Octocorallia
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A fully expanded soft coral of the genus Dendronephthya on the north coast of East Timor |
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| Octocorallia | ||||||||||||
| Haeckel , 1866 |
The Octocorallia are a subclass of the flower animals (Anthozoa). In German, the different groups depending on growth form or strength than soft - (Alcyonacea), leather - (Alcyoniidae) tubes - (Stolonifera) or horn corals, sea fans (Gorgonacea), sea fans and sea pens called (Pennatulacea) names today for the most Part no longer have taxonomic validity. A total of 3,200 of the 10,000 nettle species belong to the Octocorallia. With a few exceptions, the Octocorallia are not involved in the construction of the coral reefs because they do not have a stable skeleton.
etymology
The name Octocorallia is a reference to the eight-pointed symmetry of the individual polyps, from which there are no exceptions. In German they are also called eight-pointed flower animals , eight-pointed corals or octocorals . An older scientific name, which is actually valid according to the rules of zoological nomenclature, is Alcyonaria Dana, 1846. Since this name was also used for the actual soft corals ( Alcyonacea ) and there was often confusion, the clear synonym Octocorallia is preferred.
distribution
Octocorallia live in all oceans from the Arctic to the Antarctic, even at great depths. A gorgonian was even photographed on the wreck of the Titanic at a depth of 4000 meters. Most of them live on hard substrates such as rocky reefs or as secondary settlers on dead hard corals , the sea feathers more on muddy bottoms. Often they cover more than 50% of the available substrates and prevent the settlement and growth of other sessile organisms . The main area of distribution is the tropical coral reefs. Soft and leather corals tend to live in shallow areas of the Indo-Pacific ; in the Caribbean and deep-sea biotopes, gorgonian-like corals dominate, which are often more species-rich than hard corals. A total of 90 genera from 12 families occur in the coral reefs of the tropical Indo-Pacific. A representative of the group, the Dead Sea Hand ( Alcyonium digitatum ), also lives in the North Sea .
features
Almost all Octocorallia live in colonies that are made up of many individual polyps . Only the genus Talaroa has only one polyp. The colonies can grow over the substrate as a flat or net-like covering only a few millimeters high, but can also be mushroom-, shrub-, tree- or fan-shaped, reach diameters of up to one meter and heights of several meters and weigh several kilograms. The polyps of the Octocorallia are very uniformly built, they have eight, mostly pinnate tentacles , if present, and the gastric space is also divided into eight chambers by eight longitudinal walls ( mesenteries ). The gastric space extends up into the hollow tentacles. The polyps are on average smaller than those of the Hexacorallia . The most important polyp type is the "autozooid", a feeding polyp with feathered tentacles that is present in all genera and is the only polyp type in many. In rare cases, as in Pachyclavularia violacea , the tentacle leaflets can be secondary, or, as in Knopia octocontacanalis , have grown together so that the tentacles have a paddle-like appearance. Another type of polyp is the tentacle-free "siphozooid", a tube polyp that is smaller than the autozooid and, in large octocorallia, which are stabilized by the internal water pressure, is responsible for pumping the water in and out. In some gorgonians, the siphozooids are only responsible for pumping in the water. So-called "mesozooids" are responsible for pumping out. In sea feathers, auto and siphozooids sit on a large central primary or axial polyp called the "oozooid".
skeleton
In contrast to the hard corals, most Octocorallia do not have a solid calcareous skeleton, but only have small calcareous needles ( sclerite ) in the body as strengthening elements , which are otherwise stabilized by the water pressure inside. Careful examination of the sclerite is often the only way to distinguish between species of Octocorallia. In leather corals (Alcyoniidae), the sclerites are so dense that a certain strength is achieved and the surface of the coral feels rough to the touch. In Sinularia leptoclados and Sinularia minima , the sclerites are so densely packed in the colony base that extensive reef structures can arise that are as solid as the hard skeletons of the hard coral Acropora palmata . They are the only reef-forming leather corals. In the others, the sclerites become free with the death of the animals and they only contribute to reef formation by filling gaps.
The gorgonian-like corals have a central axial skeleton, the medulla, in which the sclerites are connected in different ways by the protein gorgonin . In the suborders Calcaxonia and Holaxonia , the flexible, horn-like axial skeleton consists of individual fibers of gorgonine, between which crystalline calcium carbonate in the form of aragonite or calcite is embedded. The medulla of the calcareous corals (Scleraxonia) consists of more or less fused sclerites, which are not reinforced by Gorgonin. The skeleton of the gorgonian-like corals disintegrates after the animals die.
