Hydra | What is a hydra, characteristics, classification, habitat, and diet

"Hydra" is a genus of cnidarians consisting of small animals with a cylindrical body, retractable tentacles, and an adhesive base or disc.
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“Hydra” is a genus that belongs to the cnidarians (coelenterates) comprised of small animals with a cylindrical body and tentacles (hydra) that live in rivers, streams, lakes, swamps, and are relatives of jellyfish, sea anemones, and corals.

The hydra is a truly fascinating creature, with great abilities such as doing somersaults and flips to move, great endurance, and an astonishing power to regenerate to such an extent that it is considered an immortal animal!

Its name is associated with a mythological animal shaped like a snake with numerous heads called the “Lernaean Hydra.”

Want to learn more about this amazing animal? Here you will find all its characteristics, where it lives, what it eats, why it regenerates, is it harmful?… Don’t go away!

What is a hydra?

A “hydra” is a small invertebrate organism (without a backbone) with a cylindrical body and several tentacles that lives in freshwater environments (rivers, lakes, ponds…).

It is also known as “polyp or freshwater hydra” because it lives fixed to a substrate.

Hidra del grupo viridissima, que presentan un característico color verde, gracias a la relación de simbiosis (asociación) que mantienen con las algas del género Chlorella; la columna es pequeña, los tentáculos son cortos y no poseen pedúnculo (pie).
Green Hydra (Hydra viridis)

Which animal group does the hydra belong to?

It belongs to the phylum “Cnidaria” or “Coelenterata,” for having stinging cells (cnidocytes) on its tentacles and mouth, and is related to jellyfish, sea anemones, and corals.

In turn, it is part of the class “Hydrozoa” (from Greek hydra, aquatic snake, and zoo, animal), the order “Hydroida,” the family “Hydridae,” and the genus “Hydra,” as it is an aquatic organism.

How did the name “Hydra” come about?

The name comes from the resemblance of the movement of its tentacles to the multiple heads of a giant creature (in the form of a snake) from Greek and Roman mythology known as the “Lernaean Hydra.”

The myth says that the animal had the virtue of renewing two heads for each one that was cut off; similarly, the hydra can regenerate! and give rise to a complete organism from small fragments of its tissues.

Gif Hydra

What characteristics do hydras have?


  • Like other coelenterates, they have radial symmetry (from a central axis, the body is organized into equal parts).
  • Their body adopts the shape of a polyp: a tube or column with two ends (oral and aboral).
  • The oral end contains the mouth, surrounded by a cycle of retractable tentacles (they contract) that radiate outward.
  • The aboral end has a foot (peduncle) that, in some species, is absent, and a base (disc) for adhering to different surfaces through adhesive cells that secrete a sticky substance.
  • The tentacles vary in number (between 6 and 8) and length depending on the species, and are equipped with stinging cells (nematocysts) that act as tiny harpoons and are activated at the slightest contact.
  • Their size varies between 2 and 30 millimeters in length and 1 millimeter in width.
  • They are generally transparent, grayish or yellowish in color, although they can display brown or green hues.
Did you know? Up to 18 different types of irritating or cnidocyst cells have been described in the hydra, powerful weapons! that paralyze and kill the prey or enemy. Click To Tweet


  • It is made up of two layers: the epidermis (external) and the gastrodermis (internal), separated by an intermediate layer or mesoglea, which has a gelatinous appearance.
  • The center of the body tube is a kind of hollow sac called the “gastrovascular cavity,” which communicates with the outside through the mouth.
  • It does not have a recognizable brain or true muscles; stimuli are captured through a primitive nerve network connected to sensory cells distributed throughout the body, especially in the tentacles.
  • They breathe through the surface of their body (cutaneous respiration).
  • Wastes are expelled through the mouth.
  • They have differentiated sexes (female and male), with male gonads (testicles) and female gonads (ovaries) located along the gastric region.
  • Certain species are hermaphroditic (having both female and male gametes).
Did you know...? Scientists from Kyushu University in Japan discovered that despite not having a brain, hydras can sleep! and develop cycles of active and sleep states that last approximately four hours each. Click To Tweet

How are hydras classified?

Hydras are currently classified into four groups: Vulgaris, Viridissima, Braueri, and Oligactis.


This is the group of common hydras; they are white, brown, or gray, do not have a peduncle (foot), the length of the stem (column) is medium, and their tentacles can be shorter, the same size, or longer than the column, reaching up to twice its length.

Specimens of vulgaris hydras

Hydra americana, hydra japónica, hydra cauliculata, hydra mariana…


Includes hydras that have a characteristic green color, thanks to the symbiotic relationship (association) they maintain with algae of the genus Chlorella; the column is small, the tentacles are short, and they do not have a peduncle (foot).

Specimens of viridissima hydras

Chlorohydra hadleyi, hydra plagiodesmica, hydra sinensis…


Includes brown-colored hydras that do not have a peduncle (foot); the column and tentacles are short.

Specimens of braueri hydras

Hydra Braueri, hydra ovata, hydra stellata…


These are brown-colored hydras; they are commonly found in the Northern Hemisphere and parts of Australia, have a peduncle (foot), and their column and tentacles are long.

Specimens of oligactis hydras

Hydra canadensis, hydra robusta, hydra pseudoligactis…

Especie de hidra oligactis; tiene pedúnculo (pie) y su columna y tentáculos son largos.
Brown hydra (Hydra oligactis)

Where do hydras live?

