| Hydra | |
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| Hydrabudding | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Cnidaria |
| Class: | Hydrozoa |
| Order: | Anthoathecata |
| Family: | Hydridae Dana, 1846 |
| Genus: | Hydra Linnaeus,1758[1] |
| Species[1] | |
List
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| Synonyms | |
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Hydra (/ˈhaɪdrə/HY-drə) is agenus of smallfreshwaterhydrozoans of thephylumCnidaria. They are solitary, carnivorous jellyfish-like animals,[2] native to thetemperate andtropical regions.[3][4] The genus was named byLinnaeus in 1758 after theHydra, the mythical many-headed beast that was defeated byHeracles, as when the animal has a part severed, it will regenerate much like the mythical Hydra's heads. Biologists are especially interested inHydra because of theirregenerative ability; they do not appear to die of old age, or toage at all.
Hydras are often found infreshwater bodies, but someHydras are found in open water. They live attached to submerged rocks using a sticky secretion from their base.[2] The genusHydra occurs on all continents except forAntarctica and the Oceanic islands, thoughHydras occur mainly in mesotrophic to eutrophic habitats.[5]
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.Hydra has a body wall made up of the ectoderm and the endoderm, which are separated by an extracellular matrix called themesoglea.[6]
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. The nematocysts are housed within a single epithelial cell together with a sensory and nerve cell.[7] 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 tohydrostatic pressure (theosmotic pressure exceeds a critical threshold),[8] firing a dart-like thread containingneurotoxins into whatever triggered the release. This can paralyze the prey, especially if many hundreds of nematocysts are fired.
InHydra, 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.[8]
Hydra has two main body layers, which makes itdiploblastic. 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[9] is abactericide recently discovered inHydra; it protects the outer layer against infection. A singleHydra is composed of 50,000 to 100,000 cells which consist of three specificstem cell populations that create many different cell types. These stem cells continually renew themselves in the body column.[10]Hydras have two significant structures on their body: the "head" and the "foot". When aHydra is cut in half, each half regenerates and forms into a smallHydra; the "head" regenerates a "foot" and the "foot" regenerates a "head". If theHydra is sliced into many segments then the middle slices form both a "head" and a "foot".[11]
Respiration and excretion occur bydiffusion throughout the surface of theepidermis, while larger excreta are discharged through the mouth by a quick radial contraction of the body column.[12][13][14]
The nervous system ofHydra is anerve net, composed of a few hundred to a few thousand neurons, which is structurally simple compared tomore derived animal nervous systems.Hydra does not have a recognizablebrain or truemuscles. Nerve nets connect sensoryphotoreceptors 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 ofneurons.[15]
It also has been described that there are three major networks extending throughoutHydra's entire body. They are activated selectively during longitudinal contractions, elongations in response to light, and radial contractions; the additional network near thehypostome, the dome-shaped jut surrounding the mouth aperture, is activated during nodding (the gentle swaying of the hypostome and its tentacles to one side).[14]
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IfHydra 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 generallysedentary orsessile, 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 thesubstrate with the mouth and tentacles and then relocate the foot, which provides the usual attachment, this process is calledlooping. In somersaulting, the body then bends over and makes a new place of attachment with the foot. By this process of looping or somersaulting, aHydra can move several inches (c. 100 mm) in a day.Hydra may also move byamoeboid motion of their bases or by detaching from the substrate and floating away in the current.
A dark-habituatedHydra 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.[16]
MostHydras canreproduce sexually under certain conditions, though they typically choose toreproduce asexually instead. Unlike many members of theHydrozoa, which alternate between thepolyp form and themedusa form (the life stage where sexual reproduction occurs),Hydra never progress beyond the polyp phase in their life cycle.[17] Instead, when food is plentiful, manyHydra opt to reproduce asexually bybudding.[18] A section of the body wall and an extension of the digestive cavity develop, creating a bud.[2] The buds grow into miniature adults and break away when mature. When aHydra is well fed, a new bud can form every two days.[19]
When conditions are harsh, often before winter or in poor feeding conditions, sexual reproduction then occurs in someHydra.[18] Either the ovaries or testes develop from interstitial cells of the epidermis, resulting in swellings in the body wall.[20] The testes release free-swimminggametes 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 nymphHydra. The maleHydra is typically smaller in size and bears 1 to 8 conical testes, while the female is larger and has 1 to 2 ovaries. SomeHydra species, likeHydra circumcincta andHydra viridissima, arehermaphrodites and may produce both testes and ovaries at the same time.[21]
The mouth of theHydra is surrounded by four to eight tentacles.[2] When feeding,Hydra extend their body to maximum length and then slowly extend their tentacles. The tentacles ofHydra 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. AHydra, 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.[22][7]
TheHydra's mouth is not permanent: When theHydra closes its mouth, the cells surrounding the open mouth fuse together. These joints are then broken when theHydra feeds again.[2]
The feeding response inHydra is induced byglutathione (specifically in the reduced state as GSH) released from damaged tissue of injured prey.[23] There are several methods conventionally used for quantification of the feeding response. In some, the duration for which the mouth remains open is measured.[24] Other methods rely on counting the number ofHydra among a small population showing the feeding response after addition of glutathione.[25] Recently, an assay for measuring the feeding response inHydra has been developed.[26] In this method, the linear two-dimensional distance between the tip of the tentacle and the mouth ofHydra 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 theHydra feeding response.[26]
Hydra, as a carnivorous cnidarian, mainly feeds on small aquatic invertebrates.Hydra can also eat worms, young insects, larval mollusks, bluegill larvae,[27] tiny crustaceans (e.g.,Daphnia,Cyclops,ostracods,[28]cladocerans, andcopepods), and algaes (e.g.,Cocconeis placentula,Cyclotella meneghineana, andNavicula zanoni).[29][30] Though some species ofHydra exist in amutual relationship with various types of unicellularalgae. The algae are protected from predators byHydra; in return,photosynthetic products from the algae are beneficial as a food source toHydra[31][32] and even help to maintain theHydra microbiome.[33] One example is theHydra viridissima (green hydra) having a symbiotic relationship with thegreen algae of the genusChlorella.
