Daphnia (water flea)

Daphnia spp. · Aquatic life

Under the microscope · Photo: Ivan Sinkov / CC BY

NativeCladoceran (water flea)

Also called Water flea, Daphnia magna, Daphnia rosea, Daphnia pulex, Large cladoceran, Daphnia pulicaria, Daphnia mendotae

How to identify Daphnia (water flea)

Jerky, hopping swimmers about 1 to 5 mm, easy to see in a clear jar
Clear, oval shell with a pointed tail spine at the back end
One large black eye and big branched antennae used for rowing
Green or brown gut and eggs or embryos visible in a pouch on the back
Lab counts and measures them; average length is a common index

Similar species

Simocephalus is similar in size but has a rounded rear with no tail spine and rests on plants; Ceriodaphnia and Bosmina are much smaller, under about 1 mm.

Profile

Season	Builds in early spring, often peaks in May and June and can clear the water for a few weeks; usually crashes in midsummer as young fish feed and blue green algae take over, with a smaller fall peak.
Starts	About 45 to 50 F water in spring
Light	In clear lakes moves deep by day to avoid fish and comes up at night
Depth	Open water, surface to near bottom
How it spreads or reproduces	Females clone themselves through summer; resting eggs in a dark saddle (ephippium) survive drying and freezing and ride on birds, boots and boats
Habitat	Open water of ponds, lakes and reservoirs; D. magna mostly in fishless ponds, stock ponds and wastewater lagoons.

Why it matters in a lake

The most important algae grazer in most ponds and a top food for young fish. Plenty of large Daphnia means few small plankton eating fish and usually clearer water; a pond with only small water fleas often has too many sunfish, perch or other small plankton eating fish.