Hydra (genus)

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Hydra species
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Cnidaria
Class: Hydrozoa
Order: Anthoathecata
Family: Hydridae
Dana, 1846
Genus: Hydra
Linnaeus, 1758 [1]
Species [1]
  • * Hydra baikalensisSwarczewsky, 1923
  • * Hydra beijingensisFan, 2003
  • * Hydra canadensisRowan, 1930
  • * Hydra cauliculataHyman, 1938
  • * Hydra circumcinctaSchulze, 1914
  • * Hydra daqingensisFan, 2000
  • * Hydra ethiopiaeHickson, 1930
  • * Hydra hadleyi(Forrest, 1959)
  • * Hydra harbinensisFan & Shi, 2003
  • * Hydra hymanaeHadley & Forrest, 1949
  • * Hydra iheringiCordero, 1939
  • * Hydra intabaEwer, 1948
  • * Hydra intermediaDe Carvalho Wolle, 1978
  • * Hydra japonicaItô, 1947
  • * Hydra javanicaSchulze, 1929
  • * Hydra liriosomaCampbell, 1987
  • * Hydra madagascariensisCampbell, 1999
  • * Hydra magellanicaSchulze, 1927
  • * Hydra marianaCox & Young, 1973
  • * Hydra minimaForrest, 1963
  • * Hydra mohensisFan & Shi, 1999
  • * Hydra oligactis Pallas, 1766
  • * Hydra oregonaGriffin & Peters, 1939
  • * Hydra oxycnidaSchulze, 1914
  • * Hydra paludicolaItô, 1947
  • * Hydra paranensisCernosvitov, 1935
  • * Hydra parvaItô, 1947
  • * Hydra plagiodesmicaDioni, 1968
  • * Hydra polymorphaChen & Wang, 2008
  • * Hydra robusta(Itô, 1947)
  • * Hydra rutgersensisForrest, 1963
  • * Hydra salmacidisLang da Silveira et al., 1997
  • * Hydra sinensisWang et al., 2009
  • * Hydra thomseniCordero, 1941
  • * Hydra umfulaEwer, 1948
  • * Hydra utahensisHyman, 1931
  • * Hydra viridissima Pallas, 1766
  • * Hydra vulgaris Pallas, 1766
  • * Hydra zeylandicaBurt, 1929
  • * Hydra zhujiangensisLiu & Wang, 2010

Hydra ( /ˈhdrə/ HY-drə) is a genus of small, fresh-water organisms of the phylum Cnidaria and class Hydrozoa. They are native to the temperate and tropical regions. [2] [3] Biologists are especially interested in Hydra because of their regenerative ability – they do not appear to die of old age, or to age at all.



Schematic drawing of a discharging nematocyst Hydra nematocyst firing 01.png
Schematic drawing of a discharging nematocyst

Hydra has a tubular, radially symmetric body up to 10 mm (0.39 in) long when extended, secured by a simple adhesive foot known as the basal disc. Gland cells in the basal disc secrete a sticky fluid that accounts for its adhesive properties.

At the free end of the body is a mouth opening surrounded by one to twelve thin, mobile tentacles. Each tentacle, or cnida (plural: cnidae), is clothed with highly specialised stinging cells called cnidocytes. Cnidocytes contain specialized structures called nematocysts, which look like miniature light bulbs with a coiled thread inside. At the narrow outer edge of the cnidocyte is a short trigger hair called a cnidocil. Upon contact with prey, the contents of the nematocyst are explosively discharged, firing a dart-like thread containing neurotoxins into whatever triggered the release. This can paralyze the prey, especially if many hundreds of nematocysts are fired.

Hydra has two main body layers, which makes it "diploblastic". The layers are separated by mesoglea, a gel-like substance. The outer layer is the epidermis, and the inner layer is called the gastrodermis, because it lines the stomach. The cells making up these two body layers are relatively simple. Hydramacin [4] is a bactericide recently discovered in Hydra; it protects the outer layer against infection. A single Hydra is composed of 50,000 to 100,000 cells which consist of three specific stem cell populations that will create many different cell types. These stem cells will continually renew themselves in the body column. [5] Hydras have two significant structures on their body: the "head" and the "foot". When a Hydra is cut in half, each half will regenerate and form into a small Hydra; the "head" will regenerate a "foot" and the "foot" will regenerate a "head". If the Hydra is sliced into many segments then the middle slices will form both a "head" and a "foot". [6]

Respiration and excretion occur by diffusion throughout the surface of the epidermis, while larger excreta are discharged through the mouth. [7] [8]

Nervous system

The nervous system of Hydra is a nerve net, which is structurally simple compared to more derived animal nervous systems. Hydra does not have a recognizable brain or true muscles. Nerve nets connect sensory photoreceptors and touch-sensitive nerve cells located in the body wall and tentacles.

