Water fleas

Water fleas are crustaceans and, like other crustaceans, have an external shell. Their most striking feature are the two pairs of antennae of which the second pair is very large and is used for swimming. The second pair is very small and is often (almost) invisible, they look a bit like the “nose hairs” of the water flea. These antennae contain sensitive senses.

Water fleas have one striking eye, and often a much smaller light sensitive organ called ocellus. The body is usually surrounded by a shell that protects the water flea. Inside the shell are the sieve legs that the water flea uses to generate a water stream and filter food from it. In the body you can usually see the intestinal tract and in many species you can also see the heart beating, very cool to see!

On its back the water flea has a breeding area in which the eggs are developed. On the abdomen, inside the shell, is the anus and often also a pair of claws and/or combs for cleaning the filter space and defence.

Most species of water fleas will be found in fresh water, but species of genera such as Podon, Pleopis, Penilia and Evadne also occur in salt water.

Most water fleas in the plankton eat algae which they filter out of the water with their sieve legs, but some are predators which also eat other water fleas. They can be very effective filterers, and large amounts of water fleas can turn a cmurky pond crystal clear.

Like many zooplankton, water fleas are only common for part of the year. If there is enough food and the conditions are favourable, water fleas can reproduce asexually. The higher the temperature the shorter the development time; at summer temperatures a water flea can develop from egg to adult within one week! When conditions become more unfavourable (colder, less food, drought) males are made, which fertilise the eggs from which resting eggs (ephippia) are produced, which for example can survive the winter or a period of drought.

Water fleas are of great importance as food for many other aquatic animals such as fish. Water fleas can adapt to avoid being eaten by fish. When they smell fish, they hide between the water plants during the day. Over time, some water fleas that live in water with fish develop a sharp “pointy hat” so they are less likely to be eaten!

Because water fleas are very sensitive to various contaminants in the water and are easy to breed, they are used in many studies, and in programmes to monitor water quality.

A water flea (Daphnia cucullata) from the Amsterdam IJ, with pointy head against predation by fish.