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Life in Freshwater

Water Fleas (Daphnia spp)

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Daphnia - water flea

IDENTIFICATION: Daphnia is just one of the families within the freshwater crustacean order of Cladocera. Known as water fleas they range from very small, microscopic creatures to almost 1 cm although the normal size is around a few millimetres. They have a folded carapace which is oval in shape. The fold is at the back so the legs can protrude through the opening at the front. They have a single large eye at the top infront of the long antennae. Crustaceans, generally, have two pairs of antennae but it is the second pair in water fleas that is especially well developed. These are the means of locomotion and their beating swims the animal through the plankton in a jerky fashion. The abdomen is small and usually has a claw at the end and projects through the bottom of the carapace. This can be used to move them forward. The legs are not involved with locomotion and are along the front of the thorax, underneath the carapace.Water fleas are a common planktonic animal of ponds and lakes. In fact any water body which is still, including small stagnant marshes and ditches. They are a group widespread across Europe and exist in many different forms and varieties.

ECOLOGY: These are an important group as they form the diet of many aquatic species. They are the principle food of young dragonflies and fish. The water fleas themselves feed on detritus and bacteria.
Males are rare in populations. Females use asexual reproduction or parthenogenesis to produce the bulk of the young. The eggs are retained within the carapace where they hatch into small water fleas. This produces offspring very quickly and helps in the rapid colonisation of a pond. If the conditions change, e.g. the water begins to dry up or the onset of winter arrives, sexual forms appear in the population to produce specialised egg sacs called ephippia. With the death of the water flea the sac drops to the bottom where it waits protected until conditions improve. The sac may be blown by the wind or be caught on the bottom of the leg of a water bird and dispersed to a new area. Once conditions are suitable it then hatches.











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