. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. COMPETITIVE DISPLACEMENT OF MUD SNAILS 549 Ilyanassa distribution The population of Ilyanassa obsoleta in the study area was estimated to contain millions of snails. The population was dominated by adult-sized individuals (Fig. 1 A; see Scheltema, 1964) with sparse recruitment in 1979, 1980, and 1981. Beginning in May, as the water warmed, they moved onto solid objects of the mid intertidal zone (Table III) to lay egg capsules, preferring isolated eel grass plants, drift algae, and small islands of marsh peat and avoiding bo
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. COMPETITIVE DISPLACEMENT OF MUD SNAILS 549 Ilyanassa distribution The population of Ilyanassa obsoleta in the study area was estimated to contain millions of snails. The population was dominated by adult-sized individuals (Fig. 1 A; see Scheltema, 1964) with sparse recruitment in 1979, 1980, and 1981. Beginning in May, as the water warmed, they moved onto solid objects of the mid intertidal zone (Table III) to lay egg capsules, preferring isolated eel grass plants, drift algae, and small islands of marsh peat and avoiding both the eel grass bed and marsh where periwinkles were numerous. The adult population moved about the 1 km stretch of shoreline of the study site in the mid intertidal zone from March to November. Although Jenner (1956) reports that mud snails in the Harbor aggregate after reproduction ceases, the study population remained in dense aggregations throughout the year, foraging upon their own shells (illustrated by Morse, 1921) and on each other's shell epiflora. Isolated individuals were always rare although individuals moved freely between the aggregations (see also Borowsky, 1979) of which there were usually two or three. General patterns of movement were directed toward the marsh during spring tides from April through July, and toward the eel grass during spring tides occurring in the summer and fall. In a nursery area near the marsh there was a small group (2-5 thousand individuals) comprised of fast growing, immature snails (<17 mm) which separated from the adults in June and roamed about near the marsh until late August or September when they rejoined the adult aggregations. During winter months the population of Ilyanassa hibernated 5 cm in the sediment in the mid intertidal zone. With littorines present in the low intertidal eel grass bed, the mud snails did not migrate to the subtidal zone as has been reported for populations in other areas (, Batchelder, 1915; Sind
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