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The Effect of Salinity Changes On a Filter Feeding Organism BY JOHN SEASE
INTRODUCTION THE AREA USED for this study is located in the Quaise salt marsh on Nantucket Island, Massachusetts. An estuary, open to the harbor, winds through the marsh, with several tributary streams, each connecting the estuary to small ponds. At the innermost extremity of the estuary is the last in this series of ponds. Where the estuary flows into this pond, sand deposition has built up a small delta. Soft-shell clams (Mya arenaria) are found in this area, but not the surrounding mud. It has long been known that clams need a fairly hard substrate to survive; soft mud or shifting sand could smother them (Kellogg 1903). Although the estuary winds through the marsh for a con siderable distance, the rise and fall of the tides is considerable at the delta, as much as 80 cm. With each rising tide, water of high salinity (approximately 29%) moves up the estuary from the harbor. Fresh water is introduced to the system from the opposite end of the pond, and because fresh water input from this source is relatively constant, it follows that extensive mixing must occur at some point in the pond during the tidal cycle. An additional source of fresh water is seepage from the underlying fresh water table. A typical marsh can be described as floating, the tangled web of plant roots forming a support ing mat over the liquid-like mud and peat below (Teal 1969, p. 2). This water would very likely be fresh, as a pond is the inter section of the water table with the ground surface. The presence of nearby fresh water is indicated by a stand of cattails (Typha latifolia) on the marsh margin. These plants are unable to survive in a highly saline environment. At question here is to what extent do water of varying salinities mix, and what is the resulting effect on the clam population in the sand delta. The Soft-Shell Clam and its Environment The clam lives in the sand, often penetrating as deep as 30 cm, although usually somewhat less, and feeds by reaching its siphon to the sand surface. Cilia within the siphon create an incurrent flow of water, which carries with it food and oxygen. More cilia pick out food particles and pass them to the mouth and then to the stomach. Wastes are removed by the excurrent water. Spawn is also carried out in this manner and fertiliza tion is external. The larvae remain free-swimming for up to two weeks before settling to the bottom. For several days the small clam will hold to the bottom with a thread-like byssus before finally burrowing into the sand. Like many other marine organ-
