Feeding and bacteriolytic responses of the deposit-feeder Abarenicola pacifica (Polychaeta : Arenicolidae) to changes in temperature and sediment food concentration

Deposit-feeders can respond to seasonal fluctuations in food concentration both functionally (e.g, by adjusting feeding rates) and physiologically (e. g. by changing the concentration of bacteriolytic agents in gut fluids). La boratory feeding experiments were carried out (11 to 21 July 1997) with the arenicolid polychaete worm Abarenicola pacifica (Healy and Wells). Objecti ves were to test for separate and interactive effects of sediment food conc entration and temperature (6, 11, and 16 degrees C) on deposit-feeder funct ional (feeding rates) and physiological (bacteriolytic activity of gut flui ds) responses. Food concentration was varied experimentally using sieved (1 mm) natural sediments (Md phi = 2.00; 0.6% organic) mixed with combusted ( 500 degrees C, 8 h) sediments for final concentrations of 25, 50, and 100% natural sediment. Sediment food quality was measured as: (1) bioavailable a mino acids (EHAA), (2) chlorophyll a (chl a), and (3) bacterial abundance. Feeding rates were inferred from egestion rates (ER, g h(-1)) and analyzed with respect to worm size. Bacteriolytic activity of midgut fluids was assa yed turbidimetrically against two bacterial isolates, after worms had fed o n experimental sediments for 15 d. Temperature and food concentration both significantly affected feeding rates, with maxima occurring at 50 and 100% natural sediment mixtures, and at high (16 degrees C) temperature. ER was p ositively, but not significantly correlated with EHAA and chl a; a positive , significant correlation was detected between ER and sediment bacterial ab undance. Overall, functional responses agreed with earlier compensatory int ake models for deposit-feeders. However, the size and direction of these re sponses was temperature-sensitive, suggesting that these models need to be adjusted for changes in absorption rates. No effects of ambient temperature or food concentration on bacteriolytic rates were observed, possibly due t o compensatory mechanisms or the presence of multiple bacteriolytic agents in gut fluids.