MACROFAUNAL INVOLVEMENT IN THE SUBLITTORAL DECAY OF KELP DEBRIS - THE SEA-URCHIN PSAMMECHINUS-MILIARIS (GMELIN) (ECHINODERMATA, ECHINOIDEA)

P. miliaris occurs in the Clyde Sea area in large numbers (< 18 individuals per 100 g-1 weed dry wt) on sublittoral beds of detached Laminaria saccharina. Its role in weed decomposition was examined. Younger urchins grew faster than older individuals on a diet of rotting weed but not on fresh weed. Large seasonal variation existed, however, with fast growth occurring in June-Aug. and little, or no, growth in Dec.-Feb., irrespective of diet. Starved controls did not grow. Correcting for seasonality, rotting kelp still promoted faster growth of young urchins than did fresh weed. Larger (older) individuals showed no difference. Urchins fed fresh weed had significantly longer gut retention times. Protein absorption efficiency was higher on fresh than rotting weed, varying with weed protein content and size of urchin. Very young indiviudals can only digest high protein weed efficiently, e.g., material derived from near the frond meristem. Organic C content of rotting weed was significantly lower than fresh weed. C absorption efficiencies were significantly higher on fresh weed which related to organic C content. Standard-sized urchins fed rotting weed produced larger dry weights of feces per day, reflecting increased ingestion rate. In closed-system choice experiments urchins preferred rotting weed kinetically. Size-frequency analysis of field populations suggested that weed beds are principally colonized by larval settlement from the plankton. Mature Psammechinus have evolved different strategies for exploiting fresh and rotting weed. Fresh weed is relatively difficult to digest and long gut retention times allow high protein absorption efficiencies to be attained. Rotting weed has microbial protein and a lower organic C fraction. Some bacterial protein is apparently unavailable and lower protein absorption efficiencies result. Gut retention time is shortened and more food is passed through the gut. Growth remains equivalent. Substratum digestion is of paramount importance for Psammechinus feeding on either fresh or rotting weed, cf. the ''classical'' microbe-stripping detritivore of Fenchel.