Great water flea
Daphnia magna
Daphnia magna is one of the largest species of water fleas, primarily inhabiting stagnant, nutrient-rich freshwater bodies. It features a nearly transparent carapace through which internal organs and the brood chamber are visible. Movement is achieved through jerky strokes of the significantly enlarged second antennae, giving them the common name water flea.
Details
Identification
Large, oval body; long apical spine; large compound eye; paddle-like second antennae; transparent body.
Social behavior
Often occurs in extremely high densities (swarms), but does not exhibit complex social interactions.
Diet
Filters algae, bacteria, and organic detritus from the open water column.
Hunting strategy
Passive filter feeding via rhythmic movements of the bristled thoracic appendages.
Spawning substrate
Eggs are carried in the dorsal brood chamber beneath the carapace.
Overwintering
Production of resting eggs (ephippia) that overwinter in the sediment and can survive freezing and desiccation.
Ecology
Ecological role
Keystone species in the food web; converts primary production (algae) into animal biomass for fish consumption.
Natural predators
Juvenile fish, predatory zooplankton (e.g., Leptodora), backswimmers, and dragonfly larvae.
Competitor species
Other Daphnia species, copepods (Cyclops), and small mysid shrimp.
Ecosystem service
Contributes to water clarity by controlling algal blooms and increasing Secchi depth.
Threats
Excessive eutrophication, pesticide runoff into water bodies, and climate change (thermal stress).
Scientific profile
Profile
Distinguishing features
Largest native Daphnia species. Characterized by a long apical spine at the posterior end of the carapace and fine combs of spines (pectens) at the base of the post-abdominal claws. The first antenna is very short in females.
Reproduction
Cyclical parthenogenesis: Asexual reproduction via diploid summer eggs during favorable seasons. Under deteriorating environmental conditions (cold, drought, food scarcity), males and haploid resting eggs are produced.
Role in food web
Keystone species; acts as an efficient link between primary producers (algae) and higher trophic levels; can control algal blooms through high filtration rates (clear-water phase).
Protection & threats
Status not on standard scale
Main threats
Loss of small water bodies through filling or drainage; input of insecticides (especially neonicotinoids) from agriculture; overstocking with fish in small ponds.