Everything You Need To Know About The Hydra

In Greek mythology, the hydra was a mythological monster that lived in the basin of the lake of Lerna. It is a serpentine creature that towered above its enemies, that had multiple heads that could regenerate any of them if they were cut off.

Everything You Need To Know About The Hydra

Allowing two to grow in the stump of one. It was only slain by the Greek hero Heracles, thanks to the assistance of his nephew Iolaus suggesting burning the stumps as they were cut.

For many fans of mythology, it might please them to know that hydra exists in the real world as well.

Well, at least kind of.

The organism that we call the hydra isn’t quite as impressive, in terms of size, its ferocity, or any other factor that we normally attribute to the creature of legend. 

However, that doesn’t mean that this animal is not interested in its own right. What it lacks in size, it makes up for in being a fully fleshed-out genus with a variety of characteristics that are fascinating to explore.

Plus, unlike the Greek hydra, this one is a beast that you can observe for yourself!

In this guide, we are going to explain as much as we possibly can about this amazing little organism that lives in our natural world, from a solid definition.

How the many species are classified, as well as characteristics of this genus as a whole, and tons of other details to absorb.

There is a lot to cover about this little creature, so let’s get started!

What Are Hydras?

Before we go any further into explaining the behavior and biology of this family of species, we should probably first explain what exactly a hydra is.

Hydra in the real world, are a genus of small-bodied freshwater creatures that are part of the wider phylum of living organisms known as Cnidaria.

A group that includes a variety of hard and soft-bodied life forms that includes sea anemones, many coral species, and most species of jellyfish, and were first recorded in the fossil record some 700 million years ago.

This means that the ancestors of the hydra predate the formation of most of the species of animals alive today, from lizards to the elephant, and from humans to a woodlouse.

Unlike many of its relieves in the same phylum, however, these small, soft bodies animals tend to live in freshwater. Like jellyfish, they have many small, thin tentacles, that protrude from their thin, almost rod-shaped bodies.

Like jellyfish, they also use stingers on the ends of their tentacles to catch prey

As we have already mentioned, the hydra is a small, organism, very rarely growing larger than 10 millimeters in length, and can stay secured by using their simple foot that has adhesive properties, similar to some mollusk species such as snails and slugs.

Classifications Of The Hydra

The Hydra genus was first described and documented in 1758 by Swedish scientist and zoologist Carl Linnaeus.

The Species

As we have already briefly mentioned, the hydra isn’t so much a single species of animals but is instead a genus of many related species, and new species of being discovered that belong to this genus every year.

These are just some species of hydra that we know exist today:

  • H. vulgaris, described in 1766
  • H. iheingi, described in 1939
  • H. circumcincta, described in 1914
  • H. baikalensis, described in 1923
  • H. beijingensis, described in 2003
  • H. sinensis, described in 2009

The Kingdom

Outside of their genus, Hydra is a part of the kingdom Animalia, the group that all eukaryotic multicellular life forms depend on receiving their necessary energy from other life forms.

The Phylum

As we have already mentioned, they are also part of the phylum Cnidaria, the family that includes predominantly water-borne living creatures such as most species of corals, and jellyfish.

Like many other members of their phylum, they have tentacles that they use to catch prey and have a single opening in their body through which most functions take place.

Like many other animals, Hydra a radially symmetrical, meaning that their body parts, like many other creatures in the animal kingdom, are arranged symmetrically around a central point or axis.

The Class

The class that Hydra belongs to is known as the Hydrozoa class, a group of very small predatory life forms that can either live as single solitary animals or as part of a colony.

Virtually all Hydrozoa have an exoskeleton, a space inside the creature for internal digestion, two body forms as part of its biology, and have cnidocytes that are detectable within them, which Hydra has.

Whilst most Hydrozoa is known to lie in marine aquatic conditions, several species are known to live in freshwater conditions. Most hydra falls into this latter category.

The Order

Below the class of Hydrozoa, Hydra is part of the order Hydroida, which encompasses a group of Hydrozoa that shares the following characteristics:

  • Their bodies are tubular-shaped.
  • Their life-cycle consists of three stages
  • They have a body that is broadly made of two separate layers, separated by a gel-like substance known as mesoglea. In Hydra, the two layers are:
    • The Epidermis makes up the outer layer of the animal.
    • The gastrodermis is an internal layer that lines the stomach of the organism.
  • Hydroida is also usually created young via asexual reproduction. 

Structure Of The Hydra’s Body

We have already briefly discussed the structure of most Hydra’s bodies, both as animals, and how they relate to other organisms in the animal kingdom.

Body Anatomy

To reiterate, their bodies are very tube-like, by their order, Hydroida. The tentacles of the animal tend to be arranged in and around the head pole of the creature.

It is this center point that is surrounded by many moving neck-like tentacles that the hydra probably found its namesake from, passingly resembling the many necks of the giant reptilian creature to the people who first categorized it.

