Marmokak,[1] Medusozoa azpifilumeko izaki gelatinotsuen medusa faseari ematen zaion ohiko izena da. Azpifilum hau, Cnidaria filumeko klado handia da.[2][3][4][5]

Marmoka
Sailkapen zientifikoa

FilogeniaAldatu

TaxonomiaAldatu

Medusozoa azpifilumaren barnean dibertsitate nahikoa dago.[6] Hiru klasetan sailkatu daiteke.[7]

  • Acraspeda
    • Cubozoa klasea (Kutxa marmokak) - Götte, 1887
    • Syphozoa klasea (Benetako marmokak) - Marques & Collins, 2004
    • Staurozoa klasea (Marmoka zurtoinduak) - Marques & Collins, 2004
  • Polypodiozoa klasea (Parasitikoa) - Raikova, 1994

EboluzioaAldatu

The phylum Cnidaria is widely accepted as being monophyletic and consisting of two clades, Anthozoa and Medusozoa. Anthozoa includes the classes Hexacorallia, the hard corals, and Octocorallia, the soft corals, as well as Ceriantharia, the tube-dwelling anemones. There is strong support for this group having been the first to branch off from the ancestral line.[8]

Medusozoa includes the classes Staurozoa, Cubozoa, Scyphozoa and Hydrozoa, but the relationships between these are unclear. Analysis using ribosomal RNA subunits suggests that within Medusozoa, Staurozoa was the first group to diverge, with Cubozoa and Scyphozoa forming a clade, a sister group to Hydrozoa. Further study involving the order of mitochondrial genes supports this view, and their possession of linear mitochondrial genomes is striking evidence of the monophyly of medusozoans.[9]

The affinities of the class Polypodiozoa, containing the single species Polypodium hydriforme, have long been unclear. This species is an endoparasite of fish eggs and has a peculiar life cycle. It has traditionally been considered to be a cnidarian because of its possession of nematocysts, but molecular studies using 18S rDNA sequences have placed it closer to Myxozoa. Further studies involving 28S rDNA sequences suggest that it is either part of the hydrozoan clade Leptothecata, or a sister taxon to Hydrozoa, and does not group with myxozoans.[10]

Arrasto fosilakAldatu

Since jellyfish have no hard parts, fossils are rare. The oldest conulariid scyphozoans appeared between 635 and 577 mya in the Neoproterozoic of the Lantian Formation in China; others are found in the youngest Ediacaran rocks of the Tamengo Formation of Brazil, c. 505 mya, through to the Triassic. Cubozoans and hydrozoans appeared in the Cambrian of the Marjum Formation in Utah, USA, c. 540 mya.[11]

AnatomiaAldatu

 
Marmoka baten zeharkako sekzioa.

Marmokek morfologia oso ezberdinak dituzte, baina Medusozoa azpifilumeko marmokek simetria tetrameroa (lauko multiplotan ebakia) aurkezten dute.[6] Gorputza onddo baten itxurakoa izaten dute. Txapelaren ertzean 4-8 garro dituzte. Azpiko aldean hodi bat edukitzen dute eta haren muturrean du ahoa. 1-1,8 m zabaleko diametroa izaten dute.

El tejido que forma su cuerpo se denomina mesoglea y, a diferencia de los pólipos, es típicamente muy grueso; suele ser gelatinoso, pero puede alcanzar consistencia cartilaginosa en algunas especies.

La cavidad gastrovascular de estos animales posee un estómago central del que parten bolsas gástricas o diversos canales radiales, que pueden continuarse dentro de los tentáculos; de este modo, los nutrientes pueden distribuirse con mayor facilidad por todo el cuerpo. Gorputzaren %95 ura izaten da.[12]

NematozistoakAldatu

Pozoitsuak dira gehienak, eta oso pozoi bizikoak batzuk.[13]

Bizi-zikloaAldatu

 
Knidarioen bizi-zikloa, medusa fasearekin (marmoka) agerian (12-14).

Medusozoa azpifilumeko partaideen ezaugarri bereizgarria, beste knidarioekiko, marmoka edo medusa fasea biltzen duen bizi-ziklo konplexua da. Bizi zikloan fase sexual (marmoka bezala) eta asexualak (polipo bezala) igarotzen dituzte, denbora gehiena fase sexualean bizitzen dutelarik.[14]

Scyphozoa taldeko planulak polipo sesil bilakatzen dira, asexualki ugaltzen direnak, kimutuz. Jarraian marmoka bilakatu edo estrobilazio deritzon prozesuan (asexualki) ugaltzen dira, haien gorputzak segmentatuz.[14]

Cubozoa taldeko planulak polipo sesil bilakatzen dira (asexualki ugaltzen direnak) eta gero marmoka bilakatzen dira (sexualki ugaltzen direnak).[14]

Staurozoa taldean, marmoka fasea ez da erabatekoa. Organismo helduak sustratuari eutsita mantentzen diren polipo handiak dira, baina partzialki desberdinztatzen dira marmoka helduetan. Ondorioz, sorturiko gametoek planula ez-igerilaritan bilakatzen dira, herresteka mugituz.[14]

