Daphnia

Daphnia are small, mostly planktonic, crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although fleas are insects and thus only very distantly related). They live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.

Biology

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 the five or six pairs of legs lie. 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 ciliary muscles and even watch blood corpuscles being pumped round the circulatory system by the simple heart. The heart is at the top of the back, just behind the head. and their average heart rate is approximately 180bpm under normal conditions. Daphnia, like many animals, are prone to alcohol intoxication, and make excellent subjects for studying the effects of the depressant on the nervous system - due to the translucent exoskeleton, and the visibly altered heart rate. They are tolerant of being observed live under a cover slip and appear to suffer no harm when returned to open water. This experiment can also be done using caffeine and watching an increase in heart rate.

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 trapped food particles are formed into a food bolus which then moves down the digestive tract until voided through the anus located on the ventral surface of the terminal appendage. 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 until the end of the summer. One or more juvenile animals are nurtured in the brood pouch inside the carapace.The newly hatched Daphnia must moult 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 fertilisation of the eggs. Fertilised 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 specialised abdominal appendage which is used to grasp a female from behind and prise open her carapace and insert a spermatheca. Their appearance is for the creation of resting or winter eggs, allowing for the survival of the population through harsh conditions.

In addition to the production of eggs capable of overwintering this switch to sexual reproduction has also been proposed to allow greater offspring variation (through genetic recombination) which may be useful in varied or unpredictable conditions, this idea is often proposed under the name of the lottery model.

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 3°C while some organisms live for only 29 days at 28°C. 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 six 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 Odonata- dragonflies and dameselflies.

They are easy to culture in the laboratory, and D.magna has been a model species for developing the Dynamic Energy Budget theory. They are frequently used to test the effects of toxicants on reproduction and survival.

Daphnia are sold by aquatic retailers in both live and freeze-dried form as food for aquarium fish. Their tiny size renders them edible in live form even for fish as small as the neon tetra

Daphnia magna, has an LD50, concentration that kills one half of the population, of about 10,000 ppm Total dissolved solids for a 96 hour exposure.

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Crustaceans | Fishkeeping

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Biology

Reproduction

Lifespan

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