The hydra
A single Hydra in an aquarium is hardly dangerous and fascinatingly. But if they reproduce strongly and appear in measures they can get dangerous for shrimps and young cancers and also young fishes.
Often they are also designated as fresh water polyps. The type hydra, an animal from the tribe of nettle animals (Cnidaria). Fresh water polyps become up to 3 cm big and settle not only in fresh- but also in flowing water. In exceptions also in brackish water.
In an aquarium you mostly find the white Hydra vulgaris and the green Hydra viridis. There is another third hydra type, the brown Hydra oligactis. She occurs rarely in home aquariums because she prefers rather cold water. In total five Hydra types occur in Europe .
They are mostly brought through aquarium plants. In an aquarium they often sit on plant leaves or at the aquarium disk. Often it is meant to be an alga and no more attention is paid to it but hydras are able to move themselves within the aquarium. Therefore the hydra inclines to the side until it reaches the ground with its tentacles. Then it stitches itself with the sticking cells that can be found at the mouth and tightens the rest of the body what is similar to the moving of a caterpillar. They are able to move about 2 cm a day. By forming a blowhole at the foot gland the Hydra can even move upwards.
The food of a Hydra consists of small cancers, water fleas, insect larves and other small animals that occur in the water. Also young shrimps and young cancers can end as booty. In this case the Hydra does not hunt actively but she waits until the floating booty sticks to the tentacles.
If a booty animals affects one of these tentacles, it sticks there and is paralyzed by the poison fired out of the nettle capsules. This nettle capsule is flung with such an impact and quickness that it can even permeate the tank of crustaceans. The process only lasts about 3 ms.
After that the tentacles with the booty are led in direction of the mouth and the booty is digested.
What to do against Hydras?
Most effectively seems to be a treatment with the German spitz sludge snail (Lymnaea stagnalis).
Also sea salt or iodine-free table salt is supposed to destroy the Hydra. Therefore you can give 20g on 100 litres of aquarium water. It is not dangerous for plants and shrimps according to the statement of shrimp holders. Even in higher proportioning above a few hours. |
