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Hydra Fact Sheet

Hydras are coelenterates, freshwater relatives of the jellyfish and corals.  In aquariums many people find them a nuisance, so this fact sheet is in two parts.  The first part on the left is about Hydra as a problem in aquariums and the second part  on the right is about them as interesting animals which could be kept for their own sake.  Hydras can be as big as half an inch long, but many types are much smaller.

As far as I know Hydras do not transmit any parasites or diseases to fish or Humans.

Hydras as a Problem in Aquariums

Hydras are carnivorous and have stinging cells.  They will compete with fish for live food.  Although they are too small to eat adult fish, I wonder if they stress smaller fish like the Neon Tetra by stinging them when they accidentally touch the hydras.

Another problem is simply that some people do not like their appearance. 

However, the biggest problem comes with infestations in breeding tanks when people are trying to breed egg laying fish.   The hydras eat the live food put into the tank for the baby fish; multiply quickly and eat the baby fish.

There are several ways suggested to get rid of Hydra infestations.  Most of these ways would certainly kill baby fish, and there are none that I would regard as being safe with them.  So if you expect hydras are going to be a problem, you should get rid of them from your breeding tank before breeding.

Ways of Removing Hydra


In a normal aquarium, the way I like best is to use fish that eat them.  The most common one used is the Blue Gourami.


A temperature of 41 degrees C (106 degrees F) for three days will apparently kill hydra.  It will also kill nearly all types of fish, so these would need to be removed.  Some plants will survive this heat treatment.


Naturally things like Chlorine bleach will kill hydra as well as most other things.  This can be used for sterilizing non living things like rocks, and has even been used successfully for destroying pests and diseases in whole aquariums.  Of course it will kill any plants and fish.  Considerable effort is needed to ensure that there is no residue left before living things are reintroduced.

Apart from such drastic chemicals there are a number that can be used with fish, but not with absolute safety.

Copper can be used, but maintaining the correct level is difficult in a freshwater aquarium.  Also some studies suggest that many fish are more susceptible to copper than Hydra.

Formaldehyde at 3 drops per US Gallon appears to kill most of the hydra without killing fish.  However it might not kill all of them with one dose.  Formaldehyde will lower the pH of the water as it degrades into formic acid.  The pH would need to be watched and adjusted as needed.

Flubendazole is an anthelmintic drug.  It will get rid of some internal parasites of fish as well as Hydra, ick (White spot), velvet and gill flukes.  It is possibly the safest of the chemicals that will get rid of Hydra.  A guide to the dosage needed is 1/2 gram of 10% Flubendazole per 20 litres (5 US gallons)

With any chemical, remember that the different types of fish are likely to have different tolerances, and the different species of Hydra may also vary considerably in their susceptibilities to the chemical.

It is also worth considering whether you actually need to get rid of hydra from your aquarium.  Normal maintenance; not allowing your aquarium to get dirty, will help to prevent a large build-up of Hydra.






Hydra as an Animal

Hydras are interesting creatures.  They do not have a brain, but have a nerve net distributed over their whole body.  Despite this they have a range of behaviours.

Collecting Hydra

Hydras of various species are very widespread in freshwater habitats.  Some of them are so small that they are likely to be overlooked unless you are looking very carefully.  Another thing that can make them difficult to see is the fact that they contract to what looks like a small blob of jelly if they are disturbed.  Hydra viridis can be small enough to live on the underside of the “leaves” of Lemna Minor, the Lesser Duckweed.  Since this species is usually bright green from the algae in its cells it is extremely difficult to see in this situation.

Many people have hydra in their aquariums, with the aquarium owner being completely unaware of the Hydra, and the hydra causing no problems.

To collect Hydra, the simplest way is to go to a lake or similar body of freshwater and gently pass a fine net through plants growing in the water of the lake.  With reasonable luck you will have caught more than one type of Hydra in your net.


