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Lesser Freshwater Mussels

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Introduction

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Fingernail clam (Sphaerium corneum).
Photo: © Alexander Mrkvicka, Vienna.		  
 
Fingernail clam (Sphaerium rivicola). Photo: M. Kohl.
The expression Lesser Freshwater Mussels unites those freshwater mussels not a part of the larger mussels of the Unionoidea order. They also are noticeably smaller than those sometimes giant species.

European lesser freshwater mussels belong in the order Veneroidea. Another part of this order also are the saltwater-living (marine) Venus clams (Veneridae), which owe their name to the Roman goddess of love, Venus. One example is the "Vongola" (Chamelea gallina), which lives from Northwestern Europe as far as the Mediterranean. The limnic (freshwater-living, as opposed to the saltwater-living marine species) members of the Veneroidea order in Europe comprise two superfamilies: The fingernail clams (Sphaerioidea), as well as the introduced zebra or wandering mussels (Dreissenoidea). Among those, there are the Zebra Mussel (Dreissena polymorpha) and the Quagga Mussel (Dreissena bugensis). Both of those inflict noticeable damage in European and North American waters, as well on water infrastructure such as tubes and outlets, as in the natural ecological equilibrium by displacing local species.

The Asian Clam (Corbicula), introduced more recently, also are a part of the fingernail clam group (Sphaerioidea).

Fingernail Clams (Sphaeriidae)

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Fingernail clams are small mussels present in fresh and brackish water. The Sphaeriidae in Europe contain fingernail clams s. str. (Sphaerium), as well as pea clams (Pisidium).

MolluscaBase eds. (2025): Sphaerioidea Deshayes, 1855 (1820).
Naturportal Südwest: Sphaeriidae (Kugelmuscheln) (In German).
Fechter, R.; Falkner, G. (1990): "Weichtiere", Mosaik-Verlag München, p. 268 f. (In German).


The foot of a Pea Clam (Pisidium spec.) is long and lancet
shaped. Photos: © Alexander Mrkvicka, Vienna.		  
A pea clam's siphons are short and protrude only a short way
over the mantle rim. In fingernail clams, they are much longer.		  
In both genera, the pallial seams have joined to form long, tube shaped siphons (see pictures). Fingernail clams have a long, tongue-shaped foot, which they can use to move like a snail. They also move by getting hold of a water plant and then pulling themselves towards it.

Common Fingernail Clam (Sphaerium corneum): Picture by Vladimir Semaschko on INaturalist.

Fingernail clams are hermaphrodites, which keep their 2 to 15 eggs inside the mother's gills, until the juveniles are fully developed. Central European species are Sphaerium corneum and Sphaerium rivicola. Under good conditions, the latter can grow to 2 cm shell length, making it the largest European Sphaerium species

Pea clams (Pisidium), on the other hand, are smaller and more spherical than fingernail clams. With the exception of Pisidium amnicum, which can grow to a size of over 1 cm, all pea clams stay noticeably smaller than 1 cm. Pea clams are found in lakes and ponds in depths of more than 40 m and can sustain cold and dryness. For this reason, pea clams are sometimes also be found in puddles and the group is spread as far north as Lapland. Because of the high ecological variability, pea clams are quite hard to identify on species level.

Pea Clams (Pidisium) from the Timavo Spring near Triest.

Zebra Mussels (Dreissenidae)

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Zebra mussels (Dreissena polymorpha). Photo: Lars Peters.
Zebra mussels are found in western Eurasia and along the American East Coast. From the Tertiary, the group is known by numerous species, however, by today, only ten species are left. The Zebra Mussel (Dreissena polymorpha) is also known as Wandering Mussel or Three-edged Mussel (translated literally from German). Its original area of distribution is the Pontic part of the Black Sea coast. There it lives in rivers that are tributaries to the Caspian and Black Sea.

MolluscaBase eds. (2025): Dreissenoidea J. E. Gray, 1840.
Naturportal Südwest: Dreissenidae (Dreikantmuscheln) (In German).
Fechter, R.; Falkner, G. (1990): "Weichtiere", Mosaik-Verlag München, p. 268 f. (In German).


