Rib jellyfish
| Rib jellyfish | ||||||||||||
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Comb jelly ( Mertensia ovum ) |
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| Ctenophora | ||||||||||||
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The rib or ridge jellyfish (Ctenophora of Altgr. Κτείς genitive κτενός ktenos , comb 'and φέρω PHERO bear') are a strain of the animal kingdom. Its scientific name refers to the comb-like platelets with which the “ribs” that give the German its name are covered.
Even if they look like jellyfish on the surface , they are not considered real jellyfish zoologically; not least because they lack the stinging cells characteristic of these cells . The more than 100 species of comb jellies are common in the oceans around the world and set regionally a significant share of total plankton - biomass . Some species, such as the sea gooseberry ( Pleurobrachia pileus ), which is also native to the North Sea , can occur in such high numbers that they clog the fishing nets of coastal fishermen as unwanted bycatch . In contrast, only a few specimens of other species occur. The fragile structure of the comb jellyfish makes researching their way of life much more difficult. For this reason, no age information is available, although it is known that comb jellyfish can start reproducing before they reach adult size and therefore presumably have a short generation cycle.
In the classical taxonomy, the comb jellyfish were combined with the cnidarians (Cnidaria) to form the hollow animals (Coelenterata). A close relationship to the cnidarians is, however, rather unlikely, according to studies that also include molecular methods such as the comparison of DNA sequences. The actual position and relationship of the comb jellyfish is controversial.
construction
Bar jellyfish are, apart from a color caused by symbiotically living algae cells, usually colorless and often only a few centimeters in size. Exceptions are the species of the genus Cestum , which can reach a diameter of one and a half meters.
The species that occur in deeper waters, however, can also be colored, such as the red tortuga, which to this day has no scientific name. This is dark red in color and, like many other comb jellyfish, can generate light using bioluminescence . In the case of species living on the ground, the coloring may serve as camouflage. The species Eurhamphaea vexilligera can release a bright red ink that may be used to distract predators.
Rib jellyfish are almost invariably radially symmetrical ; its main body axis runs between the mouth and the organ of equilibrium called the statocyst , which is exactly opposite the mouth. This symmetry is broken outwardly in the lower part of the animals by the two tentacles , in the upper part by the construction of the digestive space, which is divided into several channels. The lower plane of symmetry is offset by 90 degrees compared to the upper one, one speaks of a disymmetry or a biradial structure of the animals.
The body consists of two transparent cell layers called the outer skin (ectodermis) and the inner skin (gastrodermis). The ectodermis, which is made up of two layers of cells, is usually covered by a protective layer of mucus that is secreted by special glandular cells. The gastrodermis encloses a cavity that serves as the stomach and is only accessible from the mouth opening, which is followed by a long, narrow throat. Captured prey is pre-digested in the throat by strong digestive enzymes and completely decomposed in the stomach. Apart from two small, so-called anal pores, which are not used for excretion, there is no separate exit from the stomach, so that the indigestible waste is removed through the mouth.
The space between the inner and outer layers is covered by a thick, transparent jelly-like layer, the mesogloea formed by collagen and connective tissue cells , which is traversed by numerous small tubules that are used to transport and store the nutrients absorbed. The position of the canals varies from species to species, but they usually run directly below the tissue they supply. The extracellular network of structural proteins is maintained by special amoeba-like cells.
The mesogloea probably also plays a role in the static buoyancy of the animals. Flagellar rosettes, which are located in the canals of the digestive system, are presumably used to pump water out of or into the mesogloea when the osmotic water pressure changes, for example because the animal swims from salty seawater into coastal brackish water. There is no special circulatory system in the comb jellyfish, nor are there any special organs for breathing ; The gas exchange and probably also the excretion of waste products of cell metabolism such as ammonia takes place over the entire body surface by simple diffusion . The body is pervaded by a simple network of nerves without a “ brain ”, which is concentrated around the throat, tentacles, “ribs” and statocysts and is connected to the muscle cells located in the mesogloea and the inner cell layer of the outer skin.
