River and pond clams
| River and pond clams | ||||||||||||
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Great pond clam ( Anodonta cygnea ) |
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| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Unionidae | ||||||||||||
| Rafinesque , 1820 |
The river and pond mussels (Unionidae) are a family of mussels in the order Unionida , which also occurs in Central Europe with a total of six species in three genera. The river mussel family is found worldwide with the exception of the Antarctic.
features
River and pond mussels are relatively large mussels, the usually evenly hinged housing of which is usually over 40 millimeters long. In the case of the genus Arconaia , the casing continues to grow bent crookedly or slightly twisted after a symmetrical youth stage; when they are alive they rest on their side. The outline of the case varies from rounded, short-ovoid, elongated-ovoid, roughly triangular or somewhat trapezoidal with a vertebra that does not or only slightly protrude. The shell can be thick to very thin-walled. It is typically smooth or provided with fine grooves or folds (growth strips), but can also be roughly ribbed.
The vertebra is usually located in front of the middle, i.e. H. the rear part of the case is usually longer than the front part. The vertebra is mostly sculpted and often shows the remains of the larval housing. Often a sexual dimorphism can also be observed in the housing; the bodies of the females are usually somewhat more bulbous than those of the males. The shape of the housing or its outline is also highly variable within a type.
The lock teeth of the shell are originally elongated, lamellar formations ("schizodonte" tooth shape), consisting of two main teeth and two side teeth in the left valve and one main tooth and one side tooth in the right valve. But they can be greatly modified or almost completely regressed (tribe Anodontini). The surface line is usually entire, the foot ax-shaped.
In the gills, neighboring gill filaments are fused together to form leaf-like structures through tissue bridges ("owl lamella branch"). They are also fused on the outside with the jacket to form a (openwork) partition (diaphragm). The space between the gill leaves is divided into different chambers by partitions (septa). The outflow and inflow of water from the filtering gills is directed through these chambers. The animals are of separate sex. The brood sac (Marsupium) of sexually mature females takes up the entire area of the inner and / or outer gills.
The family is difficult to distinguish from some closely related families based on purely morphological criteria.
development
The ontogenetic development via a special larval form (glochidium larva) is characteristic of the Unionidae family, but also of the closely related family of freshwater pearl mussels (Margaritiferidae). There are only minor differences in the size and shape of the glochidium larvae.
The gametes (sperm) expelled by the male individuals , mostly agglomerated in aggregations (Spermatozeugmata), are absorbed by the female mussel with the respiratory flow and fertilize the eggs in the body. Then, unlike most marine mussels, free larvae ( Veliger ) are not released into the water, but the young larvae remain in the mother animal; the animals do brood care . The larvae sit in the interlamellar spaces between the gills. For this purpose, a special brood space, the marsupium , is formed during sexual maturity . In the Unionidae, one gill of the pair (the outer one) or all four gill leaves can be involved. Finally, a more developed larval stage, the glochidium , is released into the open water. Glochidia are parasitic stages that nestle on the skin or in the gill tissue of fish. They can not only be transported by the fish ( phoresy ), but also absorb nutrients from the tissue, so they are real parasites . Glochidia are between 70 and 350 micrometers long, two-lobed larvae that resemble an opened small conch shell. They are not actively buoyant, but they are drifted by the water current or, in the absence of a current, sink to the bottom until they are kicked up by the water current of a fish swimming by. In the species of the subfamily Unioninae, the glochidium has two strong teeth on the shell edge with which it anchors itself in the gill tissue. Toothless glochidia occur in the other subfamilies. The fish encapsulates the glochidium in a cyst. Most glochidia are host-specific, the specificity being determined by the more or less strong defense reaction of the host fish. In some species there are special formations. So the glochidia North American are Ptychobranchus subtentum wrapped in Eisäckchen that blackflies dolls, a popular fish prey imitate. If a fish bites into it, the glochidia are released and penetrate the gill tissue. The number of glochidia per mother is very variable between the species, figures vary between a few thousand (9,000 for Unio crassus , 200,000 for Unio pictorum ) up to millions, e.g. B. 200 million for Anodonta woodiana .
After completion of the development, the young mussels developed from the glochidia leave the fish host and sink to the bottom. These juvenile mussels live on or just below the substrate surface, where they actively crawl around with their feet, which are quite long in this phase. The older mussels then dig into the substrate.
Biogeography and systematics
The family of river and pond mussels occurs in all fauna kingdoms worldwide (with the exception of Antarctica). In 2007, 673 valid species were recognized in a large research project. The river and pond mussels are among the most species-rich mussel families in the world. The family is currently divided into two subfamilies with seven tribe, whereby a large number of genera could not be assigned to any subfamily or tribe ( incertae sedis ). In the overview:
- Family river mussels (Unionidae)
- Subfamily Unioninae Rafinesque, 1820
- Tribus Unionini Rafinesque, 1820
- Tribus Anodontini Rafinesque, 1820
- Subfamily Ambleminae Rafinesque, 1820
- Tribus Amblemini Rafinesque, 1820
- Tribus Lampsilini from Ihering, 1901
- Tribus Pleuroblemini Hannibal, 1912
- Tribus Quadrulini from Ihering, 1901
- Tribus Gonideini Ortmann, 1916
- incertae sedis Ambleminae
- Subfamily Unioninae Rafinesque, 1820
- incertae sedis Unionidae
The following table gives an overview of the geographical occurrence of the individual genera.
supporting documents
literature
- Michael Amler, Rudolf Fischer, Nicole Rogalla: mussels. 214 S., Haeckel-Bücherei, Volume 5. Enke Verlag, Stuttgart 2000 ISBN 3-13-118391-8 .
- Daniel L. Graf, Kevin S. Cummings: Paleoheterodont diversity (Mollusca, Trigonioida + Unionoida): what we know and what we wish we knew about freshwater mussel evolution. Zoological Journal of the Linnean Society, 148: 343-394, 2006.
- Fritz Haas : Superfamilia Unionacea. In: Willi Hennig (editor): Das Tierreich, Delivery 88. X + 663 S., Walter de Gruyter and Co., Berlin, 1969 (in the following abbreviated as Haas, Superfamilia Unionacea, with the corresponding page number)
Individual evidence
- ^ Matthias Schäfer (Ed.): Brohmer - Fauna of Germany. A book of identification of our native fauna. 23rd, revised edition. Quelle & Meyer Verlag GmbH & Co., Wiebelsheim 2010, ISBN 978-3-494-01472-2 , p. 95.
- ↑ Fluted Kindneyshell on Unio Gallery, University of Missouri
- ^ Gerhard Bauer & K. Wächtler: Ecology and Evolution of the Freshwater Mussels, Unionoida. Ecological Studies, Vol. 145. Springer Verlag. ISBN 978-3-540-67268-5 . Table 6.1 p.95.
- ^ Daniel L. Graf & Kevin S. Cummings (2007): Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida). Molluscean Studies 73 (4): 291-314. doi : 10.1093 / mollus / eym029
- ↑ Mussel project poster
- ^ Daniel L. Graf: Palearctic freshwater mussel (Mollusca: Bivalvia: Unionoida) diversity and the Comparatory Method as a species concept. Proceedings of the Academy of Natural Sciences of Philadelphia, 156 (1): 71-88, 2007