Hydrozoa taldeko organismoek bizi-ziklo forma dibertsoenak aurkezten dituzte. Normalean medusak lateralki eratzen edo kimutzen dira polipoetatik. Behin helduak direnean, arrabak eratzen dituzte gametoak uretara askatuz. Ondorioz sorturiko planulak polipo bezala gera daitezke sustratuan edo ur-zutabean mantendu marmoka moduan.[6] Halaber, Hydrozoa talde batzuetan polipo edota marmoka fasea falta dira bizi-zikloan.[14]

EkologiaAldatu

DietaAldatu

Gainontzeko knidarioak bezala Medusozoak haragijale edo parasitikoak dira (heterotrofoak); gehienbat planktonaz[15] (krustazeoak, arrain txikiak, arrain arrautzak eta larbak) eta batzuetan marmokaz elikatzen direlarik.[16] Pasiboki ehizatzen dute elikagaiak eskuratzeko, haien tentakuluak erabiliz sare bezala. Tentakuloetan aurkitzen diren nematozistoek harrapakinak hil edo paralizatzen dituzte, eta behin eskuratuta, tentakuloan flexionatzen dituzte jana ahora eramateko. Gainera, marmoken igeri mugimenduak ehizan laguntzen die, kanpaia zabaltzean ura xurgatu eta harrapakinak 'xurgatuz' tentakuloetarantz.[14][17]

HabitataAldatu

A common Scyphozoan jellyfish seen near beaches in the Florida Panhandle Most jellyfish are marine animals, although a few hydromedusae inhabit freshwater. The best known freshwater example is the cosmopolitan hydrozoan jellyfish, Craspedacusta sowerbii. It is less than an inch (2.5 cm) in diameter, colorless and does not sting. Some jellyfish populations have become restricted to coastal saltwater lakes, such as Jellyfish Lake in Palau. Jellyfish Lake is a marine lake where millions of golden jellyfish (Mastigias spp.) migrate horizontally across the lake daily.

Although most jellyfish live well off the ocean floor and form part of the plankton, a few species are closely associated with the bottom for much of their lives and can be considered benthic. The upside-down jellyfish in the genus Cassiopea typically lie on the bottom of shallow lagoons where they sometimes pulsate gently with their umbrella top facing down. Even some deep-sea species of hydromedusae and scyphomedusae are usually collected on or near the bottom. All of the stauromedusae are found attached to either seaweed or rocky or other firm material on the bottom.

Some species explicitly adapt to tidal flux. In Roscoe Bay, jellyfish ride the current at ebb tide until they hit a gravel bar, and then descend below the current. They remain in still waters until the tide rises, ascending and allowing it to sweep them back into the bay. They also actively avoid fresh water from mountain snowmelt, diving until they find enough salt.

Giza erabileraAldatu

 
Aequorea victoria

BioteknologianAldatu

1961ean, Osamu Shimomurak Proteina Berde Fluoreszentea (Green fluorescent protein, GFP edo avGFP) eta aequorina proteina erauzi zituen Aequorea victoria hidromedusa bioluminiszentetik.[18][19] Naturan, GFP aequorinarekin batera aurkitzen da medusaren txapelaren ertzeko granulo baztuetan. Aequorinak erreakzio bioluminiszentea (argi urdina) eratzen du kaltzio ioiarekin (Ca2+) elkar-eragitean. Ostera, eratzen den argi horrek GFP kitzikatzen du, modu sekundarioan, argia berde bilakatuz.[20][21] A. victoria espezieaz gain, GFP beste organismoetan (koral, anemona, zoantadio eta kopepodo, besteak beste) aurkitu da.[22]

 
Proteina Berde Fluoreszentea (Green fluorescent protein, GFP).

Shimomurak proteina eskuratu eta hiru hamarkadatara, Douglas Prasher-ek GFP-aren genea sekuentziatu eta klonatu zuen.[23] Jarraian, Martin Chalfie-ek GFP gene markatzaile edo 'reporter gene' bezala nola erabili asmatu zuen, beste zelula edo organismoetan adierazi eta erraz detektagarria den fenotipo bat (fluoreszentzia, esaterako) emango duten geneak.[24][25] Roger Tsien-ek later chemically manipulated GFP to produce other fluorescent colors to use as markers. 2008an Shimomura, Chalfie eta Tsien-ek Kimikako Nobel Saria irabazi zuten GFP-arekin egindako ikerkuntzagatik.[26]

Hala, gaur egun ekarpen hauek guztiak bateratu eta GFP ingeniaritza genetikoan erabiltzen da gene markatzaile bezala. Aztertzea interesatzen zaigun genea bektore batean barneratzen da GFP geneari lotuta. Hala, bektore hori zelula ostalari batean barneratzen da eta lerro zelular edo organismo bat eratu. GFP genea intereseko geneari lotuta dagoenez, bata adierazten denean, bestea ere bai. Ondorioz, intereseko proteina adierazten den ehun edo organoetan GFP adieraziko da (fluoreszentzia berde moduan).[27][28]

ErreferentziakAldatu

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