SOMATIC AND GERMINAL GROWTH OF THE INFAUNAL BRITTLE STARS AMPHIURA-FILIFORMIS AND AMPHIURA-CHIAJEI IN RESPONSE TO ORGANIC ENRICHMENT

The effects of a short-term organic enrichment on growth of the 2 brit tle stars Amphiura filiformis (O. F. Muller) and Amphiura chiajei (For bes) were studied in undisturbed sediment cores obtained from a coasta l and an offshore site of the Kattegat and Skagerrak, and then maintai ned in the laboratory as microcosms. Growth was measured as arm regene ration (somatic growth) and oocyte size (germinal growth). Macrofaunal abundance and biomass were higher in the offshore sediment compared t o the coastal site. Organic enrichment was studied by adding concentra ted phytoplankton (diatoms) to mimic the sedimentation pulse of a spri ng bloom in terms of quality and quantity. The experiment was run for 2.5 mo (April to June 1993). Somatic growth of A. filiformis increased in response to the phytoplankton addition in the coastal site sedimen t but not in the offshore site sediment. Arm regeneration in A. chiaje i increased in both sediments in response to the food pulse. A positiv e correlation between regenerated arm length and oocyte size was obtai ned for both species. The results show that the brittle stars were cap able of increasing their growth and gonad development in response to t he short-term organic enrichment. The difference between sites also su ggests negatively density dependent growth indicative of space or food competition in the densely populated offshore sediment.