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(Introduction Mollusca-Bivalvia
Bivalves (8,000 species) are the second richest mollusc class after the Gastropoda (60,000 species). Freshwater bivalves live in rivers and lakes around the world except Antarctica. They represent three subclasses, 19 families, 206 genera and about one thousand species (Bogan 2008). European freshwater bivalves belong to the orders Unionoida and Cardiida (Carter et al. 2011). All the European unionoids are included in the superfamily Unionoidea, the freshwater mussels or naiads. The European cardiids belong to the following three superfamilies: Cardioidea, Cyrenoidea and Dreissenoidea. Among the Unionoidea (families Margaritiferidae and Unionidae) there are the most imperilled animal groups on the planet while the Cardioidea includes the cosmopolitan genus Pisidium (family Sphaeriidae); the Cyrenoidea includes the Asiatic clam (Corbicula fluminea) (family Cyrenidae) and the Dreissenoidea includes the famous invasive zebra mussel (Dreissena polymorpha) (family Dreissenidae). Most freshwater bivalves are filter or/and suspension feeders, living buried in the substratum where they can move thanks to their muscular foot and can clear large quantities of water. Sphaeriids can creep among weeds and rocks and unionoids can move few meters in one day. The zebra mussels live adhered to the substratum by a byssus.
Family Sphaeriidae
Bogan 2008 recognizes 34 species of this family in the Palearctic. Species identification in European Sphaeriidae (Fig. 2) can be as difficult as in Unionoida, and the miniaturization of the species of the Family Sphaeriidae can complicate the taxonomic task. In this group, the shell shape, position of the umbos, sculpture, tumidity, porosity, texture, lustre and thickness should be useful characters, although the hinge gives normally the principal key for a correct identification. The most important diagnostic hinge characters are: shape of cardinal teeth, shape and length of lateral teeth, length of the hinge plate (relative to overall shell length), shape of the ligament pit and the presence or absence of callus (Ellis 1978, Killeen et al. 2004). Molecular taxonomy has not yet reached the European sphaeriids as has happened with other faunas (Lee and Foighil 2003).
Pea clams (Sphaeriidae) are ubiquitous in freshwater ecosystems; they have a minimal size of 2 mm and a life span of one year. They are always hermaphrodite and incubate the fertilized eggs into their gills until the juvenile stage (Araujo and Ramos 1997, Araujo and Ramos 1999).
Family Cyrenidae
Often also named by its synomy Corbiculidae. In Europe only lives the genus Corbicula (). The number of species considered under this genus is not yet known, we recognize two hyper variable species, Corbicula fluminea (Müller, 1774) (Fig. 3) and Corbicula fluminalis (Müller, 1774), although their taxonomical status is not clear. In the 20th and 21th centuries Corbicula clams were introduced in North America, South America, Europe and North Africa. Their populations are also hermaphrodite with two different reproductive strategies: incubate the fertilized eggs into their gills until the juvenile stage or can present planktonic larvae (Korniushin and Glaubrecht 2002).
Family Dreissenidae
In Europe live two genera and two species, Dreissena polymprpha (Fig. 4) and Mytilopsis leucophaeta, both famous invasive bivalves. Although they are considered freshwater molluscs, they can survive in brackish water, for example Dreissena in the Black and Caspian seas or Mytilopsis leucophaeta in North America, where autochthonous populations live in ecological equilibrium (Heiler et al. 2010, Kennedy 2011). Since the XIX century, the zebra mussel has spread to the European continent helped by river transport, as the first mass invasion of a Ponto-Caspian species in Europe (Bij de Vaate et al. 2002). In 1980s, mussels transported on cargo ships caused the spread of D. polymorpha to the North American continent. Species on this family can be identified by its mytiliform form, sometimes quadrate, with the hinge edentulous and the umbos anterior or terminal. The umbo cavity is bridged by a septum or myophore. They have a byssus which allows them to form dense colonies. They have separated sexes and planktonic larvae.
Recentrly Bilandzija (Bilandžija et al. 2013) have described two new species of the genus Congeria. In this way, the family Dreissenidae in Europe also includes the genus Congeria with three species, C. kusceri, C. jalcizi and C. mulaomerovici, all living in caves of the Dinaric karst.
Families Unionidae and Margaritiferidae
The assemblage fauna of European Unionoida includes only two familes, Margaritiferidae (Fig. 5) and Unionidae (Fig. 6), is not very diverse in comparison with other areas of the world, for instance the Nearctic, with 300 species and where a single river may have more species than all those in Europe. In the Palearctic there are recognized between 60 and 100 species of Unionoida (Graf 2010, Graf and Cummings 2013, Bogan 2008). Up to the beginning of the twentieth century, the number of described taxa in Europe was about 1,500 species due to an overestimation of species richness based on shell characters (Graf 2010). Since then, taxonomy and systematics of European freshwater mussels were reconstructed starting with the seminal paper from Haas (Haas 1969) who considered 58 taxa in the West Palearctic. Currently, this figure is changing reflecting the use of molecular taxonomic tools which are unrevealing previously hidden lineages (Araujo et al. 2009, Reis and Araujo 2009, Prié and Puillandre 2013). Here we consider 16 native species, belonging to two families and six genera, but the work in not yet finished, with many of the Haas (Haas 1969) subspecies needed to be clarified.
Freshwater mussels, also known as naiads, can grow to lengths of 25 cm and live more than a century. They are one of the most imperilled animal groups on the planet, yet they play an extremely important role in the ecology of freshwater ecosystems as a main component of the freshwater biomass (Vaughn and Hakenkamp 2001, Strayer et al. 2004). The dramatic changes taking place in freshwater ecosystems during the last century have played a part in the large-scale disappearance of these and other animals (Lydeard et al. 2004). Some species fulfil criteria of indicator, flagship and umbrella species, making them ideal targets in aquatic conservation, as it is the case of the freshwater pearl mussel in Europe Margaritifera margaritifera (Geist 2010).
In addition, one of the most amazing traits about freshwater mussels is their specialized reproductive strategy: the eggs are fertilized in the mussel gills (marsupium), where also occurs the segmentation until the glochidium, which has a temporary but obligatory parasitic stage in which the larvae (glochidia) attach to the external surface of a suitable host prior to metamorphosis to the free-living juvenile stage. The males release to the water the sperm, which will be siphoned for the females for fertilization (Kat 1984, Bauer 1987).
The form of the Unionoida shell can vary according to the biotype, the environmental influences giving rise to changes through which the identification of a shell can be made more difficult.