A kind of beroe
|Chun , 1879|
Nuda (also Atentaculata) is the scientific name of a class of rib jellyfish (Ctenophora) and indicates the characteristic feature of representatives of this taxon , the lack of tentacles . They are classically contrasted with the tentacle-armored tentaculata . The group, which includes the two genera Beroe and Neis , still has no German name.
The pigments located in the Mesogloea - the jelly-like intermediate layer between the outer and inner skin (epidermis and gastrodermis) - give many Nuda species a slightly pink color, while Neis cordigera also has a strong orange-red pattern. Nuda representatives reach a length of up to about thirty centimeters, with Neis cordigera often being named as the largest species. In the Beroe species, the sac-like body is slightly compressed perpendicular to the plane of the throat, in Neis this flattening is even more pronounced due to the formation of two "side wings" (more on this in the Systematics section).
The eponymous and defining feature of the Nuda is the complete absence of tentacles. In contrast to individual Tentaculata species, in which the tentacles can be severely stunted, tentacle sheaths are also not present at any stage of life.
Mouth opening and muscles
Since prey is swallowed whole with the mouth, this and the throat directly below it have a large diameter compared to other comb jellyfish. In order to reduce the water resistance when swimming and especially when chasing prey, it can be tightly sealed by a zipper-like mechanism so that the front end of the comb jellyfish takes on a streamlined shape . With this seal, opposite sides of the throat, the “lips”, are not only brought into close contact, but also closely interlocked through temporarily established cell-cell connections. The nerve network of all rib jellyfish is compressed within the “lips” of the nuda into a bundle consisting of around 40 nerve cell processes ( neurites ).
The mouth is opened and closed by the interaction of three different types of muscles: In the mesogloea there are both ring-shaped muscle fibers around the longitudinal axis of the animal and radial muscle fibers running radially from the inside to the outside; they work together with longitudinal, i.e. longitudinally aligned fibers that lie in the epidermis. Only smooth muscles are found in Nuda species .
Characteristic finger-shaped growths , the macrocilia, are located directly inside the mouth opening in the lining of the throat . These structures, first described by JGF Will in 1844 and given their current name by George Adrian Horridge in 1965, are cone-shaped bundles of two to three thousand flagella ( cilia ) surrounded by a common plasma membrane , which resemble the comb platelets of the ribs are connected to a functional unit. The individual cilia, between 35 and 60 micrometers long and five to ten micrometers thick in diameter, together form a hexagonal structure in cross-section and are connected to each other by cross braces perpendicular to the common direction of impact; however, they do not have a separate membrane that would separate them from their neighboring flagella. Each cilium shows the structure of nine outer and two inner microtubules typical of organisms with a real cell nucleus .
The flagellum cells, on which the cilia arise from individual basal bodies , are inclined at an average of thirty degrees to the side facing away from the pharynx and are stacked on top of each other like roof tiles. As a result, the macrocilia always point towards the interior of the throat. This is also where the effective blow is directed, which transports prey parts into the stomach in synchronized waves like a conveyor belt; the pharyngeal muscles promote this process.
Macrocilia terminate in a three-pronged tip; this is stiff enough to tear the soft outer wall of even larger prey such as other rib jellyfish; proteolytic (protein-decomposing) enzymes that penetrate the resulting wounds then quickly render the victim incapable of escape. Macrocilia also take on the function of teeth and can be seen as the nuda's catching structures corresponding to the tentacles of the tentaculata.
Inner canal system
As with all comb jellyfish, channels run from the central stomach into all parts of the body. Each ridge rib is supplied by its own meridional canal, which is located directly below it. In the Nuda species it has numerous finely branched outgrowths, some of which unite with the throat, while others end blind. Towards the end of the mouth, the meridional canals converge in a circular circumoral ring canal around the mouth .
Statocyst and pole plates
As with all rib jellyfish, on the side facing away from the mouth, there is the equilibrium organ, the statocyst , which also sets the rhythm for the comb plates arranged on the ribs. In contrast to the Lobata, for example, the impact impulse is not passed on through the flagellated strips running between the comb plates, but rather because the individual comb plates tip over one after the other, influencing the following plate like dominoes.