A Hydra is a carnivorous animal.  It feeds on small living things.  However some hydras especially, Hydra viridis, have zoochlorellae algae cells in their bodies.  These algae photosynthesize; producing sugars and other things which are used both by the algae and by the Hydra.  The species of Hydra with algae can live for a considerable time without catching any prey.  Presumably the Hydra also supplies the Algae with its mineral requirements.  Some of these things like nitrates would actually be waste products of the Hydra’s metabolism.

This arrangement is similar to the marine corals, which are related to hydra.


Hydras catch small animals.  In many cases they are eating microscopic creatures like ciliates, but a large Hydra can eat adult Daphnia .

The tentacles at the top of a Hydra touch something in the water and stinging cells called nematocysts are triggered, shooting out tiny darts at the thing that touched the hydra.  These darts are attached to the hydra polyp by very fine threads.  The darts are barbed and attach themselves to the potential prey, injecting poison.  This is the same mechanism used by jellyfish in the sea.  Many jellyfish have stings that are painful to Humans and some can be fatal.  As far as I know, Humans cannot feel the stings of Hydras.

The food item is then reeled in and transferred to the hydra’s “mouth” at the top, surrounded by the tentacles.  The food is then digested as much as the Hydra is capable of and the nutrients adsorbed.  The undigested parts of the Hydra’s meal are expelled through the “mouth”.


Hydras do not have specialized organs for absorbing the dissolved Oxygen from the water.  This is done through the skin.  Apparently the movement of Oxygen into the Hydra and of carbon dioxide out of it is by diffusion although active transport of the gasses cannot easily be ruled out.

The Hydras which have algae in them would also get Oxygen as a waste product of the photosynthesis carried out by the algae.  In turn the algae would use carbon dioxide which is a waste product of Hydra metabolism.


Hydras are sessile.  That is, they are attached to surfaces rather than being free floating like jellyfish.  But they do have ways of moving around.  They can move slowly by the base flowing as if it were an amoeba.  They can also move like an inchworm by looping their top and attaching it, alternating between their top and their base.  The third way they can move is to release themselves from their surface and float around hoping to find a better place to live.


Hydras do not have eyes or similar specialised light sensitive organs.  However, their bodies allow light to pass through them.  Nerves appear to have some sensitivity to light, so it is not surprising that Hydras can sense light and dark.

Most species of Hydra avoid the light, but the ones with algae in them are attracted to the light.  This is a logical thing to expect because Hydras with algae need the light for photosynthesis, while the sorts without algae are more visible both to their prey and to their predators in well lighted areas.  This suggests that there is a quite big advantage for the Hydra to be able to form a symbiotic association with algae.

Hydra Immortality

All living things can die.  More than that, the animals and plants we are most familiar with will grow old and die of old age related things, this is known as senescence.  This does not apply to all living things.  A bacterium can divide into 2 near identical “daughter” cells.  That is, both the new cells have equal right to be considered as the original.   This has apparently been going on for however many years that bacteria have been in existence.

Apart from things like bacteria there are some animals which may not grow old in the same sense that most do.  It can be difficult to prove this although it is often possible to disprove it.

Some studies suggest that hydra do not die from old age and in theory could live for ever.  The life of an individual Hydra is a dangerous one and the average life is quite short.  Although it might be difficult to prove, a very old Hydra could exist.


A brain is a concentration of nerves.  In theory there is nothing to prevent an animal having more than one brain and it appears that some types of dinosaur did have two brains.

Hydras do not have brains in this sense.  Instead they have a nerve net distributed fairly evenly throughout their bodies.  This works for their life style.


The most common way for Hydras to reproduce is asexually, with a new, genetically identical, individual budding off from the parent Hydra.  Hydras can multiply very quickly under favourable conditions.

They can also reproduce sexually.  Some types form resistant eggs so that the drying out of a pond will not necessarily get rid of all the Hydras.


If a Hydra were cut up, then as long as each piece had all three types of Hydras vegetative cell, it can grow into a complete animal.