Hydra (genus)

{{Automatic taxobox | image = Hydra-Foto.jpg | image_caption = ''Hydra'' budding | parent_authority = Dana, 1846 | taxon = Hydra | authority = Linnaeus, 1758<ref name="worse">{{cite WoRMS|vauthors=Schuchert P |year=2011|title=''Hydra'' Linnaeus, 1758|db=hydrozoa|id=267491|access-date=2011-12-20}}</ref> | subdivision_ranks = Species | subdivision_ref = <ref name=worse/> | subdivision = {{Collapsible list |* ''Hydra baikalensis'' Swarczewsky, 1923 |* ''Hydra beijingensis'' Fan, 2003 |* ''Hydra canadensis'' Rowan, 1930 |* ''Hydra cauliculata'' Hyman, 1938 |* ''Hydra circumcincta'' Schulze, 1914 |* ''Hydra daqingensis'' Fan, 2000 |* ''Hydra ethiopiae'' Hickson, 1930 |* ''Hydra hadleyi'' (Forrest, 1959) |* ''Hydra harbinensis'' Fan & Shi, 2003 |* ''Hydra hymanae'' Hadley & Forrest, 1949 |* ''Hydra iheringi'' Cordero, 1939 |* ''Hydra intaba'' Ewer, 1948 |* ''Hydra intermedia'' De Carvalho Wolle, 1978 |* ''Hydra japonica'' Itô, 1947 |* ''Hydra javanica'' Schulze, 1929 |* ''Hydra liriosoma'' Campbell, 1987 |* ''Hydra madagascariensis'' Campbell, 1999 |* ''Hydra magellanica'' Schulze, 1927 |* ''Hydra mariana'' Cox & Young, 1973 |* ''Hydra minima'' Forrest, 1963 |* ''Hydra mohensis'' Fan & Shi, 1999 |* ''Hydra oligactis'' Pallas, 1766 |* ''Hydra oregona'' Griffin & Peters, 1939 |* ''Hydra oxycnida'' Schulze, 1914 |* ''Hydra paludicola'' Itô, 1947 |* ''Hydra paranensis'' Cernosvitov, 1935 |* ''Hydra parva'' Itô, 1947 |* ''Hydra plagiodesmica'' Dioni, 1968 |* ''Hydra polymorpha'' Chen & Wang, 2008 |* ''Hydra robusta'' (Itô, 1947) |* ''Hydra rutgersensis'' Forrest, 1963 |* ''Hydra salmacidis'' Lang da Silveira et al., 1997 |* ''Hydra sinensis'' Wang et al., 2009 |* ''Hydra thomseni'' Cordero, 1941 |* ''Hydra umfula'' Ewer, 1948 |* ''Hydra utahensis'' Hyman, 1931 |* ''Hydra viridissima'' Pallas, 1766 |* ''Hydra vulgaris'' Pallas, 1766 |* ''Hydra zeylandica'' Burt, 1929 |* ''Hydra zhujiangensis'' Liu & Wang, 2010}} | synonyms = *''Chlorohydra'' }}

'''''Hydra''''' ({{IPAc-en|ˈ|h|aɪ|d|r|ə}} {{Respell|HY|drə}}) is a genus of small freshwater hydrozoans in the phylum Cnidaria. They are solitary, carnivorous jellyfish-like animals,<ref name = mc>{{cite book| title = Exploring Life Sciences| date = 2000| volume = 6|page = 428|isbn = 0-7614-7141-3|publisher = Marshall Cavendish}}</ref> native to the temperate and tropical regions.<ref>{{cite book |last=Gilberson |first=Lance |title=Zoology Lab Manual |date=1999 |publisher=Primis Custom Publishing |edition=4th |name-list-style=vanc}}</ref><ref>{{cite book |title=Biology |vauthors=Solomon E, Berg L, Martin D |date=2002 |publisher=Brooks/Cole |edition=6th}}</ref> The genus was named by Linnaeus in 1758 after the Hydra, the mythical many-headed beast that was defeated by Heracles, as when the animal has a part severed, it will regenerate much like the mythical Hydra's heads. 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.

==Habitat==

''Hydras'' are often found in freshwater bodies, but some ''Hydras'' are found in open water. They live attached to submerged rocks using a sticky secretion from their base.<ref name = mc/> The genus ''Hydra'' occurs on all continents except for Antarctica and the Oceanic islands, though ''Hydras'' occur mainly in mesotrophic to eutrophic habitats.<ref>{{Cite journal |last1=Shekha |first1=Yahya A. |last2=Ahmad |first2=Sherwan T. |last3=Aziz |first3=Falah M. |date=2015 |title=A New Record of Two Species of Hydra in Iraq : An Ecological and Histological Study |url=https://doi.org/10.12816/0027062 |journal=Jordan Journal of Biological Sciences |volume=8 |issue=4 |pages=269–272 |doi=10.12816/0027062 |issn=1995-6673|url-access=subscription }}</ref>

