Hydra(/ˈhaɪdrə/HY-drə) is agenusof smallfreshwaterhydrozoansof thephylumCnidaria.They are native to thetemperateandtropicalregions.[2][3]The genus was named byLinnaeusin 1758 after theHydra,which was the many-headed beast of myth defeated byHeracles,as when theanimalhas a part severed, it will regenerate much like the mythical hydra's heads. Biologists are especially interested inHydrabecause of theirregenerative ability;they do not appear to die of old age, or toageat all.
| Hydra | |
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| Hydrabudding | |
Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Class: | Hydrozoa |
| Order: | Anthoathecata |
| Family: | Hydridae Dana, 1846 |
| Genus: | Hydra Linnaeus,1758[1] |
| Species[1] | |
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List
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Morphology
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Hydrahas a tubular,radially symmetricbody 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, mobiletentacles.Each tentacle, or cnida (plural: cnidae), is clothed with highly specialised stinging cells calledcnidocytes.Cnidocytes contain specialized structures callednematocysts,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 containingneurotoxinsinto whatever triggered the release. This can paralyze the prey, especially if many hundreds of nematocysts are fired.
Hydrahas two main body layers, which makes it "diploblastic".The layers are separated bymesoglea,a gel-like substance. The outer layer is theepidermis,and the inner layer is called thegastrodermis,because it lines the stomach. The cells making up these two body layers are relatively simple.Hydramacin[4]is abactericiderecently discovered inHydra;it protects the outer layer against infection. A singleHydrais composed of 50,000 to 100,000 cells which consist of three specificstem cellpopulations that create many different cell types. These stem cells continually renew themselves in the body column.[5]Hydrashave two significant structures on their body: the "head" and the "foot". When aHydrais cut in half, each half regenerates and forms into a smallHydra;the "head" regenerates a "foot" and the "foot" regenerates a "head". If theHydrais sliced into many segments then the middle slices form both a "head" and a "foot".[6]
Respiration and excretion occur bydiffusionthroughout the surface of theepidermis,while larger excreta are discharged through the mouth.[7][8]
Nervous system
editThe nervous system ofHydrais anerve net,which is structurally simple compared tomore derivedanimal nervous systems.Hydradoes not have a recognizablebrainor truemuscles.Nerve nets connect sensoryphotoreceptorsand 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 synapsed to epithelial or motor cells.
Motion and locomotion
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IfHydraare alarmed or attacked, the tentacles can be retracted to small buds, and the body column itself can be retracted to a small gelatinous sphere.Hydragenerally react in the same way regardless of the direction of the stimulus, and this may be due to the simplicity of the nerve nets.
Hydraare generallysedentaryorsessile,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 thesubstratewith 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", aHydracan move several inches (c. 100 mm) in a day.Hydramay also move byamoeboid motionof their bases or by detaching from the substrate and floating away in the current.
Reproduction and life cycle
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- Non-reproducing
- Creating a bud
- Daughter growing out
- Beginning to cleave
- Daughter broken off
- Daughtercloneof parent
Most hydra species do not have any gender system. Instead, when food is plentiful, manyHydrareproduce asexuallybybudding.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 reproductionoccurs in someHydra.Swellings in the body wall develop into either ovaries or testes. The testes release free-swimminggametesinto 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 nymphHydra.SomeHydraspecies, likeHydra circumcinctaandHydra viridissima,arehermaphrodites[11]and may produce both testes and ovaries at the same time.
Many members of theHydrozoago through a body change from apolypto an adult form called amedusa,which is usually the life stage where sexual reproduction occurs, butHydrado not progress beyond the polyp phase.[12]
Feeding
editHydramainly feed on aquatic invertebrates such asDaphniaandCyclops.
While feeding,Hydraextend their body to maximum length and then slowly extend their tentacles. Despite their simple construction, the tentacles ofHydraare 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. Most of the tentacles join in the attack within 30 seconds to subdue the struggling prey. Within two minutes, the tentacles surround the prey and move it into the open mouth aperture. Within ten minutes, the prey is engulfed within the body cavity, and digestion commences.Hydracan stretch their body wall considerably.[citation needed]
The feeding behaviour ofHydrademonstrates the sophistication of what appears to be a simple nervous system.
