Calyx worms
| Calyx worms | ||||||||
|---|---|---|---|---|---|---|---|---|
Barentsa discreta |
||||||||
| Systematics | ||||||||
|
||||||||
| Scientific name | ||||||||
| Entoprocta | ||||||||
| Nitsche , 1869 |
The entoprocta even chalice Animals (Entoprocta ( Gr . With inner After ), also Kamptozoa) called, are a phylum with about 250 species. They live in the water as sessile filter feeders and are 0.5 to 5 millimeters long. They got their scientific name (Entoprocta / Greek entos = "inner" + proktos = "anus") because, in contrast to the externally similar moss animals (Bryozoa or Ectoprocta), the anus lies within the tentacle ring . They live commensally on sponges and other living things. In doing so, they do not harm their “hosts”.
features
Adult anatomy
The structure of all cupworms is very uniform. They consist of a muscular stem, at the lower end of which a foot with an adhesive gland is formed, and a calyx, which represents the actual body and carries a tentacle crown. The size of the individual animals ( Zooid ) is normally around one millimeter, the smallest known species is Loxomespilon perezi with about 0.1 millimeters and the largest Barentsia robusta with about seven millimeters in height. The individual animals are bilaterally symmetrical and, when viewed superficially, resemble the polyps of the Hydrozoa , which belong to the cnidarians . In contrast to these, however, the tentacles of the cupworms are not retractable, but can only be rolled up.
The outer layer of the animals is formed by a single layer of epidermis , which is bounded on the outside by a gelatinous layer . Underneath there is a single layer of musculature , which consists of inclined fibers in the calyx area and merges into longitudinal muscles in the stem. As a body cavity, the animals have a fluid-filled pseudocoel with isolated collections of mesenchymal cells , in which there is no epithelial lining of the free space. This runs through the body from the stem to the calyx to the individual tentacles; there is no additional vascular system.
The animals' organs are located in the calyx, with the U-shaped intestinal canal taking up most of the space. The intestine has no muscles of its own and is made up of cilia inside , which transport food to the central section, the stomach. This is where digestion and the absorption of nutrients take place; in addition, the stomach roof, similar to the liver of vertebrates, is the main storage and metabolic organ and is also used for excretion by releasing metabolic end products from the body cavity into the intestinal canal. The mouth opening and the anus both open upwards into the area of the tentacle-enclosed space (atrium), as do the paired protonephridia used for excretion and osmoregulation and the ducts of the sac-like gonads .
The nervous system is simple and has a simple, dumbbell-shaped ganglion in the loop of the bowel. From this nerve cords pull to the calyx, tentacle and stalk muscles. On the surface of the calyx there are mechanoreceptors and on the inside of the tentacles there are additional senses, the animals do not have light sense organs.
Anatomy of the larvae
The calyx worm larva has an average diameter of only 50 to 100 micrometers and shows a number of similarities to the widespread Trochophora larva type. There are differences among other things in the dome-shaped formation of the upper half ( episphere ) with a crown plate at the upper, apical end. Because of this dome shape, the name “Tholophora larva” (from Gr. Tholos = dome roof, vault) was proposed (Salvini-Plawen 1980). A lash line runs along the lower edge of the episphere, which serves as the engine of locomotion for the larva during the initial swimming (pelagic) lifestyle. In most species, a sensory organ is formed on the episphere, which consists of a tuft of eyelashes and, in later unliving animals, a pair of light-sensing organs ( ocelles ) (praeoral organ). In the later solitary forms the praeoral organ is paired, in the colony-forming forms unpaired.
The lower half of the larva, the hyposphere , is completely inversed into the episphere during the swimming phase and is therefore hardly visible from the outside. This was the main reason for the initial identification with the trochophora larval type. Only when it became clear that in a second, soil-living (benthic) phase, the everted hyposphere was used as a crawling sole, the equation with the trochophora was put into perspective (in some camptozoan species, however, this second soil-living phase is missing). The mouth opening is on the edge of the hyposphere below the lash line and leads via a U-shaped intestinal canal to the anus, which is also located on the hyposphere. In addition, the Tholophora has protonephridia , which are used for excretion .
