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 colonlal organisms, the energy budgets of 2 common Mediterranean cnidarlans the hydrold Campanularja everta and the gorgonian Paramur~cea clavata, were compared C everta is a small hydroid that forms stoloniferous colonies on ephemeral substrata Adapted to a n eplblotlc hfe strategy, it produces small colonies with 6 % of welght as structural matenal 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 metabohsm supported by dally ingestion rates of more than 19% of body w e ~g h t and commensurately high respiratory and excretory rates (8 5% of body weight daily) Sexual reproduction in this gonochonc specles is precocious and concentrated in autumn when a dally effort equivalent to 4-10% of somatlc blomass is expended on reproduction The flow of energy through C e v e ~t a amounts to some 1915 cal g C-' d-', w t h a mean turnover time of 12 d P clavata forms large colonles wlth polyps 100 times larger than those of the hydrold The skeleton consists of spicules and a n organic skeletal axls 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, wlth a turnover rate of 9 yr The main energetic component ( 7 2 % ) 1s basal metabolism From March to June, daily Investment in reproduction is equivalent to 0 4-0 7 % of the tissue welght The total energy flow through P clavata is no more than 150 cal g C-' d-' The daily energy demand is 1 43 % of tissue weight or 3 % of total dry weight Although they exhibit quite ddferent ecological strategies, whlch appear to b e regulated mainly by trophic and substratum constraints, the 2 species have certaln hfe-history traits in common In annual budgetary terms reproductive effort is s~mllar In both specles, 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 hke the Mediterranean an energetic approach can provide a n important contribution to the understanding of life-history traits of modular colonlal organisms