Interaction between searching cost and growth of the bivalve-feeder Halla okudai under rearing conditions, in relation to prey size

To evaluate an interaction between searching cost and growth of the bivalve -feeder Halla okudai(Polychaeta: Lysaretidae), observation of foraging beha vior and measurement of energy budget were conducted in feeding experiments for 15 days by supplying two different sizes of prey, Ruditapes philippina rum (small, 24.2+/-1.5 mm; medium, 34.0+/-4.1 mm in shell length). The tota l searching time was longer in the small-prey group (28.9+/-2.5 h) than in the medium-prey group (16.4+/-6.4 h), while the total handling and feeding time hardly varied with prey size. The growth rate was higher in the medium -prey group (36.3+/-5.7%) than in the small-prey group (17.0+/-9.7%). In co ntrast, the scope for active rate of respiration and excretion was lower in the medium-prey group (8.5 +/-1.6% and 6.6+/-0.8%, respectively) than in t he small-prey group (20.3+/-5.5% and 9.4+/-1.5%), although there was no sig nificant difference in the standard rate of respiration, feces, and jelly-l ike material secreted during handling and feeding between both prey groups. These results indicate that the differences in searching costs between bot h prey groups [i.e. the scope for active rate of respiration (11.8%) and ex cretion (2.8%)] have great effects on the growth (19.3%).