Unionidae

Common swan mussel ( Anodonta anatina )

The river mussels ( Unionidae ) are a family of bivalve molluscs in order Unionoida, which includes, inter alia, the three native to Central Europe genera Unio, Anodonta and Pseudanodonta. With a total of six species, this family is represented in Germany.

Features

River mussels are usually relatively large clams with shell length of about 40 millimeters. The dish can be thick-walled or very thin, it is typically but not coarse grained, but smooth or with fine grooves ( growth lines ) provided. The vortex ( the area where the related bivalve shells) sitting in front of the center position, and the rear part of the shell, the shell is asymmetrical, longer than the front. Most species have an elongated oval or slightly trapezoidal shell outline with little or no previous vertebral, but there are also species with rounded or short oval shape. The teeth of the lock shell are usually elongated lamellar formations ( " schizodonte " tooth shape ), they can be completely regressed ( subfamily Anodontinae ).

River mussels are among the mussel families in which gills neighboring Kiemenfilamente have grown by tissue bridges together to sheet-like structures ( " eulamellibranche " gills), moreover, they are on the outside of the casing to a ( broken ) partition ( diaphragm) grow together. The space between the gill leaves is divided by partitions ( septa ) in different chambers. Thus, the out - and inflowing water stream of filtered gills is directed.

The family is difficult to define by some relatives on purely morphological criteria.

Development

The development path is for the family of highly characteristic and comes in a comparable form only in the closely related families such as the freshwater pearl mussel ( Margaritiferidae ) ago.

The ejected by the male individuals gametes (sperm ), usually in aggregations ( Spermatozeugmata ) clustered together, are taken up by the female mussel with the respiratory flow and fertilize the eggs internally then are not like most marine shells free larvae ( veliger ) into the water issued, but the young larvae remain ( parental care ) in the mother. The larvae sit in the interlamellar spaces of the gills. This requires a special, partitioned off by septa brood chamber, the marsupium is formed. The Unionidae just one gill of the pair is involved (the outer ). Finally, an advanced larval stage, the Glochidium, released into the open water. Glochidia are parasitic stages that attach themselves to the gills of fish tissue. And let the fish not only transport ( Phoresy ), but also take nutrients from the tissue, so are true parasites. Glochidia are 70-350 microns long, bivalve larvae, resembling an unfolded small conch shell. They are a little active buoyant, usually they can but fall to the ground until they are stirred up by the flow of water past a floating fish. For species of the subfamily Unioninae the Glochidium has the shell edge two strong teeth that anchor themselves in the gill tissue, toothless Glochidia occur in the other sub- families. The fish encapsulates the Glochidium into its gill in a cyst. Most glochidia are host specific and only accept a few related species as a host. In some species, special formations occur. Thus, the glochidia of the North American Ptychobranchus are subtentum wrapped in Eisäckchen, the blackflies dolls, a popular fish prey mimic. Biting into a fish, the glochidia are released and penetrate into the gill tissue. The number of glochidia per dam is highly variable between species, estimates range from several thousand ( 9,000 in Unio crassus, 200,000 at Unio pictorum ) up to millions values, eg 200 million in Anodonta woodiana

After completion of the development which developed from the glochidia juvenile mussels leave the fish host and let himself fall to the ground. This young mussels living on the substrate surface on which they are quite active crawl about by means of cilia. The older shells then bury themselves into the substrate.

Biogeography and systematics

Unionidae are worldwide in all Faunenreichen ( except Antarctica ) before. In a large research project valid species are recognized currently 673. The river mussels are therefore among the most species-rich mussel families worldwide. The family is currently divided into three sub-families, where a number of species can not be assigned them ( incertae sedis ). In the following list all species are taken into account, that occur in Europe.

• Subfamily Ambleminae. 342 species. Only in America ( Neotropics ), no European Art
• Subfamily Anodontinae - pond mussels Genus Anodonta Lamarck - pond mussels Anodonta anatina ( Linnaeus), 1758 ( = piscinalis Nilsson ) - Common pond mussel
• Anodonta cygnea ( Linnaeus), 1758 - Swan mussel
• Anodonta woodiana (Lea, 1834) (syn: Sinanodonta woodiana ) ( neozoon, introduced from East Asia )

• Potomida littoralis ( Cuvier, 1798)

• Pseudanodonta complanata Rossmaessler - Flattened duck mussel

• Microcondylaea compressa ( Menke, 1830).

• Genus Unio Philipsson, 1788 - river mussels Unio crassus Philipsson, 1788 - river mussel
• Unio mancus Lamarck, 1819
• Unio pictorum (Linnaeus, 1758) - painter's mussel
• Unio Tumidus Philipsson, 1788 - Great River mussel

In Spain may come two more species added, and Unio Unio ravoisieri durieui

The systematics of the foot shells is sometimes messy and difficult penetrable. While in some regions, particularly tropical, hardly are aware of all kinds, especially the European fauna suffers more at About descriptions. Many researchers in the past and z.T. Presence represented here very narrow species concepts, has been where very large weight given to minor modifications in the form and shape of the shell. This is, although hereditary factors known to be involved in a very large extent due to variable environmental factors ( phenotypic plasticity ). In the Western Palearctic be, for example, currently three species of the genus Anodonta recognized (plus an introduced from East Asia ). In this fauna but 577 species have been described to date, which must now be considered as synonyms to these three. An agent saw in his revision here even on only a single type.

Today, DNA sequences are used to delimit species ( molecular phylogeny ). However, when using the gene segments often routinely used mitochondrial DNA caution is required here. Due to the high developmental age of the group ( the order is attested since the Triassic ) here are numerous sections due to the numerous mutations phylogenetically no longer meaningful, their comparison corresponds to that of random sequence.