Daphnia

Daphnia spec.

Daphnia (Daphnia ) are a genus of crustaceans (Crustacea ) that are provided to the claws tails ( Onychura ). The only between 1 mm and 5 mm long animals are, among others, often referred to as " water fleas ", and are easy to breed.

Morphological structure

The body Daphnia is enclosed in a bilobed shell ( carapace ), which leaves only the head with the strong rudder antennas free. The two shell halves join at the back and form a sort of keel which ends with a sting. The shell is formed by a duplication of the skin, which is faceted.

At the head striking is the large compound eye. It is movable by the six muscles and is formed during the development of two parts. Another smaller eye, the so-called Naupliusauge (see also: Naupliuslarve ) is much smaller and not a compound eye. Both eyes are the "brain" of Daphnia connected via optic nerve to the cerebral ganglion.

The second antennae are transformed into large and conspicuous rowing organs. They consist of a root element and two branches with bristles that fan out at the " rowing ". The bifurcated antennas go back to the original gap leg of crustaceans. In the strain members of the two second antennas enter strong muscles which allow movement of the antenna. Are much smaller that the first two antennas. They sit above the mouth and carry on their end sensory hairs ( Ästhetasken ). These serve as chemoreceptors. In the males, these antennas are larger and have an additional bristle probably perceive mechanical stimuli. Daphnia belong to the group of Mandibulata and thus have two strong mandibles (a type of pine). They are easy serrated wedge-shaped and inward.

Five pairs of "legs" are below the shell. Also they have Spaltfußcharakter are weichhäutig and get their strength by the blood pressure of the animal. Such supported limbs is also called Turgorextremitäten. You are busy at the edge with bristles and each have a small bag -like appendage that acts as a gill. Daphnia breathe but also by the surface of its Turgorextremitäten and its entire body surface. The legs are not meant for locomotion, because this task already take over the second antenna. The number pairs of legs 3 and 4 rather serve food. Through a fast and rhythmic beat of the legs of water and food particles contained therein " swirled " into the interior of the shells. The bristles on the legs act as a filter and catch the food on. This is then conveyed in a channel between the legs upwards towards the throat and from there into the body or into the intestine.

The rear part of the animal (abdomen or pleon ) is slightly curved. Here are the anus and a pair of inwardly curved claws.

Ecology

At certain seasons it can lead to a mass occurrence. Daphnia reproduce asexually mostly ( parthenogenesis ). Therefore, 98 percent of the Daphnia females. If the environmental conditions in their habitat negative change ( drought, cold, shorter days, lack of food, but also toxins ), but also males are born, which fertilize the eggs of females. These eggs, surrounded by a resistant to heat, cold and drought shell ( ephippium ), also called " winter eggs " or latency eggs ( resting eggs ) called, up to two years of drought survive and hatch under favorable environmental conditions again. In the sediment, the durability of the eggs is even higher: from 40 years old layers of Lake Constance hatched in laboratory experiments even daphnia. This ability gives the Daphnia the ability to colonize drying or " upset " waters quickly.

Ecotoxicology

Daphnia live in lakes and ponds. They are sensitive to pollutants in the water and leave on the impairment of their ability to move a statement about toxins in the water. The standardized test for this purpose is called daphnia.

Types (selection)

• Daphnia cristata Sars, 1861
• Daphnia cucullata Sars, 1862
• Daphnia galeata Sars, 1863
• Daphnia hyalina Leydig, 1860
• Daphnia longispina O.F. Müller, 1776
• Daphnia magna Straus, 1820
• Daphnia pulex Leydig, 1860

Daphnia as a model organism in research

Some species of the genus Daphnia are used as a model organism in biological research. In the course of using the genome of some species (D. pulex, D. magna) sequenced. It includes 31,000 genes.

Use in the aquarium

Daphnia are often used as ornamental fish feed for aquarium fish. As live food they are regularly offered in the pet trade. In addition, they are available in frozen or freeze-dried deal. They contain valuable fiber and loose in a change of feed frequently from the spawn.