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Gastropods and bivalves are the only molluscs that, apart from the sea, can be found also in fresh water: In rivers and creeks, ponds and lakes. While life conditions in the sea, perhaps except for the coastal areas, are rather uniform, habitats on land show a mosaic of small ecological spaces, varying with area and time and so requesting a strong ability to adapt from their inhabitants.
 Painter's mussel (Unio pictorum). Picture: Niels Sloth, Biopix.dk. |
In the course of evolution this has caused the consequence that groups related to each other become isolated by ecological adaptation, which leads to the development of species and to the evolution of a large number of species especially adapted to the varying ecological conditions. Because of that, terrestrial and fresh water snail species are much more numerous, the same is the case with bivalves.
Compared to the exclusively sea-living rest of the mollusc classes, gastropods and bivalves make by far the largest portion of species (see table). Of the about 55,400 species of molluscs known as of today, gastropods and bivalves contain 53,000 species, of which 43,000 are gastropods, the only class also living on land – that is more than three quarters of all known mollusc species.
The Central European fresh water bivalves by their size and form can be divided in two groups, all of which have different roots in sea-living groups:
 Duck mussel (Anodonta anatina). Picture: Niels Sloth, Biopix.dk. |
The relatives of river mussels (Unionacea) are the largest molluscs on Earth outside of the sea. The largest Central European species is the large pond mussel of swan mussel (Anodonta cygnea). It may grow to a size of up to 26 cm. Among the relatives of river mussels, apart from pond mussels, there are also the river mussels (Unio) and the fresh water pearl mussels (Margaritifera). Especially concerning their larval development, those mussel groups are very different from their sea-living relatives. They develop past a parasitic larval stage called a glochidium. To be able to develop into juvenile mussels, glochidia need a certain species of host fish in whose tissue they can remain to grow.
The relatives of river mussels also are referred to as naiads. This uncommonly poetical term points to the world of ancient Greek mythology, where naiads were fountain goddesses, among other things seeing to the cleanness of their creek. In a similar way, like blue mussels in the Wadden Sea, the large river mussels have a great importance for the cleanness of the water they live in.
Further information on large fresh
water mussels.
 Zebra or wandering mussels (Dreissena polymorpha). Picture: Lars Peters. |
Pea mussels and fingernail clams (Sphaeriacea) on the other hand are the smallest mussels on Earth, the largest Central European species is the river fingernail clam (Sphaerium rivicola) with a shell maximally 25 mm in size. Most pea clams are noticeably smaller; the largest pea clam species is Pisidium amnicum with 7 – 11 mm. Fingernail and pea mussels are hermaphrodites and reproduce by ovovivipary, larvae hatching inside the female's body. Juvenile clams are finally born.
The relatives of wandering mussels (Dreissenacea) were much richer in species during the Tertiary. Similar to blue mussels, they produce a byssus thread, by which they can attach themselves to the ground. That way the wandering mussel (Dreissena polymorpha) was able to spread on ships from the Caspian Sea region over all of Europe, even to America.
 Trigonia sp., a fossil bivalve from the Trigoniacea, today's large mussels' ancestors. Picture: Armin Bauer. |
Today this bivalve species invades waters almost everywhere and may become a pest, as it not only overgrows stones and rocks, but also other mussels, crayfish and other animals. The wandering mussel may also inflict economical damage, when it jams water power plants and dams.
Further information on lesser fresh
water mussels.
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Systematic Overview: Class Bivalvia
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Systematically, larger and lesser mussels are in two different subclasses of the mollusc class Bivalvia.
The larger mussels are in the Palaeoheterodonta subclass evolved since the Mesozoic. Present day's only sea living relatives of this subclass are the Trigoniacea, a small group, the remainder of which can be found near Australia.
The relatives of river mussels make an own order, the Unionoida.
In contrary to that, the lesser mussels belong to the subclass Heterodonta, together with sea-living relatives, such as clams (Veneracea) and cockles (Cardiacea). The relatives of fingernail clams (Sphaeriacea) and wandering mussels (Dreissenacea) together with Venus clams belong to the Veneroidea order.
