensional live structure composed mainly by heterotrophic organisms such as gorgonians, bryozoans, and sponges), confirming the validity of the utilization, by marine ecologists, of the conceptual tools developed for terrestrial forest sciences. It is suggested that the impact of mature populations on benthic-pelagic coupling and biogeochemical cycles, simulated for long-lived species, could be validated by studying fast-growing species, in order to understand the importance of the preservation of complex structures and long-lived eco-engineering organisms in benthic habitats. © Inter-Research 2012 Support for this work was provided by a FPI fellowship from the ‘Ministerio de Educación y Ciencia’ and a Ramón y Cajal Contract (RyC-2007-01327) to S.R., by a CICYT grant and by the MAST-III-ELOISE European Union METRO MED Project. L.B. was supported by a Marie Curie Intra-European Fellowship within the 7th European Community Framework Programme (Project No. 221072 Peer Reviewed