Immediately below the statocyst is an indentation lined with sensory cells; the epithelial tissue contained in it continues towards the mouth in two opposite, narrow flagellar ligaments, which run in the pharynx of the comb jelly and are known as pole plates . When they strike, they cause a flow of water over the statocyst and, in the Nuda species, are covered with finger-like protrusions, the papillae , whose function is still unclear, but presumably also serves the sensory perception.
distribution and habitat
The group occurs worldwide in all oceans and seas, where the animals live free-swimming (pelagic) in the plankton. Unlike other groups of comb jellyfish, they almost exclusively use their comb ribs for locomotion.
Nuda species feed on soft-bodied, free-swimming animals, primarily other rib jellyfish that can be larger than themselves. The prey is actively sought and usually devoured as a whole; if they are oversized, the animals can use their macrocilia as teeth in order to tear pieces of tissue out of the victim.
Meaning as a neozoon
Once in the late 1980s ctenophora Type years Mnemiopsis leidyi probably through ballast water into the Black Sea was introduced and there is a population explosion went through in the course of anchovies - fishing collapsed completely, the ecosystem stabilized again in 1997 when another ctenophora style emerged , the Beroe ovata , one of the Nuda , which feeds almost exclusively on Mnemiopsis Leidyi . The Beroe population also initially experienced an explosion in the number of individuals until both stocks collapsed rapidly. Nevertheless, both Mnemiopsis Leidyi and Beroe ovata are now established as neozoa in the Black Sea. The same phenomenon is taking place in the Caspian Sea at the beginning of the 21st century .
All species reproduce sexually and have both female and male gonads, so are hermaphrodites . Although no detailed information is available, it is assumed that self-fertilization is rather the exception in Nuda species. Young animals emerge from the fertilized eggs that already look like a miniature version of the adult animal and should therefore not be called larvae. They are already missing tentacles and tentacle sheaths , otherwise they resemble the Cydippea stage of the tentacle-occupied cortex jellyfish.
Fossil Nuda species are not known, so that the phylogenetic development of the group must be inferred from a comparison with other modern representatives of the comb jellyfish. In the traditional system, the Nuda are compared to the class of the Tentaculata, which includes all species that, in contrast to the Nuda species, have at least rudimentary tentacles. According to the preliminary results of molecular genetic and morphological studies, this classification probably does not reflect the actual relationships within the comb jellyfish. Although the system is still in flux and no independent assessment of the findings is available, a family from the order Cydippida , Haeckelidae, seems to belong to the closer relationship of the Nuda species; neither Cydippida nor Tentaculata would be monophyletic taxa, so they did not include all descendants of their last common ancestor and would therefore not be recognized as a valid taxon by modern systematics.
The monophyly of the nuda itself, on the other hand, is largely undisputed, since the complete loss of tentacles and the presence of macrocilia can be regarded as common derived features (synapomorphies).
A distinction is made between about 25 species , which are divided into two genera , Beroe and Neis , which are placed in the same family Beroidae and order Beroida. Family and order were introduced in 1825 and 1829 respectively by the German naturalist Johann Friedrich Eschscholtz .
- The genus Beroe includes almost all species of the class and is distributed worldwide. One of the best known is the melon jellyfish ( Beroe gracilis ) , which is also widespread in the North Sea . In Beroe species, the (aboral) side turned away from the mouth is rounded.
- The genus Neis is monotypical, so it contains only one species, Neis cordigera (Lesson 1824), and occurs exclusively in the waters around Australia. In this species there are two arch-like extensions on the aboral side, so that the aboral sensory pole in the middle, in contrast to the Beroids, is not at the extreme end of the body (note the species name cordigera : heart shape). The arch-like appendages extend along the flanks of the animal and are divided in two longitudinally, so that the impression of two lateral "double wings" is created. A rib band runs along the edges of these four “wings”. The remaining four rib bands run over the (relative to the "side wings") "upper" and "lower" halves of the body. The rib jellyfish typical figure eight of the rib bands is preserved, even if the bands are formed in two groups of different lengths due to the special body shape.
- GI Matsumoto, GR Harbison: In situ observations of foraging, feeding, and escape behavior in three orders of oceanic ctenophores: Lobata, Cestida, and Beroida. Marine Biology. 117 , 1993, p. 279.
- S. Tamm, SL Tamm: A giant nerve net with multieffector synapses underlying epithelial adhesive strips in the mouth of Beroe. in: Journal of Neurocytology. 24 , 1995, p. 711.
- S. Tamm, SL Tamm: Dynamic control of cell-cell-adhesion and membrane-associated actin during food-induced mouth opening in Beroe. in: Journal of Cellscience. 106 , 1993, p. 355