Subgeneric differentiation within Eurycercus (Cladocera, Chydoridae) and a new species from Northern Sweden

Abstract

Eurycercus, although quite uniform in casual gross morphology, is now regarded as a highly differentiated group of Cladocera, split into three subgenera: Eurycercus for the species lamellatus, Teretifrons subgen. nov. for glacialis, and Bullatifrons subgen. nov. for macracanthus and pompholygodes sp. nov. At least four other species await description. E. pompholygodes differs from macracanthus most strikingly in having a less well developed clasping hook on trunk limb I and in having more teeth on the postabdomen. The latter character, being size-dependent like many others, can be described only by a regression. Other important characters are the regressions of the number of denticles on the postabdominal claw, the size of the median head pore, the number of setules in three clusters on trunk limb I, and the number of denticles on scraping spines 2, 3, and 4 of trunk limb II. Biologically the species has five pre-reproductive instars in parthenogenesis, six in gamogenesis. Once reproductive capacity is achieved, a brood of parthenogenetic young is produced each instar but a brood of gamogenetic eggs only every other instar, successive broods being separated by a barren instar. The gradual ontogeny of males, particularly of their antennule and trunk limb I, to functional maturity in instar III is described. Two parasites occur, both selective for large individuals, and one seemingly important in helping control population structure. The reproductive potential of all four species is described by regressions and related to egg size and size at maturity.