What is a Hydra?

Mary McMahon
Mary McMahon

A hydra is a small multicellular organism in the genus of the same name. These tiny animals are found in fresh water all over the world, and they have a number of distinctive traits that make them interesting to scientists. People who are interested in seeing one for themselves can try taking a sample of some local fresh water and looking at it under a microscope; in addition to hydras, they may see an assortment of interesting aquatic creatures including water bears, diatoms, and rotifers.

Hydras use poisonous tentacles to catch prey.
Hydras use poisonous tentacles to catch prey.

The body of a hydra is formed in the shape of a tube, and the organisms demonstrate radial symmetry, meaning that they are symmetrical along multiple planes when viewed head on. One end has a foot called a basal disc; the animals secrete an adhesive substance to attach themselves to substrates like rocks and plants. The mouth opening is on the other end of the tube, and it is surrounded by tentacles that have small stinging cells for stunning prey. These cells can be found in many members of the Cnidaria phylum; jellyfish are perhaps the most famous stinging representatives.

Hydras reproduce via budding, which involves a miniature adult growing to maturity on the body of another.
Hydras reproduce via budding, which involves a miniature adult growing to maturity on the body of another.

To eat, a hydra extends its tube shaped body and traps prey in its tentacles. It feeds on a range of other small invertebrates, with waste products from the digestion process being secreted through the mouth opening. The creatures can reproduce sexually or asexually, depending on their environment, and they also exhibit hermaphroditic tendencies that allow them to produce eggs and then fertilize them.

Depending on the extent of the damage, these animals are able to partially regenerate themselves after injuries; in the 1800s, biologists mistakenly believed that they could force a hydra through a sieve and they individual pieces would regenerate. While this is not, in fact, the case, they are remarkably hardy. Unlike other animal species, they also do not age; a 1998 paper by Daniel Martinez detailed extensive research on this topic, and other researchers have since followed suit.

The largest examples are still so small that observers need microscopes to discern their features. Along with numerous other tiny aquatic organisms, hydras demonstrate the incredibly diverse life that can be found on every corner of the Earth. While these creatures might seem extremely bizarre to humans, they have survived for millions of years, enduring changing environments and animal populations with remarkable adaptability.

Even the largest hydra require a microscope to view.
Even the largest hydra require a microscope to view.
Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


@Oceana – I wish that jellyfish would confine their stinging to the prey they are about to eat. I can hardly swim in the ocean for a few minutes without getting stung, and they know that I am far too large to consume!

The tentacles sometimes leave purple marks on my legs. I get goose bumps in the area that last for a long time, and the pain and burning is intense.

I use baking soda paste to soothe it a little. I also make sure that there aren't any actual tentacles still clinging to my skin. If there are, I scrape them off first.

I have to remember not to touch the tentacles while removing them, though, or they will sting my fingers. I use a credit card or my driver's license to scrape them off.


Sea anemones are my favorite kind of hydra. I love how colorful they are, and they just seem so docile. They attach themselves to rocks and wait for prey to swim by, rather than hunting it down like jellyfish do.

I don't know if sea anemones even possess the ability to sting. There really isn't much chance of running into one, unless you are snorkeling.

Their tentacles look like brightly colored hair waving in the current. They add visual appeal to the ocean floor.


@kylee07drg – No, they can't survive in tap water. The chlorine that the water companies use to disinfect the water will kill them.

If you have a pond or a lake near your home, you could use water from that for your aquarium. Hydra might survive in pond water, even if it is from a different pond than the one they came from.


Can I keep freshwater hydra alive in an aquarium filled with tap water? I figure since they don't require sea water to live, they should do okay in water from my faucet.


These are great little guys to introduce to a classroom of kids. They look very interesting, although they don't always do very much. I think it might be better to keep the story about the scientists dismembering hydras in the hope that they could regenerate for older kids though.

Children tend to get attached to the little creatures they can see through a microscope and you can't really blame them. I found that kind of environment fascinating back then, and I still do now.


@MissDaphne - My first thought was of the mythological animal as well, although I've actually seen real hydras under the microscope so I'm not sure why my brain automatically went for the fantasy.

They may have been named because of their supposedly miraculous regenerative powers, but they might also have been called after the hydra because they look superficially like one, with their tentacles.

If you have a look at hydra images, you will see the similarity to the myth.


What part of the Hydra maintains the water current? If you answer the colenteron (gastrovascular cavity) then what does the internal cavity refer to?


@MissDaphne - The hydra Wikipedia article didn't say if that's where they get their name from, but it would make sense. Apparently, you can cut one in half and wind up with two hydra--sort of like what happens with a starfish, but not quite as impressive because a starfish is a much more advanced organism than the hydra.


Do they get their name from their ability to regenerate? The hydra in classical mythology had nine heads, and two more would grow in its place if one was chopped off. Killing it was one of the labors of Hercules, I think.

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