Hydras are found on all continents except Antarctica, as well as on continental islands (Japan, Greenland).

They are present in all types of freshwater habitats, including calm water rivers, swamps, streams, clean water lakes, ponds… attached (by their disc or base) to the leaves of aquatic plants, among rocks, or on the bottom, adhered to some substrate.

In some cases, they adapt to aquarium environments.

Can a hydra move?

Usually, hydras are immobile or sessile but, on occasion, can detach from the substrate and move slowly, especially when hunting.

There are various ways by which a hydra can move:

  • By doing a somersault, in which it bends its body and stands on its tentacles to roll forward.
  • In a loop, by which it leans and adheres to the substrate with the mouth and tentacles (similar to the movement of a caterpillar) and then relocates the surface where it will fix itself. This is the most common form.
  • By pulling hard with its tentacles.
  • Dragged by the water current while floating, thanks to a gas bubble it creates in its disc.
  • By climbing, joining its tentacles to adhere to some object and then letting go.
Did you know...? When feeling threatened, the hydra retracts its tentacles and body so it looks shorter and more rounded. Click To Tweet

What kind of diet does a hydra have?

The hydra is a carnivorous animal, and its diet includes aquatic invertebrates (such as daphnia) that float in the water, freshwater worms, and small fish.

Did you know...? In the absence of prey, a 'hydra viridissima' can survive for 3 months using the nutrients produced by the algae living in its tissues through photosynthesis. Click To Tweet

How does a hydra feed?

To catch its prey, the hydra extends its toxin-loaded tentacles (cnidocysts) waiting to make contact and, upon doing so, fires the stinging tips and paralyzes it.

At that moment, a special group of cells change shape to reveal a mouth!

Gif Hidra

Then, the hydra takes the prey with its tentacles, introduces it into the gastric cavity, and secretes digestive enzymes that break down the tissues to absorb nutrients.

The indigestible parts are expelled through the mouth.

Did you know...? The hydra can stretch its body up to four times its normal length to catch its prey. Click To Tweet

How does a hydra reproduce?

Individuals of the Hydra genus reproduce both sexually and asexually, depending on a seasonal pattern: under difficult conditions (winter), sexual reproduction takes place, while under favorable conditions, asexual reproduction predominates.

Sexual reproduction

This is predominant in the winter season, under conditions of low temperature and a lack of sufficient food sources.

The process begins with the release of sperm by one organism into the water, which swims until it finds another’s ovary for the fertilization of the egg.

The fertilized egg remains attached to the mother and transforms into a capsule surrounded by a hard protective cover (embryotheca) that can be smooth or have spines.

Subsequently, the capsule dissolves, and a larva (planula) emerges from the embryo.

Did you know...? Under unfavorable conditions, the fertilized egg of the hydra produces a protective cover, and its development remains halted until the environment becomes suitable again. Click To Tweet

Asexual reproduction

This is the most common form of reproduction for hydras; it occurs through budding, a process in which, through divisions of its cells, buds or offshoots (ranging from 1 to 6) emerge at the base of the adult hydra, representing identical organisms to it in miniature version.

The offshoot receives its nutrients from the parent hydra; upon completing its development, it detaches to become an independent organism.

Why can a hydra regenerate?

There is a strange and incredible process that allows a hydra to regenerate and, from a piece of itself, rebuild a new organism! How does it happen?

It turns out this animal has a network of highly resistant protein fibers called the “cytoskeleton” that acts as a memory storing information about the hydra’s body and directs cells to organize themselves and form a new body.

The cytoskeleton is capable of finding a balance between maintaining its old form and adapting to new conditions, and the steps for a complete organism to develop from a small piece of tissue happen in just 4 days!

That is why it is said that they are immortal, as it regenerates its old cells with new ones indefinitely.

Gif Hidra
Did you know...? By cutting a hydra in half, the head part will regenerate a foot, and vice versa, the foot part will regenerate a head; if you cut it into many parts, the middle cuts will form both a head and a foot. Click To Tweet

How many years can a hydra live?

Under conditions suitable for its development, a hydra can reach up to 1400 years of life!

Is a hydra harmful to humans?

Despite having toxic cells (nematocysts) capable of immobilizing and killing prey, hydras cannot cause any harm to humans.

In case of contact with a hydra, the main symptoms that may appear are irritation or a burning sensation, red spots on the contact area (petechiae), and, occasionally, blisters, nausea, and muscle cramps.

As treatment, it is recommended to wash the area with seawater, acetic acid, or vinegar, and in case of any complications, visit a specialist doctor.

Did you know...? Fully understanding the hydra's regenerative capacity will allow scientists to incorporate this property into the cure for traumas or degenerative diseases in human patients. Click To Tweet


Hiroyuki, K., Sungeon, J., Junko, S., Etsuko, A., Jongbin, H., Yoshitaka, Ch., and Taichi, I. (2020). A sleep-like state in Hydra unravels conserved sleep mechanisms during the evolutionary development of the central nervous system. Science Advances. https://doi.org/10.1126/sciadv.abb9415

Olivares, E. (2021). You don’t need a brain to be able to sleep, and this animal proved it. Science. https://bit.ly/3tdtLSC

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