Freshwater Water Flea (Daphnia)

 

This freshwater water flea is a member of the order Cladocera, family Daphniidae. Daphnia is a large, well-known genus with many species, up to 5 mm long and mostly reddish or greenish in colour.

Water fleas are small crustaceans ranging from 0.2 - 10 mm in length, with the thorax (bearing 5 or 6 pairs of limbs) and the abdomen enclosed within a folded oval carapace. The head is seperate from, but continuous with, the carapace. They have large eyes and complex antennae. The first antennae are small and unbranched, but the second antennae are more prominent and branched , armed with bristles and used for swimming in most species (see above).

The males are often smaller than the females and are less common. Some species of water flea occur throughout the year but others are only present during the warmer months. The latter hatch from 'winter eggs' in the spring and at first only females are present. These reproduce rapidly, the 'summer eggs' developing without being fertilized by males (parthenogenesis). Later, towards the onset of winter, or other unfavourable conditions such as the pond drying up, males appear and fertilize the thick-shelled winter eggs produced by the females at this time. In some families, for example Daphniidae, the winter eggs are carried by the female in a special saddle-shaped pouch (ephippium) which is shed at the following moult. The ephippium persists and protects the eggs during the winter when the adults die off. Winter eggs are resistant to freezing and dessication and are widely dispersed.

Cladocerans are an imortant component to any freshwater ecosystem. Many species occur seasonally in vast numbers, providing a valuable food supply to many other creatures and they in turn consume large quantities of planktonic algae. They are found in all kinds of standing waters - lakes, ponds, ditches, marshes - as well as canals and slow-flowing rivers. Some species are planktonic, others clamber about on vegetation or amongst the sediment. The jerky swimming motion of most species is highly characteristic and is responsible for the common name of water flea.

 

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