Tentaculata
| Tentaculata | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Tentaculate comb jelly Mertensia ovum |
||||||||||||
| Systematics | ||||||||||||
|
||||||||||||
| Scientific name | ||||||||||||
| Tentaculata | ||||||||||||
| Eschscholtz , 1825 | ||||||||||||
| Orders | ||||||||||||
As Tentaculata refers to a class of comb jellies (Ctenophora) that are characterized by the possession of tentacles distinguished. They are classically contrasted with the tentacled nuda . The taxon still has no German name (see wreath probe ). There is clear evidence that the class is paraphyletic, that is, does not include all descendants of their common ancestor. Should this prove to be correct in further investigations, it would not be recognized as valid by the systematics predominant today, the cladistics .
construction
The basic construction plan of all rib jellyfish is varied in various ways in the various orders. In particular, the eponymous tentacles are formed to different degrees and accordingly play a more or less important role in food procurement.
Both the species of the order cydippida as well as those of the ground-dwelling platyctenida have two well-developed, with long transverse threads, the Tentillen studded tentacles that spring in Tentaktelscheiden and are actively used to catch prey. They are covered with colloblasts , the adhesive bodies of the rib jellyfish. The species of the genus Haeckelia also use stinging cells (nematocysts), which they have taken over from their prey.
In the Thalassocalycida , Ganeshida and Cestida, however , the tentacles are relatively short and greatly reduced. The elongated, belt-shaped Cestida instead carry their colloblast-occupied tentils in four furrows, which arise at the base of the tentacle sheaths and run outwards along the side of the mouth.
The lobata finally capture their food in the first place with two muscle-streaked mouth flaps between which the so-called auricular bands are springing to the tentacle sheaths and are occupied with numerous Tentillen. The tentacles themselves play only a subordinate or no meaning at all when catching food; accordingly, they are either greatly reduced or, as in the species of the genus Ocyropsis, even completely absent.
distribution and habitat
Tentaculata species are found in all oceans worldwide, both near the coast and on the open sea, and both near the surface and at greater depths. The species of the order Platyctenida live on the sea floor (benthic).
nutrition
All species are predatory; Their prey includes microorganisms from the marine plankton , cnidarians , small crabs and fish . One species is likely to be parasitic .
Reproduction
Asexual reproduction occurs in the Platyctenida species living on the seabed, otherwise all Tentaculata species reproduce sexually. Almost all of them are hermaphrodites , so they have both male and female gonads. A young animal called Cydippea emerges from the fertilized egg cell, which, equipped with comb ribs, usually looks like the adult animal and has a similar structure in all orders. Only in the Platyctenida does a kind of metamorphosis occur, i.e. a major restructuring of the body during the transformation into an adult animal, so that in this case the Cydippea stage can be described as larva.
Systematics
The tentaculata comprise the largest part of the species diversity within the comb jellyfish and have an astonishing variety of types within the framework of the common basic plan. There are traditionally six orders :
- The Cydippida are plankton-living spherical to oval comb jellyfish with well-developed tentacles and tentacle sheaths. This subheading includes the sea gooseberry ( Pleurobrachia pileus ), the species of the genus Haeckelia which have taken over the nettle cells from their prey, and the parasitic genus Lampea .
- The lobata are characterized by two large, muscular mouth lobes. Their tentacles are greatly reduced or even absent; instead, two grooves each with tentacles run from the tentacle sheaths to the mouth lobes where the prey is caught.
- The Ganeshida consist of only two species in one genus, Ganesha , which look like young Lobata comb jellyfish and have two small mouth lobes.
- Thalassocalycida is a monotypic order, so it only includes a single species, Thalassocalyce inconstans , which on the surface looks like a jellyfish of the hydrozoans and has two short tentacles with tentacles.
- Cestida are characterized by a very unusual body shape: The tentacle plane is very short, the vertical plane of the pharynx is extremely long, which gives the body a belt-like appearance. Instead of swimming with the partially stunted comb ribs, Cestida use wave-like muscle movements.
- The Platyctenida are the most modified group within the comb jellyfish. With one exception, they have completely lost their comb ribs; their body is extremely flattened in the plane perpendicular to the oral statocyst axis, so that they look more like flatworms (plathelminths).
The relationships within the class have not yet been clarified; Preliminary results indicate that, firstly, it is not a monophyletic taxon, i.e. does not include all descendants of the last common ancestor of all animals and, secondly, that the order Cydippida is even polyphyletic, i.e. a completely unnatural grouping.
Morphological and molecular genetic results suggest the following systematics, which, however, should still be viewed as uncertain:
| Tentaculata |
|
|||||||||||||||||||||||||||||||||
|
|
The position of the Ganeshida is unknown.
literature
- EE Ruppert, RS Fox, RP Barnes, Invertebrate Zoology - A functional evolutionary approach , Brooks / Cole 2004, chap. 8, p. 191, ISBN 0-03-025982-7