Some species ofHydraexist in amutual relationshipwith various types of unicellularalgae.The algae are protected from predators byHydra;in return,photosyntheticproducts from the algae are beneficial as a food source toHydra[13][14],and even help to maintain theHydramicrobiome.[15]
Measuring the feeding response
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The feeding response inHydrais induced byglutathione(specifically in the reduced state as GSH) released from damaged tissue of injured prey.[16]There are several methods conventionally used for quantification of the feeding response. In some, the duration for which the mouth remains open is measured.[17]Other methods rely on counting the number ofHydraamong a small population showing the feeding response after addition of glutathione.[18]Recently, an assay for measuring the feeding response in hydra has been developed.[19]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 theHydrafeeding response.[19]
Predators
editThe speciesHydra oligactisis preyed upon by theflatwormMicrostomum lineare.[20][21]
Tissue regeneration
editHydrasundergomorphallaxis(tissue regeneration) when injured or severed. Typically,Hydrasreproduce by just budding off a whole new individual; the bud occurs around two-thirds of the way down the body axis. When aHydrais cut in half, each half regenerates and forms into a smallHydra;the "head" regenerates a "foot" and the "foot" regenerates a "head". This regeneration occurs without cell division. If theHydrais sliced into many segments, the middle slices 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.[22]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.[6]
Hydrasare capable of regenerating from pieces of tissue from the body and additionally after tissue dissociation from reaggregates.[22]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.[23]
Non-senescence
editDaniel Martinez claimed in a 1998 article inExperimental GerontologythatHydraarebiologically immortal.[24]This publication has been widely cited as evidence thatHydrado notsenesce(do not age), and that they are proof of the existence of non-senescing organisms generally. In 2010,Preston Esteppublished (also inExperimental Gerontology) a letter to the editor arguing that the Martinez data refutes the hypothesis thatHydrado not senesce.[25]
The controversial unlimited lifespan ofHydrahas attracted much attention from scientists. Research today appears to confirm Martinez' study.[26]Hydrastem cells have a capacity for indefinite self-renewal. Thetranscription factor"forkhead box O"(FoxO) has been identified as a critical driver of the continuous self-renewal ofHydra.[26]In experiments, a drastically reduced population growth resulted from FoxOdown-regulation.[26]
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 asfruit fliesandnematodes,their lifespan is significantly decreased. In experiments onH. vulgaris(a radially symmetrical member of phylumCnidaria), when FoxO levels were decreased, there was a negative effect on many key features of theHydra,but no death was observed, thus it is believed other factors may contribute to the apparent lack of aging in these creatures.[5]
DNA repair
editHydra are capable of two types ofDNA repair:nucleotide excision repairandbase excision repair.[27]The repair pathways facilitate DNA replication by removing DNA damage. Their identification in hydra was based, in part, on the presence in itsgenomeof genes homologous to ones present in other genetically well studied species playing key roles in these DNA repair pathways.[27]
Genomics
editAnorthologcomparison analysis done within the last decade[as of?]demonstrated thatHydrashare a minimum of 6,071geneswith humans.Hydrais becoming an increasingly better model system as more genetic approaches become available.[5]Transgenic hydrahave become attractive model organisms to study theevolutionofimmunity.[28]A draft of thegenomeofHydra magnipapillatawasreported in 2010.[29]
The genomes ofcnidariansare usually less than 500 Mb (megabases) in size, as in theHydra viridissima,which has a genome size of approximately 300 Mb. In contrast, the genomes ofbrown hydrasare approximately 1 Gb in size. This is because the brown hydra genome is the result of an expansion event involvingLINEs,a type oftransposable elements,in particular, a single family of the CR1 class. This expansion is unique to this subgroup of the genusHydraand is absent in the green hydra, which has a repeating landscape similar to other cnidarians. These genome characteristics makeHydraattractive for studies of transposon-driven speciations and genome expansions.[30]
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.
Hydra genome shows a preference towards proximalpromoters.Thanks to this feature, manyreporter cell lineshave 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 thecentromeresof different chromosomes and the centromeres andtelomeresof the same chromosome. It presents a great number of intercentromeric interactions when compared to other cnidarians, probably due to the loss of multiple subunits ofcondensin II.It is organized in domains that span dozens to hundreds of megabases, containing epigenetically co-regulated genes and flanked by boundaries located withinheterochromatin.[31]
Transcriptomics
editDifferent 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 predatemetazoans.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 ofcnidariaormedusozoa,like nematocysts. Interstitial cells contain translation factors with a function that has been conserved for at least 400 million years.[31]
See also
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- 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
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