==Morphology== thumb|left|Schematic drawing of a discharging nematocyst

''Hydra'' has a tubular, radially symmetric body up to {{convert|10|mm|abbr=on}} 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. ''Hydra'' has a body wall made up of the ectoderm and the endoderm, which are separated by an extracellular matrix called the ''mesoglea''.<ref>{{Cite journal |last=Galliot |first=Brigitte |date=2006 |title=Regeneration in Hydra |url=https://doi.org/10.1038/npg.els.0001096 |journal=Encyclopedia of Life Sciences |pages=3 |doi=10.1038/npg.els.0001096|isbn=978-0-470-01617-6 |url-access=subscription }}</ref>

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 clad with highly specialised stinging cells called cnidocytes. Cnidocytes contain specialized structures called nematocysts, which look like miniature light bulbs with a coiled thread inside. The nematocysts are housed within a single epithelial cell together with a sensory and nerve cell.<ref name=":3">{{Cite journal |last1=Scappaticci |first1=A.A. |last2=Kahn |first2=Fhyzeedon |last3=Kass-Simon |first3=G. |date=2010 |title=Nematocyst discharge in Hydra vulgaris: Differential responses of desmonemes and stenoteles to mechanical and chemical stimulation |url=https://doi.org/10.1016/j.cbpa.2010.06.177 |journal=Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology |volume=157 |issue=2 |pages=184–191 |doi=10.1016/j.cbpa.2010.06.177 |issn=1095-6433}}</ref> 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 due to hydrostatic pressure (the osmotic pressure exceeds a critical threshold),<ref name=":5" /> 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.

In ''Hydra'', different types of nematocysts are distinguished: the desmonemes for prey attachment; the isorhizas with spines in the interior of the nematocyst capsule and the spineless atrichous isorhizas; and the large stenoteles, with a prominent stylet apparatus at the tubule base employed for piercing cuticle structures.<ref name=":5">{{Cite journal |last1=Balasubramanian |first1=Prakash G. |last2=Beckmann |first2=Anna |last3=Warnken |first3=Uwe |last4=Schnölzer |first4=Martina |last5=Schüler |first5=Andreas |last6=Bornberg-Bauer |first6=Erich |last7=Holstein |first7=Thomas W. |last8=Özbek |first8=Suat |date=2012-03-23 |title=Proteome of Hydra nematocyst |journal=The Journal of Biological Chemistry |volume=287 |issue=13 |pages=9672–9681 |doi=10.1074/jbc.M111.328203 |issn=1083-351X |pmc=3323026 |pmid=22291027 |doi-access=free |bibcode=2012JBiCh.287.9672B }}</ref>

''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<ref>{{Cite journal |vauthors=Jung S, Dingley AJ, Augustin R, Anton-Erxleben F, Stanisak M, Gelhaus C, Gutsmann T, Hammer MU, Podschun R, Bonvin AM, Leippe M, Bosch TC, Grötzinger J |date=January 2009 |title=Hydramacin-1, structure and antibacterial activity of a protein from the basal metazoan Hydra |journal=The Journal of Biological Chemistry |volume=284 |issue=3 |pages=1896–905 |doi=10.1074/jbc.M804713200 |pmid=19019828|s2cid=3887876 |doi-access=free |bibcode=2009JBiCh.284.1896J }}</ref> 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 create many different cell types. These stem cells continually renew themselves in the body column''.''<ref name=":2">{{cite journal | vauthors = Tomczyk S, Fischer K, Austad S, Galliot B | title = Hydra, a powerful model for aging studies | journal = Invertebrate Reproduction & Development | volume = 59 | issue = sup1 | pages = 11–16 | date = January 2015 | pmid = 26120246 | pmc = 4463768 | doi = 10.1080/07924259.2014.927805 | bibcode = 2015InvRD..59S..11T }}</ref> ''Hydras'' have two significant structures on their body: the "head" and the "foot". When a ''Hydra'' is cut in half, each half regenerates and forms into a small ''Hydra''; the "head" regenerates a "foot" and the "foot" regenerates a "head". If the ''Hydra'' is sliced into many segments then the middle slices form both a "head" and a "foot".<ref name=":0">{{cite book |last=Gilbert|first=Scott F. | name-list-style = vanc |date=2000 |chapter=Regeneration |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK9971/ |title = Developmental Biology |publisher=Sinauer Associates | edition = 6th }}</ref>