The structure of the nerve net has two levels:

Some have only two sheets of neurons. [9]

Motion and locomotion

Hydra attached to a substrate Hydras (8).JPG
Hydra attached to a substrate

If Hydra are alarmed or attacked, the tentacles can be retracted to small buds, and the body column itself can be retracted to a small gelatinous sphere. Hydra generally react in the same way regardless of the direction of the stimulus, and this may be due to the simplicity of the nerve nets.

Hydra are generally sedentary or sessile, but do occasionally move quite readily, especially when hunting. They have two distinct methods for moving – 'looping' and 'somersaulting'. They do this by bending over and attaching themselves to the substrate with the mouth and tentacles and then relocate the foot, which provides the usual attachment, this process is called looping. In somersaulting, the body then bends over and makes a new place of attachment with the foot. By this process of "looping" or "somersaulting", a Hydra can move several inches (c. 100 mm) in a day. Hydra may also move by amoeboid motion of their bases or by detaching from the substrate and floating away in the current.

Reproduction and life cycle

Hydra budding:
Creating a bud
Daughter growing out
Beginning to cleave
Daughter broken off
Daughter clone of parent Hydra Budding.svg
Hydra budding :
  1. Non-reproducing
  2. Creating a bud
  3. Daughter growing out
  4. Beginning to cleave
  5. Daughter broken off
  6. Daughter clone of parent

When food is plentiful, many Hydra reproduce asexually by budding. The buds form from the body wall, grow into miniature adults and break away when mature.

When a hydra is well fed, a new bud can form every two days. [10] When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction occurs in some Hydra. Swellings in the body wall develop into either ovaries or testes. The testes release free-swimming gametes into the water, and these can fertilize the egg in the ovary of another individual. The fertilized eggs secrete a tough outer coating, and, as the adult dies (due to starvation or cold), these resting eggs fall to the bottom of the lake or pond to await better conditions, whereupon they hatch into nymph Hydra. Some Hydra species, like Hydra circumcincta and Hydra viridissima , are hermaphrodites [11] and may produce both testes and ovaries at the same time.

Many members of the Hydrozoa go through a body change from a polyp to an adult form called a medusa, which is usually the life stage where sexual reproduction occurs, but Hydra do not progress beyond the polyp phase. [12]


Hydra mainly feed on aquatic invertebrates such as Daphnia and Cyclops .

While feeding, Hydra extend their body to maximum length and then slowly extend their tentacles. Despite their simple construction, the tentacles of Hydra are extraordinarily extensible and can be four to five times the length of the body. Once fully extended, the tentacles are slowly maneuvered around waiting for contact with a suitable prey animal. Upon contact, nematocysts on the tentacle fire into the prey, and the tentacle itself coils around the prey. Within 30 seconds, most of the remaining tentacles will have already joined in the attack to subdue the struggling prey. Within two minutes, the tentacles will have surrounded the prey and moved it into the opened mouth aperture. Within ten minutes, the prey will have been engulfed within the body cavity, and digestion will have started. Hydra are able to stretch their body wall considerably in order to digest prey more than twice their size. After two or three days, the indigestible remains of the prey will be discharged through the mouth aperture via contractions.[ citation needed ]

The feeding behaviour of Hydra demonstrates the sophistication of what appears to be a simple nervous system.

Some species of Hydra exist in a mutual relationship with various types of unicellular algae. The algae are protected from predators by Hydra and, in return, photosynthetic products from the algae are beneficial as a food source to Hydra.

Measuring the feeding response

Addition of glutathione causes reduction in the tentacle spread in hydra. Fig1 TentacleSpread wiki.png
Addition of glutathione causes reduction in the tentacle spread in hydra.

The feeding response in Hydra is induced by glutathione (specifically in the reduced state as GSH) released from damaged tissue of injured prey. [13] There are several methods conventionally used for quantification of the feeding response. In some, the duration for which the mouth remains open is measured. [14] Other methods rely on counting the number of Hydra among a small population showing the feeding response after addition of glutathione. [15] Recently, an assay for measuring the feeding response in hydra has been developed. [16] In this method, the linear two-dimensional distance between the tip of the tentacle and the mouth of hydra was shown to be a direct measure of the extent of the feeding response. This method has been validated using a starvation model, as starvation is known to cause enhancement of the Hydra feeding response. [16]


The species Hydra oligactis is preyed upon by the flatworm Microstomum lineare . [17] [18]

