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 behavior and measurement of energy budget were conducted in feeding experiments for 15 days by supplying two different sizes of prey, Ruditapes philippinarum (small, 24.2 ± 1.5 mm; medium, 34.0 ± 4.1 mm in shell length). The total 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 contrast, 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 the small‐prey group (20.3 ± 5.5% and 9.4 ± 1.5%), although there was no significant difference in the standard rate of respiration, feces, and jelly‐like material secreted during handling and feeding between both prey groups. These results indicate that the differences in searching costs between both prey groups [i.e. the scope for active rate of respiration (11.8%) and excretion (2.8%)] have great effects on the growth (19.3%).