Distribution and dynamics of interstitial Tardigrada at Woods Hole, Massachusetts, U.S.A.

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Abstract
Year-round studies were made of marine interstitial Tardigrada at Crane's Beach. The dominant assemblage of tardigrades, in typically low diversity, included Batillipes pennaki, B. bullacaudatus, B. dicrocercus, and Stygarctus granulatus. Despite peripheral co-occurrences, each species occupied a specific portion of the intertidal beach. S. granulatus occurred deep in sand in the upper one half of the beach, while the Batillipes species were more superficial. Abundant B. pennaki was flanked n its mid-beach position by common B. bullacaudatus in the upper half of the beach and by seasonal B. dicrocercus in the lower half. In winter, B. pennaki was found deeper in the sand; B. bullacaudatus moved closer to the low tidal line ; and S. granulatus remained deep within the sand. While year-round reproduction and availability of tardigrades occurred, densities were maximal either in spring and fall (B. bullacaudatus) or throughout the summer and fall (B. pennaki and S. granulatus). Concentrations of tardigrades at Crane's Beach were comparatively dense, with maxima of 99.0 per cm3 for S. granulatus; 20.5 per cm3 for B. pennaki; and 10.0 per cm3 for B. bullacaudatus. An estimated growth sequence, inferred from changes in the abundance of size classes in each population, suggests a life-span of three to four months. A dominance of male S. granulatus in the fall was equalled by dominance of females in spring. Within an area of 100 cm2, the distribution of S. granulatus was only weakly contagious (index of dispersion (i. d.) = 1.61). The composition of its two dominant size classes was also fairly consistent (i. d. = 1.12 & 1.07). B. mirus formed local concentrations in a small section of a 100 cm2 area, but otherwise, its distribution was also weakly contagious (i. d. = 1.96). The distribution of “large” vs. “small” sizes of B. mirus were uniform within this area (i. d. = 0.40). Preliminary evidence suggests that competition for food is unlikely between S. granulatus or B. bullacaudatus and any of the other Batillipes but may occur between B. pennaki, B. mirus and B. dicrocercus.

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