Orthonectida

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Orthonectida
two female orthonectids.

two female orthonectids.

Systematics
without rank: Holozoa
without rank: Multicellular animals (Metazoa)
without rank: Tissue animals (Eumetazoa)
without rank: Bilateria
incertae sedis
without rank: Orthonectida
Scientific name
Orthonectida
Giard , 1880

Orthonectida ( Gr .: straight swimming ) are a small, only about 25 species comprehensive tribe of the animal kingdom with uncertain phylogenetic position. These are secondary simplified tissue animals from the Bilateria .

features

They are very small, worm-shaped organisms , the body size of all species is less than a millimeter. The body of the free-living sex animals consists of a cover (epidermis) of cells arranged in a ring, which, depending on the ring with cilia, are ciliated or not ciliated. The inner tissue consists for the most part of the germ cells, bundles of muscle cells run between the envelope and the germ cells . Sometimes there is a sensory organ at the front end , which consists of three cells. There are no body cavities, an intestine or a nervous system. With the help of the eyelashes, the Orthonectida swim.

ecology

The animals live as endoparasites in various marine invertebrates , which have so far been identified as hosts: turbellarians, cordworms, polychaete annelids, mussels, snails, echinoderms, tunicata. They can do a lot of damage, e.g. B. they sometimes destroy the gonads of their hosts. The incidence (infection rate) of many hosts is very low, in some cases less than 1%. Some species were never found again even after they were first described, although numerous host organisms from the original location have been examined. The most common and best-studied species Rhopalura ophiocomae parasitizes in the brittle star Amphiphotis squamata , this species was available to Alfred Giard when the group of animals was discovered in 1880. Giard observed that almost finished larvae, which he exposed from the tissue, characteristically swam in a straight line, on which he based the name of the strain (see above), but animals that leave the host by themselves do not show this behavior.

Life cycle

Their life cycle is an alternation of asexually and sexually reproducing generations, i.e. a metagenesis . The predominant generation lives as Plasmodium within a degenerate host cell, usually a muscle cell or an epidermal cell, which is controlled by the parasite's cell nuclei . For decades there was no agreement about the nature of the Plasmodium; in different species it may consist either of the cytoplasm of the host cell or of the parasite itself. Asexual reproduction occurs through fragmentation of the Plasmodium. Inside, some of the parasite nuclei , the so-called agamonts , accumulate and divisions occur, so that the individuals described above are formed from the Plasmodium. These represent the sexually reproducing generation and leave the host, fertilization then takes place outside in the open water. The males are smaller than the females ( sexual dimorphism ), only the animals of the genus Stoecharthrum are hermaphrodites (hermaphrodites). Males and females have a genital porus, which are held against each other during copulation, whereupon the sperm penetrate the female organism in this way. The fertilized egg cells develop into swarm larvae in the mother animal . They also have a ciliated shell in which the inner cells are located. When they attack a host, they lose their shell and the inner cells ensure the formation of new plasmodia.

Systematics

In the past, the Orthonectida were combined with the Rhombozoa , with which they have some similarities, to form the taxon " Mesozoa ", which, however, is probably not a natural group, i.e. not a monophyletic taxon . Some researchers suggested that the Orthonectida emerged from unicellular ancestors independently of all other animals. The name "Mesozoa" also referred to the position apparently "between" the protozoa and the tissue animals. Alfred Giard compared the animals with the planula larva of the cnidarians. However, based on molecular pedigrees, these opinions are now considered refuted. In a study based on the 18S rRNA , the Orthonectida were located in the basal position within the tissue animals without any closer relationship to any other animal strain. Some researchers also suspect a position within the Lophotrochozoa , close to or within the Annelida.

Within the Orthonectida two families with a total of six genera are recognized today

literature

  • Eugene N. Kozloff: Orthonectida. in: Sol Felty Light & James T. Carlton (Eds.) The Light and Smith Manual. Intertidal Invertebrates from Central California to Oregon, Fourth Edition, University of California Press 2007. ISBN 9780520239395
  • Eugene N. Kozloff (1992): The genera of the phylum Orthonectida (Mesozoa) Cahiers de Biologie Marine 33 (3): 377-406.
  • Wilfried Westheide & Reinhard Rieger (Hrsg.): Special Zoology - Part 1: Protozoa and invertebrates (2nd edition). Elsevier, Spektrum Akademischer Verlag, Munich 2007. ISBN 3-8274-1575-6

Individual evidence

  1. ^ T. Cavalier-Smith (1993): Kingdom Protozoa and its 18 phyla. Microbial Revue 57: 953-994.
  2. ↑ Ben Hanelt, David Van Schyndel, Coen M. Adema, Louise A. Lewis, Eric S. Loker (1996): The Phylogenetic position of Rhopaluva ophiocomae (Orthonectida) based on 18s ribosomal DNA sequence analysis. Molecular Biology and Evolution 13 (9): 1187-1191.
  3. ↑ cf. also: Kenneth M. Halanych (2004): The new view of animal phylogeny. Annual review of ecology, evolution, and systematics 35: 229-256. doi : 10.1146 / annurev.ecolsys.35.112202.130124
  4. ↑ GS Slyusarev & RM Kristensen (2003): Structure of the ciliated cells and ciliary rootlets of Intoshia variabilis Fine (Orthonectida). Zoomorphology 122: 33-39.
  5. ^ H. Furuya & J. van der Land: Orthonectida . World Register of Marine Species . 2010. Retrieved January 16, 2013.

Web links

Commons : Orthonectida  - collection of images, videos and audio files