Respiration and excretion occur by diffusion throughout the surface of the epidermis, while larger excreta are discharged through the mouth by a quick radial contraction of the body column.<ref>{{Cite journal |last1=Cantor |first1=Marvin H. |last2=Rahat |first2=Menachem |year=1982 |title=Regulation of Respiration and Photosynthesis in Hydra viridis and in Its Separate Cosymbionts: Effect of Nutrients |journal=Physiological Zoology |publisher=The University of Chicago Press |volume=55 |issue=3 |pages=281–288 |doi=10.1086/physzool.55.3.30157891 |issn=0031-935X |jstor=30157891 |s2cid=86961916}}</ref><ref>{{Cite web |title=Olympus Microscopy Resource Center {{!}} Pond Life Video Gallery – Hydra (Coelenterata) |url=http://olympus.magnet.fsu.edu/micd/galleries/moviegallery/pondscum/coelenterata/hydra/index.html |access-date=2019-09-21 |website=olympus.magnet.fsu.edu}}</ref><ref name=":4">{{Cite journal |last1=Dupre |first1=Christophe |last2=Yuste |first2=Rafael |date=2017-04-24 |title=Non-overlapping Neural Networks in Hydra vulgaris |journal=Current Biology |volume=27 |issue=8 |pages=1085–1097 |doi=10.1016/j.cub.2017.02.049 |issn=1879-0445 |pmc=5423359 |pmid=28366745 |bibcode=2017CBio...27.1085D }}</ref>

==Nervous system== The nervous system of ''Hydra'' is a nerve net, composed of a few hundred to a few thousand neurons, 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: *level 1 – sensory cells or internal cells; and *level 2 – interconnected ganglion cells connected via synapse to epithelial or motor cells.

Some have only two sheets of neurons.<ref name="pmid28441559">{{cite journal | vauthors = Ji N, Flavell SW | title = Hydra: Imaging Nerve Nets in Action | journal = Current Biology | volume = 27 | issue = 8 | pages = R294–R295 | date = April 2017 | pmid = 28441559 | doi = 10.1016/j.cub.2017.03.040 | doi-access = free | bibcode = 2017CBio...27.R294J | hdl = 1721.1/114954 | hdl-access = free }}</ref>

It also has been described that there are three major networks extending throughout ''Hydra''<nowiki/>'s entire body. They are activated selectively during longitudinal contractions, elongations in response to light, and radial contractions; the additional network near the ''hypostome'', the dome-shaped jut surrounding the mouth aperture, is activated during nodding (the gentle swaying of the hypostome and its tentacles to one side).<ref name=":4" />

==Motion and locomotion== thumb|upright|''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.

A dark-habituated ''Hydra'' that is exposed to light will respond by elongating its body towards it, bending its hypostome-tentacle junction, and eventually somersaulting towards the light source.<ref>{{Cite journal |last1=Passano |first1=L. M. |last2=McCullough |first2=C. B. |date=1963 |title=Pacemaker Hierarchies Controlling the Behaviour of Hydras |url=https://doi.org/10.1038/1991174a0 |journal=Nature |volume=199 |issue=4899 |pages=1174–1175 |doi=10.1038/1991174a0 |issn=0028-0836|url-access=subscription }}</ref>

==Reproduction and life cycle== Most ''Hydras'' can reproduce sexually under certain conditions, though they typically choose to reproduce asexually instead. Unlike many members of the Hydrozoa, which alternate between the polyp form and the medusa form (the life stage where sexual reproduction occurs), ''Hydra'' never progress beyond the polyp phase in their life cycle.<ref>{{Cite book |author=Hickman, Cleveland P. Jr. |title=Integrated principles of zoology |year=2019 |isbn=978-1-260-20519-0 |edition=Eighteenth |location=New York, NY |oclc=1097367369}}</ref> Instead, when food is plentiful, many ''Hydra'' opt to reproduce asexually by budding.<ref name=":6">{{Cite journal |last1=Sun |first1=Shixiang |last2=White |first2=Ryan R. |last3=Fischer |first3=Kathleen E. |last4=Zhang |first4=Zhengdong |last5=Austad |first5=Steven N. |last6=Vijg |first6=Jan |date=2020-06-23 |title=Inducible aging in Hydra oligactis implicates sexual reproduction, loss of stem cells, and genome maintenance as major pathways |url=https://doi.org/10.1007/s11357-020-00214-z |journal=GeroScience |volume=42 |issue=4 |pages=1119–1132 |doi=10.1007/s11357-020-00214-z |issn=2509-2715|pmc=7394996 }}</ref> A section of the body wall and an extension of the digestive cavity develop, creating a bud.<ref name = mc/> The buds grow into miniature adults and break away when mature. When a ''Hydra'' is well fed, a new bud can form every two days.<ref>{{cite book |vauthors=Patton WK |chapter=Hydra (coelenterate) |title=Grolier Multimedia Encyclopedia. |publisher=Grolier Online |date=August 2014}}</ref>

When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction then occurs in some ''Hydra''.<ref name=":6" /> Either the ovaries or testes develop from interstitial cells of the epidermis, resulting in swellings in the body wall.<ref>{{Cite web |last=Neupane |first=Laxmi |date=2023-08-03 |title=Hydra: Reproduction (Budding and Sexual), Regeneration, Immortality |url=https://microbenotes.com/hydra-reproduction-regeneration/ |access-date=2025-10-04 |website=microbenotes.com |language=en-US}}</ref> 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''. The male ''Hydra'' is typically smaller in size and bears 1 to 8 conical testes, while the female is larger and has 1 to 2 ovaries. Some ''Hydra'' species, like ''Hydra circumcincta'' and ''Hydra viridissima'', are hermaphrodites and may produce both testes and ovaries at the same time.<ref>{{cite book |vauthors=Holstein T, Emschermann P |date=1995 |title=Cnidaria: Hydrozoa Süsswasserfauna von Mitteleuropa. Bd 1/2+ 3 |location=Stuttgart |publisher=Spektrum Akademischer Verlag |isbn=978-3-8274-0836-5}}</ref>

