The Ecological Relationships of Certain Freshwater Copepods

Abstract

This paper is a defence of the author''s statements on the interrelationships of Eurytemora lacinulata and Diaptomus gracilis, and on the seasonal occurrence of certain spp. of Cyclops, made in his book on animal ecology and adversely criticised by A. G. Lowndes [see Entry 7828 in this issue]. Evidence is brought forward to show (1) that D. gracilis occurs on the whole in older waters and appears to be lacking in waters of comparatively recent origin; (2) that E. lacinulata in Great Britain is found mainly in artificial and comparatively recent ponds; (3) that E. lacinulata cannot maintain itself, in standing water at least, in competition with D. gracilis, but that it can do so in slowly moving water; (4) that the eggs of D. gracilis are incapable of withstanding desiccation and, in consequence, the sp. has a slow rate of spread; (5) that, on the other hand, the eggs of E. lacinulata can stand desiccation and, therefore, facilitate the rapid spread of the sp. In older ponds, therefore, D. gracilis has ousted E. lacinulata. The latter, by reason of its better and quicker powers of dispersal, is enabled to take refuge from the competition of D. gracilis in newer ponds, where, for a time, it flourishes. When these newer ponds in turn are invaded by D. gracilis, E. lacinulata gradually becomes eliminated, and again takes refuge in still newer ponds. The complementary nature of the distribution of these 2 spp. is thus explained and the apparent exceptions discussed. With regard to the seasonal occurrence of certain spp. of copepods, the author cites evidence to show that Cyclops strenuus is the dominant sp. during the winter in small ponds, and that C. albidus and C. juscus are the dominant summer forms. Individuals of these spp. may occur all the year round and are not exclusively separated in seasonal occurrence, but their relative abundance exhibits a seasonal rhythm of a marked type.