Daphnia
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Subgenus Ctenodaphnia
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Daphnia are members of the Order Cladocera. They are one of the several small aquatic crustaceans called water fleas because they tend to swim by making little jumps.
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Biology
Daphnia are small, mainly planktonic, crustaceans, between 0.2 and 5 mm in length. They live in various aquatic environments ranging from acidic swamps, lakes, ponds, streams and rivers.
The division of the body into segments is nearly invisible. The head is fused, and is generally bent down towards the body with a visible notch separating the two. In most species the rest of the body is covered by a carapace, with a ventral gap in which lie the five or six pairs of legs. The most prominent features are the compound eyes, the second antennae, and a pair of abdominal setae. In many species the carapace is translucent or nearly so and as a result they make excellent subjects for the microscope. Even under relatively low power, it is possible to observe the feeding mechanism working, watch immature young moving in the brood-pouch, observe the eye being moved by the cillary muscles and even watch blood corpuscles being pumped round the circulatory system by the simple heart. They are tolerant of being observed live under a cover slip and appear to suffer no harm when returned to open water.
A few Daphnia prey on tiny crustaceans and rotifers, but most are filter feeders, ingesting mainly unicellular algae and various sorts of organic detritus including protists and bacteria. Daphnia can be kept easily on a diet of yeast. Beating of the legs produces a constant current through the carapace which brings such material into the digestive tract. The first and second pair of legs are used in the organisms' filter feeding, ensuring large unabsorbable particles are kept out while the other sets of legs create the stream of water rushing into the organism. Swimming, on the other hand, is powered mainly by the second set of antennae which are larger in size than the first set. The action of this second set of antennae is responsible for the jumping motion.
Reproduction
Daphnia reproduce parthenogenetically usually in the spring till the end of the summer. One or more juvenile animals are nurtured in the brood pouch inside the carapace.The newly hatched Daphnia must molt several times before they are fully grown into an adult usually after about two weeks. The young are small copies of the adult; there are no true nymphal or instar stages. The fully mature females are able to produce a new brood of young about every ten days under ideal conditions. The reproduction process continues while the environmental conditions continue to support their growth. Winter or drought conditions brings an end to the production of new female generations. At this time, the reproduction method changes. Parthenogenic males are produced, followed by mating and fertilization of the eggs. Fertilized eggs are termed winter eggs and are provided with extra shell layer called ephippium. The extra layer preserves and protects the egg inside from harsh environmental conditions until the more favourable times, such as spring, when the reproductive cycle is able to take place once again.
Males are only found at times of harsh environmental conditions, typically during portions of the year of scarce resources due to population overgrowth or winter conditions, and even then may make up considerably less than half the population, in some species being unknown entirely. They are much smaller in size than the female and they typically possess a spcialised abdominal appendage which is used to grasp a female from behind and prise open her carapace and insert a spermotheca. Their appearance is for the creation of resting or winter eggs, allowing for the survival of the population through harsh conditions.
Lifespan
The lifespan of a Daphnia does not exceed one year and is largely temperature dependent. For example, individual organisms can live up to 108 days at three degrees celsius while some organisms live for only 29 days at 28 degrees celsius. A clear exception to this trend is during the winter time in which harsh conditions limit the population in which females have been recorded to live for over 6 months. These females generally grow at slower rate but in the end are larger than ones under normal conditions.
Daphnia provide an important source of food for many larger aquatic organisms including various fish species (e.g. lake trout) and the immature stages of many insects including the Orthoptera- Dragonflies and Dameselflies.
External links
- Cladoceran website (http://www.cladocera.uoguelph.ca/references/) at the University of Guelph
- Daphnia Genomics Consortium (http://daphnia.cgb.indiana.edu/)