Minimal scaling of the lophophore filter-pump in ectoprocts (Bryozoa) excludes physiological regulation of filtration rate to nutritional needs. Test of hypothesis

Feeding activity and growth in different ectoprocts (Electra pilosa, E. crustulenta, Celleporella hyalina) were related to naturally occurring algal concentrations in order to reveal if physiological regulation of the lophophore filter-pump to nutritional needs takes place. It was found that filter-feeding activity in E. pilosa and E. crustulenta is influenced by the concentration of algal cells in the surrounding water. When the algal concentration becomes very low (corresponding to a few hundred Rhinomonas algal cells ml(-1)) the clearance rate of the ectoprocts is reduced due to a gradual shutting down of the number of actively feeding zooids. The zooids are inactive during starvation periods, but feeding is quickly resumed after a new addition of algal cells. Nearly 100 % of the zooids are filter-feeding within a certain range of algal concentrations, corresponding to roughly 400 to 4000 Rhinomonas reticulata cells ml(-1). The maximum individual zooidal clearance rate in E. pilosa was measured to be 0.28 ml h(-1) zooid(-1) (18 degrees C). Simultaneous clearance of 2 different sized algal cells in E. crustulenta showed that R. reliculata (6 Fun) and Tetraselmis sp. (14 Fun) are cleared from the water at approximately the same rate, whereas the removal of 4 Fun diameter Isochrysis galbana cells is approximately half this rate. The growth in colony size of C. hyalina and E. pilosa was exponential. The specific growth rate plotted as a function of algal concentration showed that the growth potential (about 10 to 12% d(-1)) may be fully utilized at about 4000 Rhinomonas cells ml(-1), corresponding to 5 pg chlorophyll a l(-1). This implies that ectoprocts in nature, as a rule, always utilize their clearance capacity and hence have no requirement for physiological regulation of the filter-pump.