The Role of Competition in the Population Dynamics of a Freshwater Flatworm, Bdellocephala punctata (Turbellaria, Tricladida)

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Abstract
The population dynamics of B. punctata (Pallas) were studied in the field, by qualitative (hand sampling) and quantitative (fixed quadrat) techniques, and in laboratory culture. B. punctata is an annual triclad. Adults, averaging 25 mm length, deposit cocoons on stones in Jan. when temperatures rise above 0.5.degree. C; these hatch by May but young do not appear on stones until July. They then grow to maturity at 18 mm by Nov. In the field, B. punctata deposits an average of 1.3 cocoons/adult, equivalent to 16 young, and this is less than its potential. Interspecific competition for food, particularly from Polycelis tenuis Ijima, may force hatchling B. punctata (3-4 mm long) to forage in suboptimal habitats where they experience severe mortality. Young B. punctata (8 mm long) escape this competition shadow and forage on stones, which act as a prey focus. The origins of the semelparous strategy adopted by B. punctata and some other triclads may lie in a selective requirement to reduce intensity of interspecific competition for hatchlings. By investing resources in reckless cocoon production early in the year these species avoid excessively severe competition from Polycelis and Dugesia [D. polychroa], which are superior competitors breeding in spring and summer. B. punctata uses similar prey to its confamilial, Dendrocoelum lacteum (Muller), but avoids competition by size differential, breeding first and remaining 1.5 times longer during the autumn growth period. A population crisis at the study site was probably due to low water temperatures in Feb. because these delayed breeding by B. punctata and hence increased competitive overlap with D. lacteum.