An exception among the Octocorallia is the blue coral ( Heliopora coerulea ), which forms a massive skeleton made of calcium carbonate. The skeleton is not formed by interconnected sclerites, but by fibers made of aragonite that fuse into thin plates. It is colored blue by stored iron salts inside. The blue coral is a living fossil and was found in abundance in all seas 100 million years ago. It is sensitive to cold and can only be found today in the Red Sea and the central Indo-Pacific. In some of the local coral reefs it is the most important reef-forming coral and dominates the coral population.
Another reef-forming octocorallia is the organ coral ( Tubipora musica ), which forms a massive, cushion-shaped skeleton, which consists of parallel, vertical chalk tubes measuring one to two millimeters in diameter, which are connected in layers by horizontal plates and have a diameter of ten centimeters can reach up to one meter.
Way of life
All Octocorallia live sessile, most species and genera in the shallow water of tropical coral reefs. But caves, vertical rocks and deep sea regions are also populated.
nutrition
Octocorallia species feed on plankton, which they capture with their numerous polyps, mainly at night. In order to catch as much as possible, the fan-shaped species are usually aligned across the main direction of the water flow. Investigations on Dendronephthya hemprichi and some other soft corals show that almost only phytoplankton is ingested, so the animals are herbivorous . In doing so, they have developed a source of food whose biomass is significantly larger than that of zooplankton . Since phytoplankton is usually much smaller than zooplankton, the feathers of the tentacles are only 60 to 80 micrometers apart. Since the nettle force of the Octocorallia species is very weak, actively swimming animal plankton cannot be held at all. Only what is not actively swimming and is the right size is caught. The phytoplankton-eating genera are often colored yellow, orange or red by carotenoids . They have to get the coloring from their plant food, because only plants can produce carotenoids.
The species found in tropical shallow water, like most stony corals, live in a mutualistic symbiosis with zooxanthellae , unicellular algae that live in the skin of the coral polyps. The corals get most of the nutrients they need from the zooxanthellae. The zooxanthellae carry out photosynthesis and use up the carbon dioxide, which is converted into carbohydrates together with oxygen . The nutrients thus formed benefit the polyps' nutrition. The coexistence of zooxanthellae and coral is an example of a symbiosis in the narrower sense, as both organisms involved gain significant advantages. A total of 12 of the 23 Octocorallia families contain some or only symbiotic algae-containing genera, genera from 17 families live without zooxanthellae.
Many Octocorallia species can probably also absorb organic substances dissolved in water directly through the skin. This nutritional strategy has been proven in Xenia and Heteroxenia , two genera whose gastric space is filled with a gel-like mass and therefore has no function.
The Antarctic soft coral Gersemia antarctica shows another nutritional strategy . It bends with its tip to the bottom and dabs organic substances from the seabed like a sea cucumber . If it has grazed the area around its location, it can move around like a caterpillar and look for new feeding grounds.
Protection against predators and competitors
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Octocorallia species are eaten by sea turtles , butterfly fish , emperor angelfish and various egg snails . As sessile animals, they cannot escape or, like hard coral polyps, retreat into a massive exoskeleton. Pointed sclerites, which are relatively large in the genus Dendronephthya for example , protrude clearly from the coral surface in the contracted coral and give the animal colony the appearance of a pincushion, provide a certain mechanical protection against eating. Leather corals are also relatively well protected due to the high proportion of sclerites in the total mass of the colony. In some species of the genus Sinularia , the sclerites can make up 75% of the colony mass.
Overall, however, the mechanical protection is not sufficient. Instead, Octocorallia species rely on chemical defense. They produce various toxic and disgusting tasting substances. In a study of 150 soft corals on the Great Barrier Reef , 90% of the species showed a deterrent effect on fish, and toxic substances were detected in 60% of the species. The main toxins used are various terpenes , especially sesquiterpenes and diterpenes . The latter were poisonous for test fish at a concentration of 5 to 20 ppm . The amount of chemical defense in tissue can be quite large. In Lemnalia humesi , a sesquiterpene content of 3 to 5% was determined, in some Sarcophyton and Lobophytum species the diterpene content is up to 10% of the dry matter .
The planula larvae of the Octocorallia also seem to contain toxic substances. In experiments, the pufferfish ( Canthigaster solandri ) spat out swallowed larvae immediately.
In addition to feeding protection, the chemical substances also serve to fight for settlement space ( allelopathy ). To do this, the toxic substances are excreted into the surrounding water. Experiments in saltwater aquariums have shown that even low concentrations are sufficient to induce Acropora and Porites hard corals to expel their zooxanthellae and thus bleach them , a process that can lead to the death of the hard corals. The substances also prevent the Octocorallia colonies from overgrowing by other organisms.