Tissue regeneration

Hydras undergo morphallaxis (tissue regeneration) when injured or severed. Typically, Hydras will reproduce by just budding off a whole new individual; the bud will occur around two-thirds of the way down the body axis. When a Hydra is cut in half, each half will regenerate and form into a small Hydra; the "head" will regenerate a "foot" and the "foot" will regenerate a "head". This regeneration occurs without cell division. If the Hydra is sliced into many segments, the middle slices will form both a "head" and a "foot". [6] The polarity of the regeneration is explained by two pairs of positional value gradients. There is both a head and foot activation and inhibition gradient. The head activation and inhibition works in an opposite direction of the pair of foot gradients. [19] The evidence for these gradients was shown in the early 1900s with grafting experiments. The inhibitors for both gradients have shown to be important to block the bud formation. The location that the bud will form is where the gradients are low for both the head and foot. [6] Hydras are capable of regenerating from pieces of tissue from the body and additionally after tissue dissociation from reaggregates. [19]


Daniel Martinez claimed in a 1998 article in Experimental Gerontology that Hydra are biologically immortal. [20] This publication has been widely cited as evidence that Hydra do not senesce (do not age), and that they are proof of the existence of non-senescing organisms generally. In 2010, Preston Estep published (also in Experimental Gerontology) a letter to the editor arguing that the Martinez data refute the hypothesis that Hydra do not senesce. [21]

The controversial unlimited lifespan of Hydra has attracted much attention from scientists. Research today appears to confirm Martinez' study. [22] Hydra stem cells have a capacity for indefinite self-renewal. The transcription factor "forkhead box O" (FoxO) has been identified as a critical driver of the continuous self-renewal of Hydra. [22] In experiments, a drastically reduced population growth resulted from FoxO down-regulation. [22]

In bilaterally symmetrical organisms (Bilateria), the transcription factor FoxO affects stress response, lifespan, and increase in stem cells. If this transcription factor is knocked down in bilaterian model organisms, such as fruit flies and nematodes, their lifespan is significantly decreased. In experiments on H. vulgaris (a radially symmetrical member of phylum Cnidaria), when FoxO levels were decreased, there was a negative effect on many key features of the Hydra, but no death was observed, thus it is believed other factors may contribute to the apparent lack of aging in these creatures. [5]


An ortholog comparison analysis done within the last decade demonstrated that Hydra share a minimum of 6,071 genes with humans. Hydra is becoming an increasingly better model system as more genetic approaches become available. [5] A draft of the genome of Hydra magnipapillata was reported in 2010. [23]

The genomes of cnidarians are usually less than 500 Mb in size, as in the Hydra viridissima, which has a genome size of approximately 300 Mb. In contrast, the genomes of brown hydras are approximately 1 Gb in size. This is because the brown hydra genome is the result of an expansion event involving LINEs, a type of transposable elements, in particular, a single family of the CR1 class. This expansion is unique to this subgroup of the genus Hydra and is absent in the green hydra, which has a repeating landscape similar to other cnidarians. These genome characteristics make Hydra attractive for studies of transposon-driven speciations and genome expansions. [24]

See also

Related Research Articles

Cnidaria Aquatic animal phylum having cnydocytes

Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.

Polyp (zoology) One of two forms found in the phylum Cnidaria (zoology)

A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are roughly cylindrical in shape and elongated at the axis of the vase-shaped body. In solitary polyps, the aboral end is attached to the substrate by means of a disc-like holdfast called a pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly. The oral end contains the mouth, and is surrounded by a circlet of tentacles.

Jellyfish Soft-bodied, aquatic invertebrates

Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella-shaped bells and trailing tentacles, although a few are anchored to the seabed by stalks rather than being mobile. The bell can pulsate to provide propulsion for highly efficient locomotion. The tentacles are armed with stinging cells and may be used to capture prey and defend against predators. Jellyfish have a complex life cycle; the medusa is normally the sexual phase, which produces planula larva that disperse widely and enter a sedentary polyp phase before reaching sexual maturity.

Cnidocyte Explosive cell containing one giant secretory organelle (cnida)

A cnidocyte is an explosive cell containing one giant secretory organelle called a cnidocyst that can deliver a sting to other organisms. The presence of this cell defines the phylum Cnidaria. Cnidae are used to capture prey and as a defense against predators. A cnidocyte fires a structure that contains a toxin within the cnidocyst; this is responsible for the stings delivered by a cnidarian.

Ctenophora Phylum of gelatinous marine animals

Ctenophora comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming, and they are the largest animals to swim with the help of cilia.

Portuguese man o war A siphonophore in the genus Physalia; also known as the Pacific man o war

The Portuguese man o' war, also known as the man-of-war, bluebottle, or blue bottle jellyfish, is a marine hydrozoan found in the Atlantic Ocean and the Indian Ocean. It is considered to be the same species as the Pacific man o' war, which is found mainly in the Pacific Ocean.

