Zooplankton – Daphnia (or Daphnids) are members of a collection of animals that are broadly termed as “water fleas”. These are predominantly small crustaceans, and Daphnia belongs to a group known as the Daphniidae (which in turn is part of the Cladocera, relatives of the freshwater shrimp, Gammarus et al, and the brine shrimp, Artemia spp). Daphnia gets their common name (water flea) from their jerky swimming movement through the water. There are over 650 species have been recognised so far, with many more which are yet undescribed. Some have also adapted to life in the ocean, the only members of the Branchiopoda class to do so.
It is important to distinguish between Daphnia and other “water fleas” such as the various species of copepod (like Cyclops spp) and ostracod (Cypridopsis et al) which do not directly come under the scope of this document but share many of the feeding habits of Daphnids. The jerky movement, general shape (and, to a lesser extent, colour) of daphnia are the best way to distinguish them without magnification.
The head of the Daphnia is angled downwards and may be separated from the rest of the body by a “cervical sinus” or notch. Daphnia have a single black compound eye, located on their midline. Their heads also bear two pairs of antennae – the first antennae are small and unsegmented appendages, while the second antennae are large, segmented, and branched, with powerful muscles. The part of the head which projects in front of the first antennae is known as the rostrum or “beak”. The mouthparts are small, and consist of an unpaired labrum, a pair of mandibles, a pair of maxillae, and an unpaired labium. They are used to eat “organic detritus of all kinds” Microalgae and bacteria.
The thorax bears five or six pairs of lobed, leaf-like appendages, each with numerous hairs or setae. Carbon dioxide expelled, and oxygen absorbed, through the body surface.