Feeding and Growth Rates of Polinices Duplicatus Preying on Mya Arenaria at Barnstable Harbor, Massachusetts

The complete year—round feeding and growth of the marine snail Polinices duplicatus eating the clam Mya arenaria were directly monitored under field conditions. The major factors affecting feeding rates were food supply, temperature, predator size or weight, and recent feeding experience. Provided with excess prey of differing sizes, Polinices of each size or age ate similar numbers of prey per time (° = 95.5 Mya per snail per yr). Feeding varied directly with temperature, peaking at 0.6 Mya per snail per day in the warmest months, but ceasing (at °5°C) for 4 mo in winter. Energy equivalents for Polinices and size—weight relations for Mya varied seasonally. Only °80% of the tissue weight or energy content of a prey was actually ingested. Because larger predators consistenly ate larger prey, they ingested more (385 kilojoules/yr for a 4th—year snail) than smaller, younger ones (°218 kJ/yr for 2nd—year snails). Wtih a specified food supply, temperature and predator weight together accounted for most ofthe variability in ingestion (milligrams or kilojoules) of Mya by experienced predators. On a year—round basis, ingestion rates of Polinices were only °1% of their own weight per day. Growth rates, unlike feeding rates, were correlated more with a snail's age than its size or weight: 2nd—year Polinices grew nearly 3—fold in diameter or °105 kJ/yr vs. 1.2—fold or °63 kilojoules for a 4th—year nails. Growth creased during the 5 coldest months (<10°C). It was also reduced when snails could interact. Without molluscan prey, Polinices failed to grow. On a Mya diet, gross growth efficiences were high (48%—19% for year—classes 2 to 4). Energy flow through Polinices was comparable to other marine invertebrate predators: annual ingestion and production per square metre were estimated at up to 151 and 71 kilojoules, respectively.