Gonophore

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Gonophore formation in various hydromeduses (illustration in Enzyclopaedia Britannica from 1911, based on drawings by August Weismann )

Gonophores are sedentary and bud-like, more rarely also detaching and mobile, outgrowths formed zooids of the fixed polyp stage of the cnidarians ( Phylum Cnidaria), especially the Hydrozoa . They form the sexually reproducing stage in the regular alternation of generations ( called metagenesis ). Gonophores be interpreted as reducing the form of a jellyfish -Stadiums in the life cycle , that is cnidarians with gonophores derived probably from ancestors who exhibited a Medusenstadium.

education

Gonophores arise as bud-like protuberances in which both epithelia , the inner gastrodermis formed by the endoderm and the epidermis formed by the outer cotyledon , the ectoderm , are involved. The formation begins in a very similar way to the constriction of a young medusa, but comes to a standstill after a more or less short time, so it is incomplete compared to the formation of the medusa. The tip section of the ectoderm thickens and forms an inwardly turned vesicle or nodule ("bell core"). Inside, the gastrodermis constricts four gastric pockets arranged in a cross shape. The further development takes place differently depending on the form or type. Eumedusoids still form a very medusa-like stage with a complete umbrella, often also with a central stomach stalk (manubrium) and radial canals, but always without tentacles, without the typical sensory organs such as statocysts and eyes and without a velum on the inner edge of the screen. Cryptomedusoids and heteromedusoids are even further reduced, they lack radial channels, the screen is often incomplete and flattened. Medusoids are mostly fixed, but in some groups, like many Leptomedusae , they are pinched off like medusae, they are then freely swimming and mobile.

With many hydrozoans , the reduction continues, only a two-layer vesicle, called a styloid or sporosac, is formed. The only remaining differentiation is then a central stalk called a spadix, on which the gonads are located between the epidermal layers. Free-swimming Sporosacs occur, but are the absolute exception, they have been reported by Dicoryne conybaerei (Anthothecata) , for example .

In some groups such as freshwater polyps , the reduction goes so far that no actual gonophores are formed. The gonads form here directly in the endoderm or ectoderm of the polyp wall.

Gonophores arise in the polyps of most hydrozoans near the tentacle rim, near the mouth end (the oral end) of the polyp. In some species they prefer to form at the base or on the stem (stolon). If gonophores develop at the oral pole, the polyps usually lose the ability to eat. Some colonies of polyps form specialized gonozooids, which are special polyps that can no longer feed themselves and are specialized in the formation of gonophores.

Gonophores are sometimes structured morphologically differently in the male and female sex (sexual dimorphism), for example in Tubularia indivisa (Anthothecata) and Laomedea flexuosa (Leptothecata).

Occur

Gonophores are typical of the orders Anthothecata (synonym: Athecata) and Leptothecata (synonym: Leptomedusae , Thecata) of the class Hydrozoa. It has evolved independently ( convergently ) many times in these groups, probably more than seventy times . Presumably, an extensive free-swimming pelagic stage in these groups was ecologically disadvantageous and was abandoned. Independently of one another, some groups then developed secondarily freely movable medusoids from a fixed gonophore.

In the Anthothecata species Sarsia lovenii , trunks with firmly attached and freely swimming gonophores were detected within the same species. In the White Sea , these forms were shown to be genetically differentiated from one another using different haplogroups . It is possible that the species is beginning to split ( cladogenesis ), i.e. two species in the process of development.

swell

  • Jean Bouillon, Cinzia Gravili, Francesc Pagès, Josep-Maria Gili, Ferdinando Boero: An introduction to Hydrozoa (= Mémoires du Muséum national d'Histoire naturelle Tome, Volume 194). Publications Scientifiques du Muséum Paris, 2006. ISBN 978-2-85653-580-6 . Pp. 79-82.
  • B. Werner: 4th tribe Cnidaria. in: HE Gruner (ed.): Textbook of special zoology. Lim. by Alfred Kaestner. Volume 1, part two. Gustav Fischer, Jena, 4th edition 1984. ISBN 3-334-60474-8 , pp. 149-153.
  • Paulyn Cartwright & Annalize M. Nawrocki: Character Evolution in Hydrozoa (phylum Cnidaria). In: Integrative and Comparative Biology , Volume 50, No. 3, 2010, pp. 456-472. doi: 10.1093 / icb / icq089

Individual evidence

  1. Andrey A. Prudkovsky, Irina A. Ekimova, Tatiana V. Neretina (2019): A case of nascent speciation: unique polymorphism of gonophores within hydrozoan Sarsia lovenii. In: Scientific Reports , Volume 9, Article number 15567, doi: 10.1038 / s41598-019-52026-7 (open access).