Nitrogen Budget of the Bivalve Mactra veneriformis, and its Significance in Benthic—pelagic Systems in the Sanbanse Area of Tokyo Bay

To examine the nitrogen budget of the suspension feeding bivalve Mactra veneriformis in the Sanbanse, a shallow (<5 m) part of Tokyo Bay, Japan, field surveys and laboratory experiments were conducted in early summer, 1998. The Sanbanse area is one of the few remaining areas of intertidal wetland in Tokyo Bay and is an area of significant importance for migrating waders and as a clam fishery and for laver culture. Three infaunal suspension-feeding bivalves, Mactra veneriformis, M. chinensis and Ruditapes philippinarum, were found as the dominant species in terms of biomass. Of these three species, Mactra veneriformis was dominant in the Funabashi area of the Sanbanse. In laboratory experiments, the filtration rate of M. veneriformis (525–918 mg of flesh dry weight), a representative size of the bivalve in the Funabashi area, was 0·95 l g 1 flesh dry weight h 1 corresponding to 408 g N g 1 flesh dry weight h 1. The excretion rates of ammonia and faeces were 35·0 g N g 1 flesh dry weight h 1 and 45·4 g N g 1 flesh dry weight h 1, respectively, at 20 C. Based on the nitrogen budget of M. veneriformis in the Funabashi area in early summer, the population excreted 8·6% of the filtered nitrogen as ammonia and egested 22·2% of the filtered nitrogen as faeces and pseudofaeces, and converted 69·2% as growth. The high growth ratio indicates that the M. veneriformis population efficiently transfers the pelagic primary production to higher trophic levels in the early summer.