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

In order to evaluate the interaction between handling cost and growth of the bivalve‐feeder Halla okudai (Polychaeta: Lysaretidae), the observation of foraging behavior and measurement of energy budget were conducted in laboratory feeding experiments by offering three different bivalves: the short‐neck clam Ruditapes philippinarum, the blue mussel Mytilus galloprovinciallis, and the Pacific oyster Crassostrea gigas. The total handling time was 1.6–2.7 times longer in the oyster‐group (25.6 ± 9.3 h) than in the clam and mussel‐groups, while the total searching and feeding time hardly varied with prey species. No significant difference in the amount of ingestion (4.5–5.3 kcal/polychaete per 15 days) was shown among the three prey groups. However, the growth increment was 1.5–2.1 times higher in the clam‐group (1.85 ± 0.65 kcal/polychaete per 15 days) than in the other two prey groups. In contrast, the scope for active rate of respiration and jelly‐like material secreted during handling was higher in the oyster‐group (0.66 ± 0.14 and 0.24 ± 0.07 kcal/polychaete per 15 days, respectively) and mussel‐group (0.62 ± 0.07 and 0.16 ± 0.05 kcal/ polychaete per 15 days) than in the clam‐group (0.48 ± 0.09 and 0.06 ± 0.01 kcal/polychaete per 15 days), although there were no significant differences in the standard rate of respiration, excretion, feces and jelly‐like material secreted during feeding among the three groups. These results indicate that growth of H. okudai increases with a decreasing handling cost, when feeding on the optimal prey species.