Latreille, 1829
Cladocera, or "water fleas", generally are small, 0.2-3.0 (18) mm, showing reduced body segmentation with two tagmata: the head (cephalon) and the trunk, the latter composed of thorax and abdomen and bearing 4-6 pairs of usually phyllopodous appendages, terminating in a flexed postabdomen with claw-like caudal rami. The carapace is single but folded along the dorsal midline, giving a bivalved appearance; the carapace encloses the entire trunk and in some species also the cephalon.
Ther antennules are uniramous, short, slender, and with a tuft of terminal sensory setae; occasionally modified in males. The antennae are biramous, strong, serving as the principal locomotive appendages.
The sexes are sometimes separated, but in Cladocera parthenogenetic reproduction is most common. The larval development is primarily epimorphic.
Ecology
Most cladocerans are freshwater or brackish water species, but several genera are marine.
Distribution in the North Sea
Cladocera are a minority in the North Sea plankton; eight species are known from the area. The key to the species atrts at Page 142: Cladocera.
The following species of Cladocera are included in this volume:
Family Bosminidae
Bosmina longispina
Family Podonidae
Podon polyphemoides
Podon leuckartii
Podon intermedius
Evadne nordmanni
Evadne spinifera
Evadne tergestina
Family Sididae
Penilia avirostris