Zebra Mussel colony on the lake floor (Dreissena polymorpha).
Photo: © Alexander Mrkvicka, Vienna.		  
It was not until the last 200 years that the Zebra Mussel has spread far outside of its original area of distribution. With economic ship traffic it has even reached the American continent. In some rivers and lakes, all stationary objects, as well as hard shelled creatures, such as crayfish and other mussels, are entirely overgrown by zebra mussels.

The Zebra Mussel develops via a planktonic larval stage resembling the veliger larva of marine mussel species. By river currents, this larval stage is distributed further. Juvenile zebra mussels build a byssus thread, same as the marine blue mussels (Mytilidae). Up to a certain age, the juvenile mussels can cut the thread and move to a different location. Then the foot is reduced and the mussel remains in the same place for the rest of its life. The adult mussels, however, can distribute further, if the underground they are sitting moves. So, zebra mussels move together with the colonies of other bivalves they are latched onto, as well as with ships they are attached to. Also, zebra mussels can move their entire colony by flapping their shell halves to turn their colony or to move it into more nutrient-rich water. Except for that they overgrow other animals, suffocating them, they can also inflict considerable damage by clogging tubes and pumps and by stopping generators. So far, there is no feasible method to get rid of zebra mussels effectively.

Molluscs With Immigration Background (Neobiota) on molluscs.at.

Another related species that was introduced from Eastern Europe into Central Europe as well as to Northern America by ships and boats to settle many rivers, lakes and other water bodies, is the Quagga Mussel (Dreissena bugensis), which in a similar manner inflicts considerable damage.

Further Information on the Quagga Mussel (Dreissena bugensis).

In the area of the Great Lakes in North America, the induced mussel species of the Dreissenidae family have become a large problem (see above). Apart from the Zebra Mussel (Dreissena polymorpha, since the 80s), which was introduced by ballast water in transport ships, the Quagga Mussel (Dreissena bugensis) likewise is found in very large numbers. The mussels have overgrown suitable substrates almost entirely and replaced domestic mussel species. They are, however, controlled by another introduced group, the Gobies (Fam. Gobiidae), which mainly feed on the pelagic larvae of the mussels. The gobies can sustain their populations due to their large offspring numbers, even if they are prey to many other kinds of fish. Other predators of zebra mussels also are Muskrats (Ondathra zibethicus), which feed on large amounts of zebra mussels, leaving the shells behind in considerable heaps found at the waterside. In Europe, muskrats were notorious as mussel predators, especially to the already threatened larger mussel species, such as the River Pearl Mussel (Margaritifera margaritifera), which from an ecological point of view, is rather alarming.

Great Lakes Now: Quadrillions of Mussels Are Invading the Great Lakes. ( YouTube Video).
Great Lakes Now: Invaders on the Menu. ( YouTube Video).

 
Ohrid Dreissena (Dreissena stankovicii).
Photo: © Alexander Mrkvicka, Vienna.

Geographical location of Lake Ohrid. Source: Wikipedia.		  
In Lake Ohrid in Macedonia (at the border to Albania and Greece), apart from a considerable number of other endemic mollusc species, there is also an endemic species of Dreisssena clam: Dreissena stankovicii lives in depths between 20 and 40 metres and is especially noticeable due to its black mantle and the thread-like mantle protrusions. Lake Ohrid is a very old lake (min. 2 mio. years) and home to at least 40 endemic species of molluscs and numerous endemites from other animal groups.

Dreissena Clams from Lake Ohrid.

Asian Basket Clam (Corbicula)

Another introduced (neozoic) bivalve group is the Corbicula genus. This, originally Asian mussel group, in German also referred to as Körbchenmuscheln (Basket Clams), has been spreading into the Central European river systems from the rivers Rhine and Weser since the 1980s. The latest proven findings are from the Upper Rhine near Waldshut-Tiengen and from Lake Constance. In the Danube River, Corbicula has been known for longer, a first finding published in 1997.

According to more current understanding and as published on MolluscaBase (see below) the Corbiculidae family cannot be maintained, and the Corbicula genus thus is placed in the Cyrenidae family.

More about the Asian Clam (Corbicula fluminea).
MolluscaBase (2025): Cyrenidae J. E. Gray, 1840.
Naturportal Südwest: Corbiculidae (Körbchenmuscheln) (In German).

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Latest Change: 06.08.2026 (Robert Nordsieck).