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

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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%).
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