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Hydra are members of the phylum Coulentarata - which includes the familiar sea anemones, corals and jelly-fish. Hydra are the only common freshwater members of this group.
Hydra are much smaller than sea anemones, mostly 1-5mm long, though some can be up to up about 25mm long, including the tentacles - though that size is unusual! It's actually quite difficult to measure or specify the size of a hydra, for they are elastic - they extend when hungry, waving their tentacles to catch prey. When they are well fed, they contract and, since they now contain a complete prey (usually a daphnia) they can be quite fat, though very short, little more than a blob of jelly!
There are many species of Hydra:
Hydra are voracious and nearly insatiable: when food is plentiful they can catch and devour a lot - when the body becomes distended. This is when they breed by division: a small bud develops on the side of the parent and grows into a miniature hydra, which then breaks away and settles elsewhere.
Hydra also breed sexually. Hydra can also breed another way: if yu cut a hydra into pieces - each piece can develop into a new hydra! This incidentally was how the hydra got its name, for in 1750 , the Abbe Trembley -n who was the first to study the creature - found out about this property and he therefore named the creature after the Hydra of Greek mythology - this had a hundred heads and, if any head was cut off, it simply re-grew!
Hydra aren't however a problem to th normal fish in a normal community tank - only in a breeding tank. Though they do eat rather a lot of the daphnia that is intended for the fish!
Hydra can be very interesting to watch and they are big enough they quite a lot can be seen with a good hand lens.
Because Hydra are so voracious and tend to find ideal living conditions in the average aquarium, where they breed very fast, they can be a nuisance as they consume food intended for the fish. They can be killed by copper - also toxic to fish, or by ammonium nitrate - not quite so toxic to the fish, if conditions are monitored carefully. Ammonium nitrate is an excellent plant fertilizer, so will get absorbed in time.
Larger fish, e.g. Blue Gouramis and Paradise fish, find Hydra delectable and will devour them voraciously. Smaller fish won't touch them, presumably finding the stinging cells (with which Hydra catch their prey) unpalatable.
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Last modified: Fri, 24 Nov 2017 12:09:56 GMT
Page first published 16th May 2003
Page written and © by Richard Torrens