Statocyst and tentacles
A specialized system of the comb jellyfish is the statocyst , which serves as a balance organ and controls the movement of the animals. It is located on the side of the body facing away from the mouth opening and consists of a collection of several hundred calcium cells called statolith on the one hand and four horizontally arranged groups of serpentine flagella on the other. If the comb jellyfish changes its position in space due to external influences, the statolith exerts more pressure on one of the four flagella groups than on the other three. This sensory impression is passed on to the outer skin, which is traversed by a total of eight longitudinal bands, the eponymous ribs. On these bands there are small platelets arranged one behind the other in a row, which consist of hundreds of fused flagella about two millimeters long and are also known as membranelles . By erecting these platelets in rows, a real stroke of the oar occurs which, if it is correctly synchronized between the eight ribs, can restore the original position. A group of flagella of the statocyst is responsible for stimulating the flapping movement in one quadrant each and, as a pacemaker, controls two comb ribs. The beat rhythm is transmitted in a mechanical way and not by nerve impulses. The flapping movement of the excited flagella continues in waves over the ribs. With suitable lighting, the resulting movement patterns generate interference colors and are visible to the naked eye, for example in sunlight, as rainbow-colored light reflections along the ribs.
Whether increased pressure on the flagella groups of the statocyst increases or decreases the beat frequency depends on the "mood" or geotaxis of the comb jellyfish: If it is positive, the frequency is decreased when the pressure increases, so that the comb jellyfish aligns itself with its mouth downwards and swims away from the surface of the water. If, on the other hand, it is negative, the frequency increases, the comb jellyfish directs its front end upwards and swims towards the surface of the water. The "mood" of the comb jellyfish is determined by sensory impressions processed by the nerve network.
Most species have two opposing retractable tentacles in front of the mouth opening, each emerging from a tentacle sheath and used to catch prey. They often carry a series of thread-like filaments on their sides, the tentils, which, unlike in the cnidarians, are not covered with nettle cells but with so-called "Lasso cells", the colloblasts . These consist of a head strewn with small adhesive bodies, a short, slim holding thread and a longer spiral thread wound around it. When touched, this construction acts like a spring that hurls the adhesive head at the prey. The colloblasts, like the tentacles as a whole, are regularly regenerated.
All comb jellyfish with the exception of the species of the genus Beroe (therefore called Nuda or Atentaculata) have tentacles. Some Lobata also use their muscle-lined mouth flaps instead of their tentacles to obtain food, which are then simply put over their prey.
Comb jellyfish are amazingly regenerative animals: even if half of an individual is destroyed, the remaining half can often restore it. This also applies to individual organs such as the statolith, which can still be regenerated even after total loss.
Locomotion
Many comb jellyfish simply let themselves be carried away by the ocean current. At short distances they can also swim actively by the stroke of their flagellated platelets, depending on the "rowing direction" in relation to the mouth opening forwards or backwards. This makes them the largest animals that still use flagella for locomotion, and with that they reach speeds of around five centimeters per second. A possible evolutionary advantage is seen in the fact that, thanks to the even stroke of the oars, neither prey nor possible predators such as other comb jellyfish become aware of them through vibrations triggered by muscle movements.
However, some species also use the muscle cells of their mouth lobes for swimming, while others move forward with undulating body movements or crawl like flatworms .
Diet and predators
Rib jellyfish are predatory. With their tentacles they catch plankton , animal larvae , worms , crustaceans , cnidarians and other rib jellyfish, but sometimes also small fish . If the tentacles are contaminated with food particles, they can be drawn in and stripped off at the mouth opening. They are carried from there to the stomach by secreted mucus or internal flagella. The species of the genus Haeckelia feed almost exclusively on cnidarians, whose stinging cells, however, are not digested, but built into the tentacles as so-called kleptocnids . This "theft" has historically caused confusion among zoologists for a long time, as it was wrongly assumed that cortisized jellyfish are also able to form nettle cells.