==Feeding== The mouth of the ''Hydra'' is surrounded by four to eight tentacles.<ref name="mc" /> When feeding, ''Hydra'' extend their body to maximum length and then slowly extend their tentacles. The tentacles of ''Hydra'' are extensible and can be four to five times the length of the body. To search for prey, ''Hydra'' extends its tentacles and slowly maneuvers them, waiting for contact. Upon contact with a prey, the stenoteles discharges neurotoxins; the desmonemes on the tentacle (nematocysts) also discharge threads that coil around the prey. Most of the tentacles join in the attack within 30 seconds. Within two minutes, the tentacles move the prey into the open mouth aperture. Within ten minutes, the prey is engulfed and digestion commences. A ''Hydra'', with a column length of approximately 3–30 mm when extended and a width of about 1 mm, can stretch its body wall to digest prey more than twice its size. After 2–3 days, the indigestible remains will be discharged through the mouth aperture via contractions.<ref>{{Cite web |date=2018 |title=Phylum: Cnidaria (Coelenterata) |url=https://uomustansiriyah.edu.iq/media/lectures/6/6_2018_11_16!01_50_11_PM.pdf |at=Feeding of Hydra}}</ref><ref name=":3" />[[File:Hydra Budding.svg|thumb|left|upright=1.35|''Hydra''<nowiki/>'s budding: {{olist |Non-reproducing |Creating a bud |Daughter growing out |Beginning to cleave |Daughter broken off |Daughter clone of parent}}]]The ''Hydra''<nowiki/>'s mouth is not permanent: When the ''Hydra'' closes its mouth, the cells surrounding the open mouth fuse together. These joints are then broken when the ''Hydra'' feeds again.<ref name="mc" />

===Measuring the feeding response===

thumb|Reduction 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.<ref>{{cite journal | vauthors = Loomis WF | title = Glutathione control of the specific feeding reactions of hydra. | journal = Annals of the New York Academy of Sciences | date = October 1955 | volume = 62 | issue = 9 | pages = 211–27 | doi = 10.1111/j.1749-6632.1955.tb35372.x | bibcode = 1955NYASA..62..211L | s2cid = 85570550 }}</ref> There are several methods conventionally used for quantification of the feeding response. In some, the duration for which the mouth remains open is measured.<ref name="pmid7957948">{{cite journal | vauthors = Bellis SL, Laux DC, Rhoads DE | title = Affinity purification of Hydra glutathione binding proteins | journal = FEBS Letters | volume = 354 | issue = 3 | pages = 320–4 | date = November 1994 | pmid = 7957948 | doi = 10.1016/0014-5793(94)01154-0| s2cid = 29262166 | doi-access = free | bibcode = 1994FEBSL.354..320B }}</ref> Other methods rely on counting the number of ''Hydra'' among a small population showing the feeding response after addition of glutathione.<ref name="pmid2888575">{{cite journal | vauthors = Venturini G | title = The hydra GSH receptor. Pharmacological and radioligand binding studies | journal = Comparative Biochemistry and Physiology. C, Comparative Pharmacology and Toxicology | volume = 87 | issue = 2 | pages = 321–4 |year = 1987 | pmid = 2888575 | doi = 10.1016/0742-8413(87)90015-6}}</ref> Recently, an assay for measuring the feeding response in ''Hydra'' has been developed.<ref name="kulk">{{cite journal | vauthors = Kulkarni R, Galande S | title = Measuring glutathione-induced feeding response in hydra | journal = Journal of Visualized Experiments | issue = 93 | article-number = e52178 | date = November 2014 | pmid = 25490534 | pmc = 4354099 | doi = 10.3791/52178 }}</ref> 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.<ref name="kulk" /> {{clear}}

