Importance of predation by crustaceans upon bivalve spat in the intertidal zone of the Dutch Wadden Sea as revealed by immunological assays of gut contents

Abstract

Predation on bivalve spat ( Macoma balthica, Cerastoderma edule and Mya arenaria) by crustaceans was studied in an intertidal area in the western Dutch Wadden Sea in 1993. In all species settlement started at the end of April. Peak numbers were reached in the beginning of June of about 4300 ind. m −2 in Macoma, 1300 in Cerastoderma and 500 in Mya. During June and July numbers dropped sharply to low levels at a mean size of 1.0 to 1.5 mm. Instantaneous mortality rates over this period varied from 0.045 d −1 in Macoma to 0.056 d −1 in both Cerastoderma and Mya. Immunoassays of gut contents of brown shrimps Crangon crangon and shore crabs Carcinus maenas positively identified both species as predators on bivalve spat. The high shrimp densities observed (100 ind. m −2) in combination with their simultaneous presence with the post-larval bivalves indicate that juvenile shrimps are likely the most important predator. The estimated predation pressure by crustaceans, and especially the brown shrimp, is sufficient to account for the observed mortality rates among bivalves. Bivalve recruitment, however, did not appear to be regulated by crustacean predation: during the period of predation, bivalve mortality was not density-dependent. Information on bivalve predation in Swedish bays support this view.