Trophic characteristics of the Manila clam (Tapes philippinarumAdams and Reeve)

Parameters of feeding and food balance in the Manila clam Tapes philippinarum and in the carpet shell Tapes decussatus were estimated using 14C-labelled algae and bacteria as food. The Manila clam has a wide feeding spectrum, consuming and digesting various kinds of algae, bacteria, and rotifers. The optimal concentrations of algae (Nitzschia, Chlorella) were found to be around 8–9 mg l−1 (wet weight) and those of bacteria, 4.5–5 mg l−1. The threshold concentration of food (where assimilation rate=respiration rate) was 1.6–1.7 mg l−1 of algae and 1.5 mg l−1 of bacteria. The filtration rate fell to between 3 and 6 l h−1 g−1 flesh carbon, or 0.1–0.9 l h−1 per specimen. The food ration per day was 3–5% of body weight, or 30–50 mg C g−1 of flesh carbon with optimal food concentrations. The assimilation of green algae and diatoms consumed as food was 50–60%, and the efficiency of the use of assimilated food for growth (k2) was 0.36–0.49, depending on the kind of food (algae). The data obtained were used to evaluate trophic conditions for farming clams in the Lagoon of Venice, by comparing food requirements with the real food concentrations found in the lagoon.