== Ecology == === Food sources === ''Hydra'', as a carnivorous cnidarian, mainly feeds on small aquatic invertebrates. ''Hydra'' can also eat worms, young insects, larval mollusks, bluegill larvae,<ref>{{Cite journal |last1=Elliott |first1=J. K. |last2=Elliott |first2=J. M. |last3=Leggitt |first3=W. C. |date=1997 |title=Predation by ''Hydra'' on larval fish: Field and laboratory experiments with bluegill (''Lepomis macrochirus'') |url=https://doi.org/10.4319/lo.1997.42.6.1416 |journal=Limnology and Oceanography |volume=42 |issue=6 |pages=1416–1423 |doi=10.4319/lo.1997.42.6.1416 |doi-access=free|bibcode=1997LimOc..42.1416E |issn=0024-3590}}</ref> tiny crustaceans (e.g., ''Daphnia'', ''Cyclops'', ''ostracods'',<ref>{{Cite web |last=Garnelio |title=Hydra or freshwater polyps - small cnidarians in the aquarium |url=https://www.garnelio.de/en/blog/aquarist-tips/hydra-or-freshwater-polyps-small-cnidarians-in-the-aquarium |access-date=2025-10-04 |website=Garnelio EN |language=en-GB}}</ref> cladocerans, and copepods), and algaes (e.g., ''Cocconeis placentula'', ''Cyclotella meneghineana'', and ''Navicula zanoni'').<ref>{{Cite web |title=Hydras |url=https://mdc.mo.gov/discover-nature/field-guide/hydras |website=Missouri Department of Conservation |at=Food}}</ref><ref>{{Cite journal |last1=Deserti |first1=María I. |last2=Esquius |first2=Karina S. |last3=Escalante |first3=Alicia H. |last4=Acuña |first4=Fabián H. |date=2017 |title=Trophic ecology and diet of Hydra vulgaris (Cnidaria; Hydrozoa) |url=https://doi.org/10.1163/15707563-00002537 |journal=Animal Biology |volume=67 |issue=3–4 |pages=287–300 |doi=10.1163/15707563-00002537 |issn=1570-7555|hdl=11336/56879 |hdl-access=free }}</ref>

Some species of ''Hydra'' exist in a mutual relationship with various types of unicellular algae. The algae are protected from predators by ''Hydra''; in return, photosynthetic products from the algae are beneficial as a food source to ''Hydra''<ref>{{Cite journal |last1=Thorington |first1=Glyne |last2=Margulis |first2=Lynn |date=1981 |title=Hydra viridis; transfer of metabolites between Hydra and symbiotic algae |url=https://www.journals.uchicago.edu/doi/10.2307/1540911 |journal=The Biological Bulletin |language=en |volume=160 |issue=1 |pages=175–188 |doi=10.2307/1540911 |issn=0006-3185 |jstor=1540911 |pmid=6164406 |s2cid=21008864 |url-access=subscription}}</ref><ref>{{Cite journal |last1=Muscatine |first1=Leonard |last2=Lenhoff |first2=Howard M. |date=1963-11-15 |title=Symbiosis: On the Role of Algae Symbiotic with Hydra |url=https://www.science.org/doi/10.1126/science.142.3594.956 |journal=Science |language=en |volume=142 |issue=3594 |pages=956–958 |bibcode=1963Sci...142..956M |doi=10.1126/science.142.3594.956 |issn=0036-8075 |pmid=17753799 |s2cid=28578967 |url-access=subscription}}</ref> and even help to maintain the ''Hydra'' microbiome.<ref>{{Cite journal |last1=Bathia |first1=Jay |last2=Schröder |first2=Katja |last3=Fraune |first3=Sebastian |last4=Lachnit |first4=Tim |last5=Rosenstiel |first5=Philip |last6=Bosch |first6=Thomas C. G. |date=6 June 2022 |title=Symbiotic Algae of Hydra viridissima Play a Key Role in Maintaining Homeostatic Bacterial Colonization |journal=Front. Microbiol. |volume=13 |doi=10.3389/fmicb.2022.869666 |pmc=9207534 |pmid=35733963 |doi-access=free |article-number=869666}}</ref> There is an entire clade of "green hydra" where the species can stably maintain a population of ''Chlorella'' internally, with the main representative being ''Hydra viridissima''. There are at least 4 species in this clade.<ref>{{cite journal |last1=Wang |first1=An-Tai |last2=Deng |first2=Li |last3=Lai |first3=Jing-Qi |last4=Li |first4=Juan |title=A New Species of Green Hydra (Hydrozoa: Hydrida) from China |journal=Zoological Science |date=September 2009 |volume=26 |issue=9 |pages=664–668 |doi=10.2108/zsj.26.664}}</ref> The other species are called "brown hydra". There exists an immature symbiotic relationship between some strains of ''Hydra vulgaris'' and ''Chlorococcum''.<ref>{{cite journal |last1=Miyokawa |first1=Ryo |last2=Kanaya |first2=Hiroyuki J. |last3=Itoh |first3=Taichi Q. |last4=Kobayakawa |first4=Yoshitaka |last5=Kusumi |first5=Junko |title=Immature symbiotic system between horizontally transmitted green algae and brown hydra |journal=Scientific Reports |date=3 February 2021 |volume=11 |issue=1 |doi=10.1038/s41598-021-82489-6|pmc=7859245 }}</ref>