Way of life
Cupworms live as filter feeders , using their tentacles to filter suspended matter from the water. These secrete a sticky mucus to which the particles stick, and the cilia of the tentacles deliver them to the mouth opening. By contraction of the longitudinal muscular strands, the individual animals perform pendulum movements of their stalk and characteristic nodding movements of the calyx in order to increase the filtration capacity.
Most species live solitary and have their feet attached to the ground, but about a third of the species also form colonies with several individuals that are connected to one another by branches from their stems.
Reproduction and development
Cupworms can reproduce both sexually and asexually. While the Pedicellinidae are basically simultaneous hermaphroditic , i.e. they permanently have both male and female genital organs, the Loxosomatidae represent protandric hermaphrodites, so they develop male and then female genital organs first.
The Barentsidae are separate sexes, both sexes occur within a colony. In principle, males with testicles always form first , the development of female zooids depends on the presence of fully developed males in the colony. The lifespan of the individual animals is about six weeks.
Asexual reproduction
Asexual reproduction takes place through budding of the ectodermal calyx wall, in which a single animal results in several genetically identical offspring. The buds arise in the calyx wall in the area of the mouth or, in the case of colony-forming forms, on the stem. Only after budding is complete, maternal mesenchymal cells migrate into the newly formed animal and form the muscles and connective tissue .
In the colony-forming species, the young remain connected to the mother animal via the stem and form a connecting stolo by increasing length at the connection point. As a result, mat-shaped or heavily branched, tree-shaped colonies form, which in some species such as Pedicellinopsis fructiosa can reach heights of up to 2.5 centimeters with thousands of individual zooids .
The regenerative capacity of the camptozoa is relatively limited and only affects the formation of new injured tentacles. In the colony-forming forms, the calyxes can be regenerated from the blastoderm of the stalk, so the stalks can serve as persistence stages (especially in Barentsiidae). Some colonies also develop severely retarded buds that only become real zooids when the rest of the colony has died.
Sexual reproduction
During sexual reproduction, the male animals release their sperm into the open water, from where they are curled up by the female animals. Fertilization takes place in the ovary of the females, from there the fertilized zygotes migrate into the atrium and are stored in special paired brood pouches on both sides of the rectum .
The embryonic development takes place via a spiral furrow , in which quartets of two smaller (micromers) and two larger cells (macromers) are formed and superposed in a spiral. Telomesoblasts , which are the cells of origin of the entire mesodermal tissue , develop from the 4d cells - it is therefore the spiral quartet 4d cleavage typical of the Spiralia . Because of this feature, a closer relationship between the cupworms and other spiralers must be taken into account; annelids and molluscs are discussed here (see section Systematics).
distribution and habitat
Cupworms can be found in coastal waters around the world. Most of the species are marine, the widespread species Urnatella gracilis is the only species that lives in fresh water. Originally it probably only occurred in North America and was introduced to Europe and Asia from there.
The individual animals very often settle on other invertebrates such as sponges , annelids , echinoderms and, more rarely, on crustaceans , where they benefit from the swirling of the water through the movement and ingestion of larger animals. Colonies live on all substrates that are exposed to a current, especially on hydroid stocks, mussel shells and rock surfaces .
Tribal history
Due to their small size and the lack of hard substances in their bodies, the phylogenetic evidence of calyx worms is very sketchy. The oldest known forms date from the late Jurassic and were found in England.
Systematics
The external systematics of cupworms has not yet been fully clarified. Originally they were completely assigned to the bryozoa by Nitsche , but this classification was rejected by most of the researchers. On the basis of ontogenetic observations, individual authors, especially Nielsen 1979, still place them in the close relationship of the Bryozoa. Ax 1999, on the other hand, suggests a classification in the relationship of the molluscs (Mollusca) as an alternative and names the resulting taxon as Lacunifera, citing the fine structure of the cuticula , the structure of the body cavity as a lacunae system and the fine structure of the cilia as arguments. Another advantage of this hypothesis is that an evolutionary connection can be established between the wired, trochophora-like larvae of some molluscs and the similar-appearing crawling larvae of calyx worms (in extreme cases, even the crawler foot of the mollusc is derived from the crawl sole of the calyx worm larva!) .
