Platyctenida
| Platyctenida | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Coeloplana willeyi |
||||||||||||
| Systematics | ||||||||||||
|
||||||||||||
| Scientific name | ||||||||||||
| Platyctenida | ||||||||||||
| Bourne , 1900 | ||||||||||||
| Familys | ||||||||||||
|
As platyctenida refers to a fine of comb jellies (Ctenophora) in the class Tentaculata whose physique deviates greatly from the basic blueprint of comb jellies, so some species at first glance more like flatworms (Platyhelminthes) look.
The taxon name was first used in 1900 by the English zoologist Gilbert Charles Bourne in the treatise on Zoology .
construction
In contrast to most other comb jellyfish, the small animals, which often do not grow to be more than an inch long, are probably pigmented in complex color patterns for camouflage, which are often coordinated with the body color of the host on which they live. They have two well-developed tentacles that often arise in their own tentacle sheaths. The tentils , transverse threads, which are covered with numerous adhesive bodies, the colloblasts , extend from them .
The most distinctive feature of the Platyctenida is the extreme flattening of the body in the plane perpendicular to the longitudinal axis of the body, the line connecting the mouth and statocyst . They are also compressed perpendicular to this, specifically in the tentacle plane formed by the longitudinal axis and the connecting line of the two tentacle sheaths, so that they look oval in cross section.
Comb ribs are usually no longer present in the adult animals and are instead replaced by rows of knob-shaped elevations, the papillae. Around the organ of equilibrium, the statocyst, there are two, sometimes four, sensory fields, the pole plates, covered with flagella.
A crawling sole on the mouth side is derived from the everted pharyngeal tissue, with the help of which the Platyctenida can move.
The digestive system consists of a complex, branched network, which is formed by numerous food ducts that start from the centrally located stomach. Usually there are also anal pores , small openings through which metabolic degradation products are presumably released into the environment.
A peculiarity of the Platyctenida is the location of the testicles in separate pockets next to these canals. The ovaries, on the other hand, lie in the canal system itself, as in all comb jellyfish.
distribution and habitat
All Platyctenida species live in warm waters, most of them as adult animals on the seabed ( benthic ), many of them ectocommensal , i.e. without disturbing the host from outside on stationary animals such as leather corals (Alcyonacea), sea feathers (Pennatulacea), salps (Thaliacea) ) or echinoderms (Echinodermata), also on mangrove roots . Only the species of the genus Ctenoplana can also occur free-swimming as part of the plankton .
Sometimes Platyctenida species are found as uninvited guests in saltwater aquariums ; but at least a small number of them are harmless there.
Diet and locomotion
To catch zooplankton, Platyctenida species use their well-developed tentacles, which allow them to protrude into the water current above the seabed. The fact that they often attach themselves to other animals is probably due to the fact that in this way they can assume an elevated position and thus increase their food yield. Sometimes they also form a kind of “chimney” by folding their body around the base of the tentacles, which allows water to flow over the tentacles towards the mouth.
For locomotion, adult Platyctenida use their crawling sole formed by pharyngeal tissue - in this respect they are more like flatworms than rib jellyfish.
Reproduction
The Platyctenida are the only comb jellyfish that can reproduce asexually . This happens quite often, in which fragments separate from the edges of their flattened bodies as they crawl, which then develop into new individuals.
Nevertheless, the possibility of sexual reproduction is also available with the Platyctenida and, in contrast to the asexual method of reproduction, primarily serves to spread the animals over a wide area.
In contrast to other comb jellyfish, fertilization takes place internally; the fertilized embryos are incubated in special either lying mouth side facing away or mouth bags and than about five to twenty-five millimeters Cydippea released stadiums. In contrast to the adult animals, these are still equipped with comb ribs, the number of which, however, is reduced from eight to six in some species.
At first they live freely swimming in the plankton and thus colonize a different ecological niche than the old animals, which is why they can be regarded as real larvae . When they finally establish themselves on the sea floor and switch to the benthic way of life of the adult animals, a corresponding metamorphosis also occurs .
In the course of this, the comb ribs are lost and are converted into papillae, the body flattens noticeably at the same time in the oral statocyst axis, which is accompanied by increased ramification of the channels of the digestive system and a reduction in the proportion of mesoglea in the body weight. The crawling sole is created by everting and inflating the pharynx lining, the embryological origin of which is therefore not comparable to that of the analogous structure of flatworms.
Tribal history
Fossil order has not been preserved, so that the relationships must be inferred by comparison with other comb jellyfish. It is considered likely that the Platyctenida form a monophyletic group, i.e. all descendants of their common ancestor. According to preliminary results of molecular genetic studies, the Platyctenida are an early group that branched off from the family tree of the rest of the comb jellyfish species; its sister taxon form all other orders with the exception of the Cydippida family Mertensiidae.
Systematics
Within the order one differentiates between the following five families with more than forty species:
- The Coeloplanidae form the family with the greatest biodiversity: They are divided into two genera with a total of twenty-six species.
- The Ctenoplanidae are in second place in terms of species number. In one genus, Ctenoplana , there are twelve species that, in contrast to the other Platyctenida, have not lost their crest and can therefore also live in the plankton.
- Tjalfiellidae is a monotypic family with only one species, Tjalfiella tristoma , that lives on sea feathers.
- This also applies to Savangiidae, into which the species Savangia atentaculata is divided.
- Finally, the Lyroctenidae family includes two species in one genus, Lyrocteis . Both live attached to sea feathers.
The numbers of species given here are only to be understood as guidelines, as it is by no means clear whether two different species names of a genus actually stand for one species, i.e. are synonyms.
literature
- Alfred Kaestner Textbook of Special Zoology. Volume I: Invertebrates 1st part . VEB Gustav Fischer, Jena 1969 (3rd edition), pp. 214-216