Hydrozoa class of cnidarians

Hydrozoa are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in fresh water. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.

<i>Obelia</i> Genus of hydrozoans

Obelia is a genus of hydrozoans, a class of mainly marine and some freshwater animal species that have both polyp and medusa stages in their life cycle. Hydrozoa belongs to the phylum Cnidaria, which are aquatic organisms that are relatively simple in structure.

Nerve net

A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a central nervous system, organisms with radial symmetry are associated with nerve nets. Nerve nets can be found in members of the Cnidaria, Ctenophora, and Echinodermata phyla, all of which are found in marine environments. Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.


In zoology, a tentacle is a flexible, mobile, elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mainly like muscular hydrostats. Most forms of tentacles are used for grasping and feeding. Many are sensory organs, variously receptive to touch, vision, or to the smell or taste of particular foods or threats. Examples of such tentacles are the eyestalks of various kinds of snails. Some kinds of tentacles have both sensory and manipulatory functions.

Morphallaxis is the regeneration of specific tissue in a variety of organisms due to loss or death of the existing tissue. The word comes from the Greek allazein, (αλλάζειν) which means to change.

Medusozoa Clade of marine invertebrates

Medusozoa is a clade in the phylum Cnidaria, and is often considered a subphylum. It includes the classes Hydrozoa, Scyphozoa, Staurozoa and Cubozoa, and possibly the parasitic Polypodiozoa. Medusozoans are distinguished by having a medusa stage in their often complex life cycle, a medusa typically being an umbrella-shaped body with stinging tentacles around the edge. With the exception of some Hydrozoa, all are called jellyfish in their free-swimming medusa phase.

<i>Hydra oligactis</i> Species of hydrozoan

Hydra oligactis, also known as the brown hydra, is a species of hydra found widely dispersed in the northern temperate zone. It is a common organism found in still waters from early Spring to late Autumn.

<i>Turritopsis dohrnii</i> Species of small, biologically immortal jellyfish

Turritopsis dohrnii, also known as the immortal jellyfish, is a species of small, biologically immortal jellyfish found worldwide in temperate to tropic waters. It is one of the few known cases of animals capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary individual. Others include the jellyfish Laodicea undulata and species of the genus Aurelia.

Sea anemone Marine animals of the order Actiniaria

Sea anemones are the marine, predatory animals of the order Actiniaria. They are named after the anemone, a terrestrial flowering plant, because of the colourful appearance of many. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. As cnidarians, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra. Unlike jellyfish, sea anemones do not have a medusa stage in their life cycle.

<i>Hydra vulgaris</i> Species of cnidarian

Hydra vulgaris, the fresh-water polyp, is a small animal freshwater hydroid with length from 10 mm to 30 mm and width about 1 mm.

Cydippida Order of comb jellies with retractable branched tentacles

Cydippida is an order of comb jellies. They are distinguished from other comb jellies by their spherical or oval bodies, and the fact their tentacles are branched, and can be retracted into pouches on either side of the pharynx. The order is not monophyletic, that is, more than one common ancestor is believed to exist.

<i>Hydra viridissima</i> Species of hydrozoan

Hydra viridissima is a species of cnidarian which is commonly found in still or slow-moving freshwater in the Northern temperate zone. Hydra viridissima is commonly called green hydra due to its coloration, which is due to the symbiotic green algae Chlorella vulgaris which live within its body. These creatures are typically 10 mm long and have tentacles that are about half of their length. They are strictly carnivorous and typically feed on small crustaceans, insects and annelids. Hydra are normally sessile and live on aquatic vegetation. They secrete mucous to attach to substrate using their basal disc.

<i>Clytia hemisphaerica</i> Species of hydrozoan

Clytia hemisphaerica is a small hydrozoan-group cnidarian, about 1 cm in diameter, that is found in the Mediterranean Sea and the North-East Atlantic Ocean. Clytia has the free-swimming jellyfish form typical of the Hydrozoa, as well as vegetatively propagating polyps.

<i>Clava</i> (hydrozoa) Genus of hydrozoans

Clava is a monotypic genus of hydrozoans in the family Hydractiniidae. It contains only one accepted species, Clava multicornis. Other names synonymous with Clava multicornis include Clava cornea, Clava diffusa, Clava leptostyla, Clava nodosa, Clava parasitica, Clava squamata, Coryne squamata, Hydra multicornis, and Hydra squamata. The larvae form of the species has a well developed nervous system compared to its small size. The adult form is also advanced due to its ability to stay dormant during unfavorable periods.


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