Like a whole series of cnidarians, the comb jellyfish sometimes live in a symbiotic relationship with various algae , which they supply with high-energy carbohydrates through photosynthesis . Parasitism appears to occur in only one species, Lampea pancerina , which is parasitic in tunicates (tunicata).
The predators of the rib jellyfish include cnidarians, sea turtles (Cheloniidae), various fish such as mackerel (Scombridae) or the sea hare ( Cyclopterus lumpus ), seabirds and not least other rib jellyfish.
habitat
All comb jellyfish live in the sea, some of them up to three kilometers deep. Their area of distribution is determined primarily by water currents, especially by the tides . Individual species also occur in the North Sea, such as the so-called sea gooseberry ( Pleurobrachia pileus ) or the melon jellyfish ( Beroe gracilis ) and the sea walnut ( Mnemiopsis Leidyi ).
The best-known species live as part of the plankton in the light-flooded water layers close to the surface. However, since they are largely transparent and extremely fragile and usually only a few millimeters in size, they are unknown to most people. The spherical Pleurobrachia species, which also include the sea gooseberry, are found on the coasts . Also Bolinopsis and Mnemiopsis , and the tentacle-less Beroe are often to be found there.
About 35 species live on the sea floor. These species are classified in the taxon Platyctenida , as they are characterized by their flattened shape and are more like snails or flatworms than "jellyfish".
Comb jellyfish represent the predominant group of zooplankton in arctic waters with Mertensia ovum .
Reproduction
Squid jellyfish reproduce, apart from the species of the order Platyctenida, which can also reproduce asexually, in principle sexually. It is almost always a hermaphrodite , so every animal has both male (testes) and female gonads (ovaries), which are located directly under the "ribs" next to the small canals of the mesoglea. In almost all species, probably triggered by external light conditions, the germ cells are released into the surrounding water through small openings in the outer skin, the gonopores , where fertilization then takes place. Self-fertilization is rather rare and apparently only occurs in the genus Mnemiopsis . Only one species, Tjalfiella tristoma , is viviparous; the young grow here in their own breeding cave.
In the fertilized egg cells, after two cell divisions, the later body symmetry of the comb jelly is already established; they develop through what is known as the Cydippea stage, which is already free- swimming and which looks very similar in all comb jellyfish and is sometimes referred to as a larva , but in reality is usually a miniature version of the adult animal. In some highly specialized groups such as the Platyctenida, however , the Cydippea and adult animals occupy different ecological niches, so that the term larva is more appropriate there.
Rib jellyfish as neozoa
Although cortis jellyfish are usually barely noticed and their impact on an ecosystem appears to be minimal, they can cause considerable damage if they enter non-native waters. The North Atlantic species Mnemiopsis Leidyi was brought into the Black Sea with the ballast water of ships in the early 1980s, where it spread rapidly. Within ten years, the anchovy fishery around the sea collapsed as the newly introduced species fed on the same plankton that the fish larvae eat. The biomass of the comb jellyfish in the Black Sea was estimated at one million tons at the height of this development. The comb jellyfish can sometimes double their weight in 24 hours. Researchers at the Helmholtz Association belonging Biological Institute Helgoland (BAH) had recently pointed out, is that the comb jellyfish due to warmer winter in the Baltic and North Sea could become a problem.
The equilibrium was leveled off again when another species of comb jellyfish, Beroe ovata , appeared in 1997, feeding on Mnemiopsis Leidyi , but the Black Sea has since been populated by both alien species. The same scenario plays out with the same species in the Caspian Sea at the beginning of the 21st century . Correspondingly serious changes can also be expected for this ecosystem. In the western Baltic Sea, the North Atlantic species Mnemiopsis Leidyi was detected for the first time in October 2006.
