DEPENDENCE OF THE SURF CLAM PSEUDOCARDIUM SACHALINENSE (BIVALVIA: MACTRIDAE) ON THE NEAR-BOTTOM LAYER FOR FOOD SUPPLY

Abstract

We investigated the food sources of the surf clam Pseudocardium sachalinense by comparing the microalgal composition in the stomach with that of the overlying water and in the bottom sediment. The microalgal composition in the stomach contents resembled that of the sediment, in which four microalgal species groups: Melosira, Coscinodiscus, solitary species of Nitzschiaceae and Naviculaceae constituted a large proportion of the total throughout the study. By contrast, the dominant microalgae in the water column changed through the year and was different from those in the stomach contents and the sediment. Since the surf clam is an obligate suspension feeder, these results indicate that it relies on the overlying water close to the bottom surface as a food source layer. Since there is little vertical transport of pelagic microalgae near the bottom, microalgae dominant even 1 m above the bottom rarely enter the food source layer. Consequently, the surf clam may rarely ingest pelagic microalgae that are produced in the upper layers of the water column. Unlike bivalves on tidal flats or mussels in intertidal rocky areas, few pelagic microalgae produced in the upper layers are supplied to infaunal suspension-feeding bivalves on open shallow sandy bottoms. So measurements of chrorophyll a in the water column do not indicate the food supply for such bivalves.