The speciesHydra oligactis is preyed upon by theflatwormMicrostomum lineare.[34][35] SomeColeps sp. have also been observed to attackHydra polyps in groups, with them attackingHydras' tentacles first before consuming the entire polyps.[36] Some other common predators include carnivorous or omnivorous fishes such asguppies,bettas, andgouramis.[37]
Hydras undergomorphallaxis (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 aHydra is 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 theHydra is sliced into many segments, the middle slices form both a "head" and a "foot".[11] 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.[38] 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.[11]
Hydras are capable of regenerating from pieces of tissue from the body and additionally after tissue dissociation from reaggregates.[38] 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.[39]
Daniel Martinez claimed in an article inExperimental Gerontology in 1998 thatHydra arebiologically immortal.[40] This publication has been widely cited as evidence thatHydra do notsenesce (do not age), and that they are proof of the existence of non-senescing organisms generally. In 2010,Preston Estep published (also inExperimental Gerontology) a letter to the editor arguing that the Martinez data refutes the hypothesis thatHydra do not senesce.[41]
The controversial unlimited lifespan ofHydra has attracted much attention from scientists. Research today appears to confirm Martinez's study.[42]Hydra stem 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.[42] In experiments, a drastically reduced population growth resulted from FoxOdownregulation.[42]
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 flies andnematodes, 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.[10]
Hydras are capable of two types ofDNA repair:nucleotide excision repair andbase excision repair.[43] The repair pathways facilitate DNA replication by removing DNA damage. Their identification inHydra was based, in part, on the presence in itsgenome of genes homologous to ones present in other genetically well studied species playing key roles in these DNA repair pathways.[43]
Hydra has more than 20,000genes, along with a set of sixactinoporin-like toxin genes found in its nematocysts.[44] Anortholog comparison analysis done in 2013 demonstrated thatHydra share a minimum of 6,071 genes with humans.Hydra is becoming an increasingly better model system as more genetic approaches become available.[10]Transgenic hydra have become attractive model organisms to study theevolution ofimmunity.[45] A draft of thegenome ofHydra magnipapillata wasreported in 2010.[46]
The genomes ofcnidarians are usually less than 500 Mb (megabases) in size, as in theHydra viridissima (green hydras), which has a genome size of approximately 300 Mb. In contrast, the genomes ofbrown hydras are 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 genusHydra and is absent in the green hydra, which has a repeating landscape similar to other cnidarians. These genome characteristics makeHydra attractive for studies of transposon-driven speciations and genome expansions.[47]
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 inAnthozoa, but the key upstream receptor component of it is missing or has diverged inHydra.[48] The genusHydra is missing some genes associated with larvae development,fluorescent proteins, and circadian rhythms that are normally found in the sea anemone genome.[49] The geneseve andemx are absent inHydra, even though they are present inNematostella and hydrozoans. These genes are expressed during the larvae development.[50] These losses in gene expression are thought to be signs of a substantial secondary gene loss during evolution.[48]
Hydra genome shows a preference towards proximalpromoters. Thanks to this feature, manyreporter cell lines have been created with regions around 500 to 2000 bases upstream of the gene of interest. Itscis-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 thecentromeres of different chromosomes and the centromeres andtelomeres 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 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.[51]
DifferentHydra 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 ofcnidaria ormedusozoa, like nematocysts. Interstitial cells contain translation factors with a function that has been conserved for at least 400 million years.[51]
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