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Life in Freshwater

Hydra

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Brown Hydra

Brown Hydra (Hydra oligactis)

There are several species of hydra, all with between 4 and 8 tentacles. These, along with the main body, can be extended or contracted. The tentacles are armed with sting cells (nematocysts) that fire off when touched. The body consists of a "bag" with an opening at the top, surrounded by the row of tentacles. The wall is made up of two layers each just one cell thick. Green Hydra, H.viridissima, has the different colour due to a symbiotic alga living inside.

The Brown Hydra is capable of hanging on in the roughest of conditions including quite fast moving rivers and on beaches of lakes where wave action can occur. A very abundant species and widespread across Europe .

ECOLOGY: Hydra are easily over looked as they are just a few millimetres long but are extremely common in just about any water body. Vegetation can be removed from the pond or river and left to stand in some water for a while and slowly the hydra reveal themselves. Once found in a water body they will seen to be in huge numbers as they bud new individuals off the side of the body when the conditions are good. This is asexual reproduction and results in genetically identical offspring. Ideal when the conditions are good: quick and simple colonisation.

Hydra with bud
Hydra with bud lower down the body

However, when conditions change (autumn with its temperature change or when the pond is drying up) the hydra develop an ovary or testis - there are no fixed sexes here. They release the sperm which swim through the water to find an egg. The resulting fertilised egg will be different from the parent and have a greater chance of surviving in a variable environment. The egg in its protective case can pass through winter or drought. Eggs develop into a tiny larva which helps dispersal.

The sting cells on the tentacles have a tiny trigger hair that when touched by prey releases a "mini-harpoon". This is barbed and on penetratingthe prey paralyses it. Hydra feed on small crustaceans like water fleas. The tentacles slowly push the immobilised prey into the mouth where it is digested. Undigested food comes out the same way.

 Classification:

 

 Kingdom

Animal

 Phylum

Cnidaria

 Class

Hydrazoa

 Order

 


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