Multiplication
Octocorallia, like stony corals, can reproduce both sexually and asexually. The colonies are mostly male or female. Some soft corals, for example Xenia or Heteroxenia, are hermaphrodites and have both female and male gonads . Colonies with separate sexes are called "gonochoric". Sexual reproduction occurs in two different ways, depending on the species.
Most soft and leather corals (such as Sarcophyton ) and some gorgonians simply expel eggs and sperm into the open water where fertilization takes place. Since the chances of fertilization are relatively small - the germ cells are quickly displaced by the current - it is important that the spawning act takes place in a coordinated manner at the same time. The spawning time is controlled by the phases of the moon or the water temperature. In regions like the Great Barrier Reef, where the hard corals expel their germ cells en masse at certain times, the Octocorallia also participate. The larvae that develop from fertilized egg cells live planktonic for days or weeks and then, often hundreds or thousands of kilometers away, attach themselves to hard substrate and then transform into founding polyps.
Another multiplication strategy only releases the sperm, never eggs, into the open water, usually a few hours after sunset. The egg cells stay in the polyp and are fertilized by sperm that the polyp has ingested with the surrounding water. The number of egg cells produced is much lower here. Days or weeks later, the finished larvae are ejected when they are ready for metamorphosis . Octocorals with this reproduction strategy are called "breeders". They include, for example, all Xeniidae , most gorgonians and the genera Clavularia and Briareum .
All planula larvae live planktonic for a shorter or longer period of time and settle when they have found a suitable substrate. You can still crawl around on a layer of mucus at first. Once they have finally established themselves, they undergo a metamorphosis and the tentacle-equipped primary polyp is created. This reproduces asexually and by budding more polyps gradually develop and the coral colony grows.
One form of asexual reproduction that creates a new colony is the regrowth of parts of the colony, branches and fragments that have been separated from the mother colony by external mechanical action. The risk stage of the planula larva is skipped. The probability of survival of a fragment in the Caribbean gorgonian Plexaura , for example, is said to be 25 times as high as that of the planula. The regrowth of a cluster of polyps of the genus Xenia only takes a few days, but for most other genera it takes much longer.
Tribal history and external systematics
The Octocorallia are a subclass of the flower animals and the sister group of the Hexacorallia , the six-pointed flower animals, to which u. a. Sea anemones and hard corals include. Fossilized , they are known only from a few sclerites from the Silurian and Jurassic and gorgonian axial skeletons from the Cretaceous . A single find of a sclerite from the Ordovician could also come from an octocoral. Axial skeletons of sea feathers have been known since the Triassic . The sea feathers are sometimes associated with the petalo organisms from the Proterozoic Ediacaran fauna . The reef-forming Helioporacea , to which the recent blue coral belongs, appeared in the Upper Cretaceous about 100 million years ago. Because of the very fragmentary fossil record, the tribal history of the Octocorallia is largely unknown.
Internal system
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Traditionally, the Octocorallia are divided into three orders of soft corals (Alcyonacea), the Helioporacea and the sea feathers (Pennatulacea). Characteristics of the colony shape as well as the skeletal structure were used for the classification.
- Subclass Octocorallia (eight-pointed flower animals)
- Order soft corals (Alcyonacea Lamouroux, 1816)
- "Subordination" or informal group Alcyoniina Lamouroux, 1812
- "Subordination" or informal group Protoalcyonaria Hickson, 1894
- "Subordination" or informal group Scleraxonia Studer, 1887
- "Subordination" or informal group Stolonifera Hickson, 1883
- Suborder Calcaxonia Grasshoff, 1999
- Subordination to Holaxonia Studer, 1887
- Order Helioporacea Bock, 1938
- Order sea feathers (Pennatulacea Verrill, 1865)
- Order soft corals (Alcyonacea Lamouroux, 1816)
Alcyoniina, Protoalcyonaria, Scleraxonia and Stolonifera represent a degree of colony architecture rather than monophyletic groups. They are used more as a convenience to divide the families of the Alcyonacea somewhat.
Phylogeny
The monophyly of Octocorallia is undisputed. A study from 2006 provided for the first time a phylogenetic analysis of the relationships within the Octocorallia and came to the conclusion that the soft corals and most of the families belonging to them are not a monophyletic taxon. Instead, the authors set up three main clades around the soft coral specialist Philip Alderslade , the first of which mainly consists of the actual soft corals ( Alcyoniina ) and the gorgonian-like Holaxonia . The families Alcyoniidae , Nephtheidae and Xeniidae belong to this clade . The tube coral genus Clavularia is the sister group of this clade. The second large clade includes the gorgonian-like Calcaxonia , the sea feathers (Pennatulacea), whose monophyly has been confirmed, and the blue coral ( Heliopora coerulea ). The third, much smaller group consists of the genera Anthomastus and Corallium, which were previously considered to be soft corals . The species Erythropodium caribaeorum, known to many marine aquarists as "monkey hair", as well as the genera Briareum and Telestula, are primal octocorals at the base of the family tree, compared to all other Octocorallia.
