Comparative Morphology and Biology of Three Species of Eurycercus (Chydoridae, Cladocera) with a Description of Eurycercus macrocanthus sp. nov.

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Abstract
Comparative studies of the three known species of Eurycercus (including the new species macrocanthus from Eastern Siberia described herein) demonstrate that macrocanthus is much more closely related to lamellatus than to glacialis in many details of morphology, ontogeny, and biology. Its most distinctive character is a great enlargement of the middle clasping hook on the first trunk limb, with the front end of the animal being completely restructured to accomodate its function. Both macrocanthus and lamellatus become reproductive at a small size and have a high reproductive potential, enabling them to survive heavy predation, as by fish, selective for large individuals, which the size‐frequency distributions of the populations suggest is occurring. On the other hand, glacialis does not become reproductive until a much larger size, and its reproductive potential is smaller than in the other species, making it extremely vulnerable to predation concentrated on large individuals. Significantly, where it occurs in the coastal North Sea region, Iceland, and Greenland it is virtually restricted to water bodies without fish. The taxon lamellatus sens. lat. appears to be a complex of closely related species.