=== Predators === The species ''Hydra oligactis'' is preyed upon by the flatworm ''Microstomum lineare''.<ref>{{Cite journal |last=Krohne |first=Georg |date=2018 |title=Organelle survival in a foreign organism: Hydra nematocysts in the flatworm Microstomum lineare |journal=European Journal of Cell Biology |volume=97 |issue=4 |pages=289–299 |doi=10.1016/j.ejcb.2018.04.002 |issn=1618-1298 |pmid=29661512}}</ref><ref>{{Cite journal |last=Krohne |first=Georg |date=2020 |title=Hydra nematocysts in the flatworm Microstomum lineare: in search for alterations preceding their disappearance from the new host |journal=Cell and Tissue Research |volume=379 |issue=1 |pages=63–71 |doi=10.1007/s00441-019-03149-w |issn=1432-0878 |pmid=31848750 |s2cid=209380951}}</ref> Some ''Coleps sp.'' have also been observed to attack ''Hydra'' polyps in groups, with them attacking ''Hydras''<nowiki/>' tentacles first before consuming the entire polyps.<ref>{{Cite journal |last1=Deserti |first1=María I. |last2=Monti Areco |first2=Florencia |last3=Acuña |first3=Fabián H. |last4=Stampar |first4=Sergio N. |date=2024-12-04 |title=Predation by Coleps sp. (Ciliophora, Prostomatea) on polyps of Hydra (Cnidaria, Hydrozoa) |url=https://www.limnetica.net/es/predation-coleps-sp-ciliophora-prostomatea-polyps-hydra-cnidaria-hydrozoa |journal=Limnetica |volume=44 |issue=2 |pages=1 |doi=10.23818/limn.44.17 |issn=0213-8409|doi-access=free }}</ref> Some other common predators include carnivorous or omnivorous fishes such as guppies, bettas, and gouramis.<ref>{{Cite web |title=How to Get Rid of Hydra from Your Freshwater Fish Tanks |url=https://www.aquariumcoop.com/blogs/aquarium/hydra |access-date=2025-10-04 |website=Aquarium Co-Op |language=en}}</ref>

==Tissue regeneration==

''Hydras'' undergo morphallaxis (tissue regeneration) when injured or severed. Typically, ''Hydras'' reproduce by just budding off a whole new individual; the bud occurs around two-thirds of the way down the body axis. When a ''Hydra'' is cut in half, each half regenerates and forms into a small ''Hydra''; the "head" regenerates a "foot" and the "foot" regenerates a "head". This regeneration occurs without cell division. If the ''Hydra'' is sliced into many segments, the middle slices form both a "head" and a "foot".<ref name=":0" /> 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.<ref name=":1">{{cite journal | vauthors = Fujisawa T | title = Hydra regeneration and epitheliopeptides | journal = Developmental Dynamics | volume = 226 | issue = 2 | pages = 182–9 | date = February 2003 | pmid = 12557197 | doi = 10.1002/dvdy.10221 | s2cid = 26953455 | doi-access = free }}</ref> 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 where the bud forms is where the gradients are low for both the head and foot.<ref name=":0" />

''Hydras'' are capable of regenerating from pieces of tissue from the body and additionally after tissue dissociation from reaggregates.<ref name=":1" /> This process takes place not only in the pieces of tissue excised from the body column, but also from re-aggregates of dissociated single cells. It was found that in these aggregates, cells initially distributed randomly undergo sorting and form two epithelial cell layers, in which the endodermal epithelial cells play more active roles in the process. Active mobility of these endodermal epithelial cells forms two layers in both the re-aggregate and the re-generating tip of the excised tissue. As these two layers are established, a patterning process takes place to form heads and feet.<ref name="Fujisawa-2003">{{cite journal | last=Fujisawa | first=Toshitaka | title=Hydra regeneration and epitheliopeptides | journal=Developmental Dynamics | publisher=American Association for Anatomy (Wiley) | volume=226 | issue=2 | date=2003-01-29 | issn=1058-8388 | doi=10.1002/dvdy.10221 | pages=182–189| pmid=12557197 | s2cid=26953455 | doi-access=free }}</ref>

==Non-senescence== Daniel Martinez claimed in an article in ''Experimental Gerontology'' in 1998 that ''Hydra'' are biologically immortal.<ref>{{cite journal | vauthors = Martínez DE | title = Mortality patterns suggest lack of senescence in hydra | journal = Experimental Gerontology | volume = 33 | issue = 3 | pages = 217–25 | date = May 1998 | pmid = 9615920 | doi = 10.1016/S0531-5565(97)00113-7 | s2cid = 2009972 }}</ref> 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 refutes the hypothesis that ''Hydra'' do not senesce.<ref>{{cite journal | vauthors = Estep PW | title = Declining asexual reproduction is suggestive of senescence in hydra: comment on Martinez, D., "Mortality patterns suggest lack of senescence in hydra." Exp Gerontol 33, 217–25 | journal = Experimental Gerontology | volume = 45 | issue = 9 | pages = 645–6 | date = September 2010 | pmid = 20398746 | doi = 10.1016/j.exger.2010.03.017 | s2cid = 35408542 }}</ref>