The following cladogram shows the likely internal systematics of the Octocorallia in a greatly simplified form:
| Octocorallia |
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use
The precious coral ( Corallium rubrum ), the innermost, finger-thick axial skeleton of great strength and bright red colored, is processed into jewelry. Related species with similar characteristics are used in Southeast Asia. The dead sea hand is fished and processed into fertilizer called sea hand guano .
Many Octocorallia species that live in symbiosis with zooxanthellae and are nourished by them can be kept in saltwater aquariums. They are easier to keep for beginners than hard corals. The colorful, plankton-eating species such as Dendronephthya and Scleronephthya are occasionally imported, but usually starve to death within six months.
swell
Individual evidence
- ↑ GC Williams & SD Cairns: SYSTEMATIC LIST OF VALID OCTOCORAL GENERA ( Memento of the original from March 27, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Integrated Taxonomic Information System Alcyonaria Dana, 1846
- ↑ a b c C. S. McFadden, SC France, JA Sánchez, P. Alderslade: A molecular phylogenetic analysis of the Octocorallia (Cnidaria: Anthozoa) based on mitochondrial protein-coding sequences , doi : 10.1016 / j.ympev.2006.06.010
- ↑ a b Philip Alderslade : Introduction to the Octocorallia subclass , in Svein A. Fossa, Alf Jacob Nilsen: Korallenriffaquarium , Volume 4, Schmettkamp Verlag, 1995, ISBN 3-928819-05-4
- ↑ a b c Katharina Fabricius: Ecology and nutrition of azooxanthellate octocorals , in KORALLE, marine aquarium specialist magazine, No. 12 December / January 2002, Natur und Tier Verlag Münster, ISSN 1439-779X
- ↑ a b Jürgen Lange, Rainer Kaiser: Lower animals of tropical and cold seas . Ulmer, Stuttgart 1991, ISBN 3-8001-7222-4
- ↑ Michael P. Janes, Lee Mei Wah: Octocoral Taxonomy Laboratory Manual . Results of the International Workshop on the Taxonomy of Octocorals March 20–26, 2005. University of Kerala, India PDF ( Memento of the original from January 11, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Götz B. Reinicke, Helmut Schuhmacher: Riffbildende Lederkorallen , in KORALLE, marine aquarium specialist magazine, No. 29 October / November 2004, Natur und Tier Verlag Münster, ISSN 1439-779X
- ↑ Volker Storch, Ulrich Welsch: Systematic Zoologie , Fischer, 1997, ISBN 3-437-25160-0
- ^ A b Svein A. Fossa, Alf Jacob Nilsen: Korallenriff-Aquarium , Volume 4, Birgit Schmettkamp Verlag, Bornheim, ISBN 3-928819-05-4
- ↑ Harry Erhardt, Horst Moosleitner: Mergus Sea Water Atlas Volume 2 . Page 207, Mergus-Verlag, Melle, ISBN 3-88244-112-7
- ↑ a b c d Matthias Bergbauer: Soft corals (Alcyonaria) - Selected ecological aspects. in the proceedings of the 5th International Seawater Symposium, 1999
- ^ A b Yossi Loya and Ramy Klein: Die Welt der Korallen , Jahr Verlag Hamburg, 1998, ISBN 3-86132-226-9
- ↑ Australian Institute of Marine Science: Soft corals and sea fans, Reproduction and Propagation ( Memento of the original from October 19, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Bernhard Ziegler: Introduction to Paleobiology, Part 2, Special Paleontology, Protists, Sponges and Coelenterates, Mollusks . Schweizerbart'sche Verlagbuchhandlung, ISBN 3-510-65036-0
- ^ Steven Weinberg: Mediterranean , Delius Klasing, 1996, ISBN 3-7688-0968-4
literature
- Marymegan Daly, Mercer R. Brugler, Paulyn Cartwright, Allen G. Collin, Michael N. Dawson, Daphne G. Fautin, Scott C. France, Catherine S. McFadden, Dennis M. Opresko, Estefania Rodriguez, Sandra L. Romano & Joel L. Stake: The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa, 1668: 127-182, Wellington 2007 ISSN 1175-5326 Abstract - PDF
Web links
- Octocoral Resource Network Published Octocoral Papers