Molecularly, however, neither a proximity of the cupworms to the moss animals nor the Ax's “Lacunifera” hypothesis could be confirmed (see also below).
If one considers the crawling larvae of the cupworms as modified trochophoral larvae, at least two evolutionary models have to be formulated with regard to the origin, in each of which a descent from annelid-like preforms (i.e. annelid worms in the broadest sense) is assumed. Due to the organization of the calyx worms as coelomless animals with spiral furrows and no recognizable structure, it is sometimes assumed that the calyx worms can be directly traced back to trochophorae which were already adult in their youth stage (progenetic trochophorae). A second version sees in the cupworms gradually dwarfed and transformed annelid worms; according to this model, the changes in the larval form would have taken place parallel to the transformation of the adult forms. A certain proximity to the annelids was confirmed by the first molecular genetic studies of the rRNA (Mackay et al. 1995), but many other Lophotrochozoa lines also show molecular genetic similarities with the annelids (including mollusks ). According to recent genetic studies, cupworms are the sister group of another sessile dwarf form, namely the Cycliophora , which was first described in 1995 (Halanych 2004). This means that the assumption of a sister group relationship to the bog animals or molluscs would be rejected, while the ancestry from ringworm-like preforms remains under discussion.
There are around 150 species of cupworm, which are classified into four families . The solitary Loxosomatidae, which are considered to be original, contain around two thirds of the known species and are compared to the other families as the basic sister group Solitaria. The representatives of the other families form colonies and are summarized as Coloniales, here again the Astolonata (Loxokalypodidae) are compared to the Stolonata. As a phylogenetic system, this results in:
| Calyx worms |
|
||||||||||||||||||
|
|
The individual taxa contain the following genera:
- Loxosomatidae : Loxosoma , Loxosomella , Loxomitra , Loxosomespilon , Loxocore
- Loxokalypodidae : Loxokalypus
- Barentsiidae : Barentsia , Urnatella , Pedicellinopsis , Pseudopedicellina , Coriella
- Pedicellinidae : Pedicellina , Pedicellina , Myosoma , Chitaspis , Loxosomatoides
literature
- Peter Emschermann: Kamptozoa (Entoprocta), cupworms. In: W. Westheide, R. Rieger (Ed.): Special Zoology. Part 1: Protozoa and invertebrates. 2nd Edition. Gustav Fischer Verlag, Stuttgart / Jena 2004, ISBN 3-437-20515-3 .
- Peter Ax: The system of the Metazoa: a textbook on phylogenetic systematics. Gustav Fischer Verlag, Stuttgart 1999, ISBN 3-437-35528-7 .
- Luitfried Salvini-Plawen : What is a trochophora? An analysis of the larval types of marine protostomes. In: Zool. Jb. Anatomy. 103, 1980, pp. 389-423.
- Kenneth M. Halanych: The new view of animal phylogeny. In: Annu. Rev. Ecol. Evol. Syst. 35, 2004, pp. 229-256.
Web links
- Animal Diversity Net (with pictures)
- Earthlife.net
- Checklist of recent Entoprocta. ( Memento from June 13, 2010 in the web archive archive.today )
swell
- ↑ C. Nielsen : Larval ciliary bands and metazoan phylogeny. In: Advances in zoological systematic evolutionary research. 1, 1979.
- ^ P. Ax: The system of the Metazoa: a text book of the phylogenetic systematics. G. Fischer Verlag, Stuttgart 1999.
- ^ LY Mackay, B. Winnepennickx, R. de Wachter, T. Backeljau, P. Emschermann, J. Garey: 18S rRNA suggests that Entoprocta are protostomes, unrelated to Ectoprocta. In: Journal of Molecular Evolution. 41, 1995.