On closer inspection, an observer can see a mouth opening on this pole, which takes the form of a peduncle and basal disk. It is through this part of the body that the substrate of the organism is attached, which can also be considered a ‘foot’ of sorts.

It is also possible to sometimes spot buds forming along the body of the hydra from time to time. These are smaller organisms that live on the surface of the large parent organism. They can be found in the ectoderm of the hydra, which allows it to asexually reproduce.


Generally speaking, when it comes to its size, the hydra usually is around 1 to 2 cm in length, although its tentacles can stretch out from their main body much further and make the creature appear much larger than it actually is.

As an example, whilst the tentacles are normally around 5cm long, over twice the size of the main body, they can extend to over 20cm long in some cases, some 10 times the body of the creature!

They are also able to can retract their body in response to danger, making them look even smaller and more rounded than they otherwise would be.

Pedal Disc

At the bottom of the creature’s rod, the aboral end of many Hydra species appears to be flat and is vital in the process that allows the organism to briefly attach itself to other objects.

The glandular section of the disc allows the hydra to secrete adhesives that will allow the animal to attach to a substrate.

Not only that, but the aboral end is also able to produce gas bubbles from this gland, allowing the Hydra to float through the water when the situation demands it.

The Head/Tentacles

On the other end of the hydra’s rod body, you’ll find the head pole and mouth, surrounded by many tentacles.

Most species tend to have between 6 to 12 tentacles, which they use to capture and trap food and move to the mouth of the body. The tentacles also carry stingers in them, perfect for immobilizing food.

The Body Walls

As we have already mentioned, the main body structure of hydras has two different tissue layers, which makes it in biology a diploblastic organism. We have also already briefly outlined the purposes for each layer, the epidermis, and gastrodermis.

The Epidermis

The epidermis acts as a physical barrier between the outside world and the internal functions and organs of the creature, in the same way that the epidermis of ours skins as humans and other orders of the kingdom Animalia do.

It also functions as a sensory layer for the animal to detect what is in its immediate surroundings, as well as cells that help regulate impulse control in its muscles and are covered in a thin cuticle.

The body is mainly composed of an epithelia-muscle cell, which allows the hydra to contract and extend parts of its body, as we have briefly touched on.

There are also several gland cells across the epidermis that, like the glands in the pedal disc, are responsible for the secretion of adhesives substances and bubbles of gas.

As a creature that can sustain injuries, the interstitial cells that are involved in both cellular regeneration and reproduction are also located in the epidermis of the hydra.

The Gastrodermis

As we have already touched on, the gastrodermis of the hydra is responsible for controlling the digestive process of the creature.

However, the gastrodermis is also responsible for virtually every other internal process of the hydra’s anatomy. Some of the cells that are found inside the hydra’s gastrodermis include:

  • Endothelial-gland cells, which, like the glands that are found in the epidermis on the outside layer of the hydra, are responsible for creating and secreting substances inside the creature. Unlike the other gland cells, however, the substances that are created by the internal glands primarily function as a lubricant or paralyzing agent.
  • Nutritive-muscle cells function very similarly to the epithelial cells found on the outside of the epidermis, and function very similarly to them as well. The contractions and relaxing of these muscles, however, assist in the nutritive and digestive functions of the gastrodermis instead, moving food and nutrients around the animal.
  • Interstitial cells, like those found on the outside of the cell, are cells that can transform and turn into other more specialized cells, such as regenerating damaged cells around the gastrodermis.
  • There are also several nerve and sensory cells found in the gastrodermis layer of the hydra, and they play a key part in the digestive process. For example, once food has been captured and eaten through the mouth part of the creature, the sensory and nerve cells detect its presence and start the digestive process.

Feeding And Hunting Habits Of The Hydra

What, you thought that scientists were going to name a herbivore after the mythological hydra?

Whilst it may not be as grand as its mythological namesake, the real-world hydra is a pure carnivore, through and through!

Being a multicellular animal that is composed of eukaryotic cells, the hydra is an organism that relies on feeding on others to survive.

Hydra usually feeds on smaller organisms, such as annelids, animals related to worms and other invertebrates, copepods, species that are related to plankton, and cladocerans, a massive order of crustaceans that include many tiny and microscopic forms.

To move and capture organisms for consumption, the hydra expands and contracts in bursts to help it move, whilst it is also attached to the substrate that it has chosen in its habitat.

To do this, a pacemaker at the base of the hydra’s hypostome initiates the beginning of the movement, allowing the tentacles of the hydra to extend out and seek out prey that is in its vicinity.

And, as we have already established, those tentacles can reach very far, up to 10 times the length of the creature itself.

Once prey is caught in the tentacles, it is then injected with toxins from the nematocysts in the tentacles, effectively stinging cells, that are located up the tentacles.

With the prey immobilized, the tentacles then lead the prey into the mouth of the creature. Once it is past the mouth, the digestive process is kick-started by nerve cells in the gastrodermis layer, and digestion can begin.

