AN ENERGETIC APPROACH TO THE STUDY OF LIFE-HISTORY TRAITS OF 2 MODULAR COLONIAL BENTHIC INVERTEBRATES

In order to study contrasted ecological strategies and life-history tr aits of modular colonial organisms, the energy budgets of 2 common Med iterranean cnidarians, the hydroid Campanualaria everta and the gorgon ian Paramuricea clavata, were compared. C. everta is a small hydroid t hat 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 alloca ted 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 supp orted by daily ingestion rates of more than 19% of body weight and com mensurately high respiratory and excretory rates (8.5% of body weight daily). Sexual reproduction in this gonochoric species is precocious a nd concentrated in autumn, when a daily effort equivalent to 4-10% of somatic biomass is expended on reproduction. The flow of energy throug h 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 l arger 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, equi valent 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 th e tissue weight. The total energy flow through P. clavata is no more t han 150 cal g C-1 d(-1). The daily energy demand is 1.43% of tissue we ight 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 tr aits in common. In annual budgetary terms, reproductive effort is simi lar in both species, substratum constraints notwithstanding, and conce ntrated in a period of the year favourable to the survival of offsprin g. The results suggest that, at least in seasonal seas Like the Medite rranean, an energetic approach can provide an important contribution t o the understanding of life-history traits of modular colonial organis ms.