Feeding activity, retention efficiency, and effects of temperature and particle concentration on clearance rate in the marine bryozoan Electra crustulenta

Various factors influencing clearance rate were elucidated on the bryozoan Electra crustulenta (Pallas). Measurements of clearance rates were performed using the algae Rhodomonas sp. (6 mum in diameter). Clearance rates were related to the area of the active zooids within the colonies in order to obtain area-specific clearance rates. Specific area was 42 % of the total colony area. Several replicates were performed with each colony to obtain maximum clearance rate (F-max). F-max increased with temperature from 90 ml h(-1) cm(-2) at 6 degreesC to 229 ml h(-1) cm(-2) at 22 degreesC. Clearance rate decreased at increasing algal cell concentration from 1600 to 19 000 cells ml(-1). The decrease in clearance corresponded to a maximum ingestion rate at particle concentrations > 8500 Rhodomonas sp. cells ml(-1). E. crustulenta zooids are capable of retaining and ingesting particles in the range from ca 5 to ca 30 pm in diameter. Smaller particles are less efficiently retained due to the structure of the feeding apparatus, the lophophore and larger particles due to the size of the mouth (30 pm in diameter). Feeding activity was observed on single zooids and it was found that zooids have periodical retraction of the lophophore. At low particle concentrations (ca 1500 cells of Rhodomonas sp, ml(-1)) the lophophore is retracted 5 x h(-1) for periods of 38 s. Zooidal activity measured as the time of protruded lophophore thus leads to an activity of 95 % of the total time. At high algae concentrations, zooidal feeding activity decreased to 70 % as the lophophore was retracted more frequently (10 x h(-1)) and for longer periods of time (107 s). Despite the decreased activity at high algae concentration, this could only account for 50 % of the decrease in clearance rate. Thus, regulatory mechanisms of the clearance rate other than retraction of the lophophore must be considered in bryozoans.