The controversial unlimited lifespan of ''Hydra'' has attracted much attention from scientists. Research today appears to confirm Martinez's study.<ref name="B–K">{{cite journal | vauthors = Boehm AM, Khalturin K, Anton-Erxleben F, Hemmrich G, Klostermeier UC, Lopez-Quintero JA, Oberg HH, Puchert M, Rosenstiel P, Wittlieb J, Bosch TC | title = FoxO is a critical regulator of stem cell maintenance in immortal Hydra | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 48 | pages = 19697–702 | date = November 2012 | pmid = 23150562 | pmc = 3511741 | doi = 10.1073/pnas.1209714109 | bibcode = 2012PNAS..10919697B | doi-access = free }}</ref> ''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''.<ref name="B–K" /> In experiments, a drastically reduced population growth resulted from FoxO downregulation.<ref name="B–K" />

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.<ref name=":2" />

==DNA repair==

''Hydras'' are capable of two types of DNA repair: nucleotide excision repair and base excision repair.<ref name="Barve2021">{{Cite journal |doi=10.3389/fgene.2021.670695 |doi-access=free |pmc=8117345 |pmid=33995496|title=DNA Repair Repertoire of the Enigmatic Hydra |year=2021 |last1=Barve |first1=Apurva |last2=Galande |first2=Alisha A. |last3=Ghaskadbi |first3=Saroj S. |last4=Ghaskadbi |first4=Surendra |journal=Frontiers in Genetics |volume=12 |article-number=670695 }}</ref> The repair pathways facilitate DNA replication by removing DNA damage. Their identification in ''Hydra'' was based, in part, on the presence in its genome of genes homologous to ones present in other genetically well studied species playing key roles in these DNA repair pathways.<ref name = Barve2021/>

==Genomics== ''Hydra'' has more than 20,000 genes, along with a set of six actinoporin-like toxin genes found in its nematocysts.<ref>{{Cite journal |last1=Chapman |first1=Jarrod A. |last2=Kirkness |first2=Ewen F. |last3=Simakov |first3=Oleg |last4=Hampson |first4=Steven E. |last5=Mitros |first5=Therese |last6=Weinmaier |first6=Thomas |last7=Rattei |first7=Thomas |last8=Balasubramanian |first8=Prakash G. |last9=Borman |first9=Jon |last10=Busam |first10=Dana |last11=Disbennett |first11=Kathryn |last12=Pfannkoch |first12=Cynthia |last13=Sumin |first13=Nadezhda |last14=Sutton |first14=Granger G. |last15=Viswanathan |first15=Lakshmi Devi |date=2010 |title=The dynamic genome of Hydra |journal=Nature |language=en |volume=464 |issue=7288 |pages=592–596 |doi=10.1038/nature08830 |pmid=20228792 |issn=1476-4687|pmc=4479502 |bibcode=2010Natur.464..592C }}</ref> An ortholog comparison analysis done in 2013 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.<ref name=":2" /> Transgenic hydra have become attractive model organisms to study the evolution of immunity.<ref>{{cite web|title= Transgenic Hydra Facility, University of Kiel (Germany)| url=http://transgenic-hydra.org/lines.htm/ }}</ref> A draft of the genome of ''Hydra magnipapillata'' was reported in 2010.<ref>{{cite journal | vauthors = Chapman JA, Kirkness EF, Simakov O, Hampson SE, Mitros T, Weinmaier T, etal | title = The dynamic genome of Hydra | journal = Nature | volume = 464 | issue = 7288 | pages = 592–6 | date = March 2010 | pmid = 20228792 | pmc = 4479502 | doi = 10.1038/nature08830 | bibcode = 2010Natur.464..592C }}</ref>

The genus ''Hydra'' is divided into two clades, the green hydras (e.g. ''H. viridissima'') and the brown hydras (e.g. ''H. oligactis'', ''H. vulgaris''). The genomes of cnidarians are usually less than 500 Mb (megabases) in size, as in the green hydras, 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.<ref>{{cite journal |last1=Wong |first1=WY |last2=Simakov |first2=O |last3=Bridge |first3=DM |last4=Cartwright |first4=P |last5=Bellantuono |first5=AJ |last6=Kuhn |first6=A |last7=Holstein |first7=TW |last8=David |first8=CN |last9=Steele |first9=RE |last10=Martínez |first10=DE |title=Expansion of a single transposable element family is associated with genome-size increase and radiation in the genus Hydra |journal=Proc Natl Acad Sci U S A |year=2019 |volume=116 |issue=46 |pages=22915–22917 |doi=10.1073/pnas.1910106116 |pmid=31659034|pmc=6859323 |bibcode=2019PNAS..11622915W |doi-access=free }}</ref>