Research history
Cib jellyfish have been known since ancient times , as they can be seen when observing them closely from ships . The first surviving drawing was made by a ship's doctor in 1671. In his classification of the animal kingdom, the Swedish taxonomist Carl von Linné put them together with other "primitive" invertebrates such as sponges (Porifera) or cnidarians as the Zoophyta (translated as animal plants), alluding to the passive, "plant-like" character of the animals . The French zoologist Georges Cuvier also maintained this classification. It was not until the 19th century that the comb jellyfish were recognized as an independent taxon.
Tribal history
The classification given in the introduction is not undisputed. According to the currently leading systematic method, cladistics , the rib jellyfish are more closely related to the mirror-symmetrical bilateria than to the cnidarians. This is also supported by the fact that they have two tentacles facing each other, which break the radial symmetry and turn it into bilateral or mirror symmetry. It also differentiates from the cnidarians the presence of real muscle tissue and the "ribs" described, consisting of hundreds of small platelets. An important indicator for the sister group relationship of the comb jellyfish with the bilateria is also the structure of the sperm . In both groups, these have a single, large acrosome and an underlying subacrosomal perforation plate . In the cnidarians there are several acrosome vesicles.
For this reason, the "classical" grouping of coelenterates is opposed to the alternative taxon of Acrosomata :
- Alternative 1
- Hollow animals (Coelenterata):
| Tissue animals (Eumetazoa) |
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- Alternative 2
- Acrosomata:
| Tissue animals (Eumetazoa) |
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In addition, a close relationship between the rib jellyfish and flatworms has been suggested; the reasons given include the similarities between these and the flattened rib jellyfish of the order Platyctenida. For most zoologists, however, these are only superficial similarities that do not indicate a close relationship.
- Alternative 3
- ParaHoxozoa / Planulozoa:
| Multi-cell (Metazoa) |
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Last but not least, the comb jellyfish in the combination of the ParaHoxozoa / Planulozoa alternative are placed at the base of the metazoa and form the most original multicellular animals here.
The soft body of the comb jellyfish, which is not covered by any hard parts, makes fossilization very unlikely in general, so that the tribal history of the comb jellyfish is very poorly documented fossil . Individual finds, Archaeocydippida hunsrueckiana and Paleoctenophora brasseli , which can be divided into the order Cydippida, are known from the Devonian Age ; In the fine-grained slate from the Hunsrück , enough details have been preserved here to enable identification. The species Matianoascus octonarius , known from the Chengjiang fauna of the lower Cambrian, is controversial in its membership of the comb jellyfish, but three species, Ctenorhabdotus capulus , Fasciculus vesanus and Xanioascus canadensis from the Cambrian Burgess slate are known.
Systematics
Currently about a hundred species are known, which are traditionally divided into the two classes of the Tentaculata (also Tentaculifera) and Nuda (also Atentaculata).
- The tentaculata comprise by far the largest part of the biodiversity; As the name suggests, they have tentacles, which, however, can be very stunted. A distinction is made between the following six orders :
- Order Cydippida , including the sea gooseberry ( Pleurobrachia pileus )
- Order Platyctenida
- Order Ganeshida
- Order Thalassocalycida
- Order Lobata
- Order Cestida , including the Venus belt ( Cestum veneris )
- Only one order, Beroida, belongs to the class Nuda , which includes the melon jellyfish ( Beroe gracilis ). Here, too, the name of the taxon indicates that Nuda species are characterized by the complete absence of tentacles.
Due to the continuing uncertainties about the classification of the comb jellyfish as a whole, it is currently unclear whether the above classification correctly reflects the actual phylogenetic relationships within the taxon. This is only the case if the tentacle-owning species evolved from tentacle-less ancestors. If instead the tentacles have been lost secondarily in the species of the class Nuda, it is very likely that the class Tentaculata represents a paraphyletic group, i.e. does not include all descendants of their common ancestor. According to the ideas of modern systematics, cladistics , it would then only be an unnatural summary of non-closely related species. Molecular genetic studies support the latter view and also see the order Cydippida as polyphyletic; If the results of these studies are confirmed, this does not even include the last common ancestor of the group and would therefore be an artificial taxon.
