Daphnia magna

The great waterflea lives in eutrophic fresh waters. If present, it can occur in dense flocs, and colours red from heamoglobine under oxygen stress. Females usually reproduce parthenogenetically: diploid females produce diploid daughters without interference by the rare and much smaller males.

Neonates have a length of 0.8 mm, reproduction starts at 2.5 mm, and well fed adult females can reach a length of 5 mm. So, adults grow a factor 2 in length, or a factor 8 in volume.

At 20°C, these crustaceans moult every 2-3 days, till death follows, usually after some 7 months in the laboratory. The first brood of neonates are released from adults' brood pouch typically at the age of 5-7 days, just prior to moulting. Shortly after moulting, new eggs are deposited in the brood pouch, so the incubation period almost equals the intermoult period.

Sixty or more neonates per brood pouch is not exceptional. This means that females allocate most of their assimilates to offspring.

Daphnids have a single large compound eye and a single small nauplius eye. The first pair of antennae is very short and close to the rostrum ("nose"). The large second pair of antennae is used for hipping around. The hart is rapidly beating in the "neck"-region.

A paired caecum (blind ending digestive tubes) is visible just after the mouth. The transition between the bright green algae in the foregut, and the brown remains in the hind gut is sharp. A thick spine on the abdomen closes the brood pouch. The maxillary gland (close to the mouth) is usually barely visible; if it is green, like here, there is a health problem.

The relative ease to culture these creatures on algae, the ability to make clones, and their short generation time make waterfleas popular research subjects, especially in ecotoxicology. Effects of toxicants on reproduction usually occur at two orders of magnitude lower concentrations than lethal effects.