Due to the simplicity of their life cycle when compared to other hydrozoans, ''Hydras'' have lost many genes that correspond to cell types or metabolic pathways of which the ancestral function is still unknown. The ancestral Toll/TLR pathway, for example, is present in ''Anthozoa'', but the key upstream receptor component of it is missing or has diverged in ''Hydra.''<ref name=":7">{{Cite journal |last1=Miller |first1=David J. |last2=Hemmrich |first2=Georg |last3=Ball |first3=Eldon E. |last4=Hayward |first4=David C. |last5=Khalturin |first5=Konstantin |last6=Funayama |first6=Noriko |last7=Agata |first7=Kiyokazu |last8=Bosch |first8=Thomas C. G. |date=2007 |title=The innate immune repertoire in cnidaria--ancestral complexity and stochastic gene loss |journal=Genome Biology |volume=8 |issue=4 |pages=R59 |doi=10.1186/gb-2007-8-4-r59 |issn=1474-760X |pmc=1896004 |pmid=17437634 |doi-access=free}}</ref> The genus ''Hydra'' is missing some genes associated with larvae development, fluorescent proteins, and circadian rhythms that are normally found in the sea anemone genome.<ref>{{Cite web |title=Hydra Genome Sequenced by J. Craig Venter Institute and Multi-National Research Team |url=https://www.jcvi.org/media-center/hydra-genome-sequenced-j-craig-venter-institute-and-multi-national-research-team |access-date=2025-10-04 |website=J. Craig Venter Institute |language=en}}</ref> The genes ''eve'' and ''emx'' are absent in ''Hydra'', even though they are present in ''Nematostella'' and hydrozoans. These genes are expressed during the larvae development.<ref>{{Cite journal |last1=Chapman |first1=Jarrod A. |last2=Kirkness |first2=Ewen F. |last3=Simakov |first3=Oleg |last4=Hampson |first4=Steven E. |last5=Mitros |first5=Therese |last6=Weinmaier |first6=Thomas |last7=Rattei |first7=Thomas |last8=Balasubramanian |first8=Prakash G. |last9=Borman |first9=Jon |last10=Busam |first10=Dana |last11=Disbennett |first11=Kathryn |last12=Pfannkoch |first12=Cynthia |last13=Sumin |first13=Nadezhda |last14=Sutton |first14=Granger G. |last15=Viswanathan |first15=Lakshmi Devi |date=2010-03-25 |title=The dynamic genome of Hydra |journal=Nature |volume=464 |issue=7288 |pages=592–596 |doi=10.1038/nature08830 |issn=1476-4687 |pmc=4479502 |pmid=20228792 |bibcode=2010Natur.464..592C }}</ref> These losses in gene expression are thought to be signs of a substantial secondary gene loss during evolution.<ref name=":7" />

''Hydra'' genome shows a preference towards proximal promoters. Thanks to this feature, many reporter cell lines have been created with regions around 500 to 2000 bases upstream of the gene of interest. Its cis-regulatory elements (CRE) are mostly located less than 2000 base pairs upstream from the closest transcription initiation site, but there are CREs located further away.

Its chromatin has a Rabl configuration. There are interactions between the centromeres of different chromosomes and the centromeres and telomeres of the same chromosome. It presents a great number of intercentromeric interactions when compared to other cnidarians, probably due to the loss of multiple subunits of condensin II. It is organized in domains that span dozens to hundreds of megabases, containing epigenetically co-regulated genes and flanked by boundaries located within heterochromatin.<ref name="A chromosome-scale epigenetic map o">{{cite journal |last1=J. F. |first1=Cazet |last2=S. |first2=Siebert |last3=H. M. |first3=Little |last4=P. |first4=Bertemes |last5=A. S. |first5=Primack |last6=P. |first6=Ladurner |last7=M. |first7=Achrainer |last8=M. T. |first8=Fredriksen |last9=R. T. |first9=Moreland |last10=S. |first10=Singh |last11=S. |first11=Zhang |last12=T. G. |first12=Wolfsberg |last13=T. G. |first13=Schnitzler |last14=A. D. |first14=Baxevanis |last15=O. |first15=Simakov |last16=B. |first16=Hobmayer |last17=C. E. |first17=Juliano |title=A chromosome-scale epigenetic map of the Hydra genome reveals conserved regulators of cell state. |journal=Genome Research |date=2023 |volume=33 |issue=2 |pages=283–298 |doi=10.1101/gr.277040.122 |pmid=36639202 |pmc=10069465 |doi-access=free }}</ref>

==Transcriptomics== Different ''Hydra'' cell types express gene families of different evolutionary ages. Progenitor cells (stem cells, neuron and nematocyst precursors, and germ cells) express genes from families that predate metazoans. Among differentiated cells some express genes from families that date from the base of metazoans, like gland and neuronal cells, and others express genes from newer families, originating from the base of cnidaria or medusozoa, like nematocysts. Interstitial cells contain translation factors with a function that has been conserved for at least 400 million years.<ref name="A chromosome-scale epigenetic map o"/>

==In popular culture== In the video game ''Terra Invicta'', the aliens attacking the Earth come to be nicknamed "hydras" due to their similarity to the real-world animal. They share a number of common traits, such as tentacles around their mouth, tissue regeneration, and lacking senescence.

== See also == {{Commons category|Hydra}} * Lernaean Hydra, a Greek mythological aquatic creature after which the genus is named * ''Turritopsis dohrnii'', another cnidarian (a jellyfish) that scientists believe to be immortal

== References == {{Reflist|35em}}

{{DEFAULTSORT:Hydra (Genus)}} Category:Hydridae Category:Negligibly senescent organisms Category:Hydrozoan genera