Feeding activity, absorption efficiency and suspension feeding processes in the ascidian, Halocynthia pyriformis (Stolidobranchia : Ascidiacea): responses to variations in diet quantity and quality

The benthic suspension feeding ascidian, Halocynthia pyriformis (Rathke, 18 06), is often exposed to high concentrations of resuspended sediment in the Bay of Fundy. Resuspended sediment can change diet quantity and quality th at may alter the ascidian's ability to feed and gain energy. The feeding ac tivity of H. pyriformis exposed to bottom sediment was examined using stand ard physiological techniques and video endoscopy. Ascidians were exposed to natural seston plus additions of bottom sediment ranging in concentration from 0 to 46 mg l(-1). For each sediment concentration, clearance rate, ing estion rate, and retention efficiency of the ascidians was estimated using flow-through feeding chambers. Samples of suspended particles and feces wer e collected to estimate absorption efficiency and absorption rate. Results indicate that with increasing sediment concentration, ingestion rate increa sed to a constant level, absorption rate increased linearly despite a logar ithmic decrease in absorption efficiency, and the retention of small partic les (2-5 mum) increased while retention of larger particles (5-15 mum) decr eased. As sediment concentration increased, squirting frequency increased a nd diameter of the siphon was reduced.