Life-history traits of Alcyonium acaule and Parazoanthus axinellae (Cnidaria, Anthozoa), with emphasis on growth

Life-hstory aspects of 2 common cnidarians, Parazoanthus axinellae and Alcyonium acaule, were studied in the northwestern Mediterranean over 2 yr.Rates of growth (i e. area gained) and shrinkage (i.e.area lost), colony fiss~on and fusion, mortality, feeding activity and preliminary data on recruitment (for A. acaule) were measured.Independent growth and shrinkage rates of P. dvinellae were measured as area gained and lost per month respectively using a photographic and computerassisted image analysis.Measurement of growth in A. acaule was based on size variations of the colonies over 2 yr.Results indicated moderate growth dynamics for P. axinellae and very slow growth by A. acaule.Growth rates and shrinkage rates in P. axinellae showed non-significant differences over time, although growth rates peaked during summer to autumn penods for both mon~tored years.In general, growth dynamics for the cnidarians studied were similar to or lower than those reported for related species from temperate and tropical habitats.Fission events were common in P. axinellae.Of the colonies monitored.29% underwent at least 1 fission event in 2 yr.Fusion events occurred less frequently; only 8 % underwent 1 fusion event.Neither fusion nor fission occurred in A acaule.Mortality rates were about 10 % yr-' for both species.Preliminary data showed low recruitment rates for A. acaule.Data on life-history traits were interpreted as evidence for d~stinct biological strategies used to persist and to occupy new substrata.P. axinellae based ~t s strategy on a greater dynamic spreading over the substrata at moderate rates, by somatic growth and by fission.In contrast, A. acaule showed relatively slow growth but greater longevity.