The following diagram shows the presumed phylogenetic relationships within the comb jellyfish on the basis of morphological and molecular genetic data (ribosomal RNA):
| Rib jellyfish (Ctenophora) |
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The position of the Ganeshida is unknown.
However, the above information is still fraught with great uncertainty - for the time being, the phylogenetic relationships within the comb jellyfish must be regarded as unexplained.
literature
- Donald T. Anderson: Invertebrate Zoology. Cape. 3. Oxford Univ. Press, Oxford 2001, p. 54. ISBN 0-19-551368-1 .
- Richard S. Barnes, Peter Calow, Peter J. Olive, David W. Golding, John I. Spicer: The invertebrates. A synthesis. Cape. 3.4.3. Blackwell, Oxford 2001, p. 63. ISBN 0-632-04761-5 .
- Richard C. Brusca, Gary J. Brusca: Invertebrates. Cape. 9. Sinauer, Sunderland Mass. 2003, p. 269. ISBN 0-87893-097-3 .
- Janet Moore: An Introduction to the Invertebrates. Cape. 5.4. Cambridge University Press, Cambridge 2001, p. 65. ISBN 0-521-77914-6 .
- Edward E. Ruppert, Richard S. Fox, Robert D. Barnes: Invertebrate Zoology. A functional evolutionary approach. Cape. 8. Thomson Brooks Cole, Belmont, Calif. 2004, p. 181. ISBN 0-03-025982-7 .
- Wilhelm Schäfer: Ctenophora, rib jellyfish. In: Wilfried Westheide , Reinhard Rieger : Special Zoology, Vol. 1: Protozoa and invertebrates . Gustav Fischer, Stuttgart 1996. p. 182. ISBN 3-437-20515-3 .
- Bruno Wenzel: Glass animals of the sea. Rib jellyfish (Acnidaria). (Neue Brehm-Bücherei; Vol. 213). Westarp Wissenschaftlicher Verlag, Hohenwarsleben 2010, ISBN 978-3-89432-659-3 (reprint of the Wittenberg 1958 edition).
- Special literature
- Mark Q. Martindale, Jonathan Q. Henry: Ctenophora. In: Scott F. Gilbert, Anne M. Raunio: Embryology. Constructing the Organism. Sinauer, Sunderland Mass. 1997, p. 87. ISBN 0-87893-237-2 .
- Mircea Podar, Steven H. Haddock, Mitchell L. Sogin, G. Richard Harbison: A molecular phylogenetic framework for the phylum Ctenophora using 18S rRNA genes. In: Molecular Phylogenetics and Evolution , Vol. 21 (2001), p. 218. ISSN 1055-7903
Individual evidence
- ↑ Tamara A. Shiganova: Invasion of the Black Sea by the ctenophore Mnemiopsis Leidyi and recent changes in pelagic community structure. in: Fisheries Oceanography. Blackwell, Oxford 1998, p. 305. ISSN 1365-2419
- ↑ The sea walnut on a conquest. Helmholtz Association: Insights into the research area Earth and Environment ( Memento from September 4, 2014 in the Internet Archive )
- ↑ Casey W. Dunn, Joseph F. Ryan: The evolution of animal genomes . In: Current Opinion in Genetics & Development . Volume 35, 2015, p. 26.
- ↑ Martin Dohrmann, Gert Wörheide: Novel Scenarios of Early Animal Evolution - Is It Time to Rewrite Textbooks? . In: Integrative and Comparative Biology . Volume 53, 2013, pp. 504, 507-508.
- ↑ George D. Stanley, Wilhelm Stürmer: The first fossil ctenophore from the lower devonian of West Germany. In: Nature , 303 : 518 (1983). ISSN 0028-0836
Web links
- Ctenophora page of the Washington State University (in English)
- Ctenophores from the São Sebastião Channel, Brazil (in English and Portuguese)
