Minnesota Profile: Water Flea (Genus Daphnia)
Not really fleas Because they typically propel themselves through water in jerky, flealike hops, daphnia are called water fleas. However, they are not fleas, but cladocerans-tiny aquatic crustaceans that belong to the order Anomopoda. To students and aquarium fanciers, daphnia are the most familiar cladocerans, but they make up only a modest percentage of all cladocerans.
Appearance Although tiny (less than 1/8-inch long) daphnia are readily visible to the naked eye. Look for them along shallow, vegetated shores of lakes and ponds. A hand lens reveals a beating heart, pulsing intestine, four to six feathery pairs of legs, and often babies in a brood chamber within a rigid, transparent carapace, which is shaped like a taco shell. A single black compound eye twitches in the head.
Life history Unfertilized eggs develop as clones of the mother. This continues from generation to generation until unfavorable conditions arise. Then a few eggs develop into males. Fertilization produces special eggs encased in modified brood chambers resistant to environmental stresses. These eggs hatch into females. Daphnia populations peak in spring and sometimes late summer, when algae, which daphnia eat, are plentiful.
Ecological significance Cladocerans occupy nearly every conceivable freshwater habitat and act as important natural water filters of microscopic life. Under perfect conditions, daphnia could filter most of a small, shallow lake in a single day. By eating algae and then being eaten by tiny fish and aquatic insects, cladocerans transfer energy and nutrients up the food chain.
Predator avoidance Several species of daphnia have large helmets and long tail spines that repel predators. Other species avoid being eaten by fish by hiding in weed beds or in dark, deep water by day, then moving to open surface waters to feed at night, when fish are less likely to see them.
Relationships to humans Daphnia are food for game fish. By eating algae, daphnia clarify lake water-enhancing lakeshore property values. When people remove aquatic plants, daphnia have fewer places to hide and are more likely to be eaten by fish. Then algae multiply, water clarity declines, and lakes lose out. In shallow lakes with daphnia, clear water promotes growth of aquatic plants, which waterfowl eat. Ducklings also eat daphnia.
Darby Nelson, aquatic ecologist and freelance writer