This is a hunting process that has been developed and adopted by virtually all hydras.

Feeding Habits Of Specific Hydra Species

Whilst hydras are almost all effective hunters and predators in their environment, there are discrepancies in how different species of hydra hunt their prey.

For example, smaller species of hydras will usually only hunt smaller prey, such as the larval stages of creatures such as cladocerans and calanoid copepods, but will not attempt to hunt the adult versions of the same animals.

By the same tokens, larger species of hydras will opt to attack and prey on larger examples of these orders of animals, rather than feeding on their larval stages

Reproduction Of The Hydra

Like every other living creature, hydras need to reproduce to stop their species from becoming a dead end.

However, unlike many other members of the kingdom Animalia, hydra has a few options when it comes to reproducing young. It can either reproduce asexually or reproduce sexually.

Asexual Reproduction In Hydra

The process that hydra reproduce through when they do so asexually, without another hydra, they do so through a process that is known as budding.

Remember when we were describing the anatomy of the hydra’s body, and we mentioned the small- bud-like organisms along its rod-shaped body? Well, those shapes are the asexual offspring of the hydra itself, hence why we called it the ‘parent’.

During budding, the offspring will develop along the basal section of its hydra parent, through mitotic cell division that is repeated from the adult hydra’s interstitial cells located in the epidermis layer.

Which are responsible for new cells being produced in the body. In this case, those cells are baby little hydras!

As the offspring hydra develops, cell differentiation kicks in, and the infant hydra starts to develop the different characteristics of the adult, such as the tentacles and parts of the mouth.

Once the offspring is fully developed, it will twist and constrict at its base that is connected to the parent, and it will separate, becoming its independent life form.

This entire process takes around 3 days and results in an offspring that shared most of its genetic characteristics with its parent.

Sexual Reproduction In Hydra

Generally speaking, asexual reproduction is the normal way that offspring.

However, when the environment it is living in becomes less suitable for the organisms, around winter or fall time, the hydra, due to a lack of plentiful food or nutrients, will instead start to develop gonads that are made in preparation for sexual reproduction.

Similar to how asexual reproduction occurs within a hydra, the interstitial cells on the epidermis layer of the animal will start developing gonads, the organs that generate sperm or egg cells in other animals.

These form a bulge on the main body of the hydra under the epidermis wall.

Interestingly, hydra will develop both testes and ovarian gonads. Testes gonads will form from the interstitial cells and the bottom of the conical-shaped body.

And the ovarian gonads will develop in other centers of interstitial cells, and develop spermatogonia and ovum, respectively.

After a certain amount of time, the testes of a hydra will rupture, allowing the sperm cells that were contained in the testes to fertilize the ovum. Once there, the fertilization of the eggs will eventually develop into a zygote.

Stages Of Hydra Pregnancy

The egg of the hydra will then go through several stages:

  • The egg will produce blastomere inside of it. This process is known as cleavage.
  • The blastomere is then produced in a narrow cavity. This is known as Blastuation.
  • The blastula will then start reorganizing into the shape of a multilayer structure, called a gastrula.
  • At this point, a cyst will begin to form around the embryos within the egg, which will allow the creature to survive and alterations to its environment. As it is being formed in an environment that is not currently suitable for hydras, the infant will remain dormant until the environment has changed again.
  • When the time comes, the embryo will increase in size and gain more tentacles, allowing it to finally hatch when the cyst next to them ruptures. Once they have hatched, they will continue to grow until they have fully matured.

Other Characteristics Of The Hydra

Photosynthesis And Symbiosis In Hydra

As we have already mentioned, the hydra is largely a carnivorous genus that gains its energy through consuming other creatures.

However, it has also been observed how its phylum, Cnidaria, is often shown to have a symbiotic relationship with many forms of algae, meaning that the two species coexist with mutually beneficial effects on one another.

The same thing has been observed to occur within hydra and several algae species.

For example, the Green Hydra species, thanks to its symbiotic relationship with algae from the chlorella genus, can generate its food and sustenance, thanks to this cooperation with the algae, allowing the hydra to photosynthesize its energy.

In terms of finding food it needs to survive, this is a massive advantage for species of hydra that can do this, as they do not rely solely on hunting to gain nutrition, so long as they are exposed to the sunlight needed for photosynthesis to occur.

A green hydra, even without direct sunlight, can survive up to 3 months using the sugars that are created through photosynthesis.

Regenerative Abilities

As with many other organisms, the interstitial cells of the hydra allow the creature to regenerate damaged parts of its body.

However, one of the most astounding things about the hydra is its ability to regenerate itself from some of the most deadly wounds, including being blended and cut in half!

This has led the hydra to become one of the most studied organisms when it comes to cellular regeneration. Currently, there is no definitive answer as to why it can do this, but researchers are hoping that the results will have a massive effect on the world of medicine.


As you can see, even though it is only a little creature, there is plenty about these fascinating organisms, both to scientists and the public as well!

Jennifer Dawkins

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