An energetic approach to the study of life-history traits of two modular colonial benthic invertebrates

In order to study contrasted ecological strategies and life-history traits of modular colonial organisms, the energy budgets of 2 common Mediterranean cnidarians, the hydroid Campanualaria everta and the gorgonian Paramuricea clavata, were compared. C. everta is a small hydroid that forms stoloniferous colonies on ephemeral substrata. Adapted to an epibiotic life strategy, it produces small colonies with 6% of weight as structural material. Growth is very fast and much energy is allocated throughout the year to growth and it develops stolons which enable it to spread readily on the algal substratum. A colony may live for 2 to 6 wk. Maintaining high growth rates requires rapid metabolism supported by daily ingestion rates of more than 19% of body weight and commensurately high respiratory and excretory rates (8.5% of body weight daily). Sexual reproduction in this gonochoric species is precocious and concentrated in autumn, when a daily effort equivalent to 4-10% of somatic biomass is expended on reproduction. The flow of energy through C. everta amounts to some 1915 cal g C-1 d(-1), with a mean turnover time of 12 d. P. clavata forms large colonies with polyps 100 times larger than those of the hydroid. The skeleton consists of spicules and an organic skeletal axis which amount to 54 and 35%, respectively, of the colonies' total biomass. Growth is slower than in C. everta, equivalent to 0.15% of tissue weight daily, with a turnover rate of 9 yr. The main energetic component (72%) is basal metabolism. From March to June, daily investment in reproduction is equivalent to 0.4-0.7% of the tissue weight. The total energy flow through P. clavata is no more than 150 cal g C-1 d(-1). The daily energy demand is 1.43% of tissue weight or 3% of total dry weight. Although they exhibit quite different ecological strategies, which appear to be regulated mainly by trophic and substratum constraints, the 2 species have certain life-history traits in common. In annual budgetary terms, reproductive effort is similar in both species, substratum constraints notwithstanding, and concentrated in a period of the year favourable to the survival of offspring. The results suggest that, at least in seasonal seas Like the Mediterranean, an energetic approach can provide an important contribution to the understanding of life-history traits of modular colonial organisms.