breathing with gills (Introduction, Fresh Water Nerites, Mud Snails, Bithynias).
Freshwater snails breathing with lungs (Pond snails, ram's horn snails, fresh water limpets, bladder snails).
Mollusc species numbers. Diagram.
Snails and mussels are the only molluscs that live not only in the sea, but also in the fresh waters on the continents – rivers and creeks, ponds and lakes. While life conditions are quite constant in the vast reaches of the sea (except from the littoral zones), life in fresh water displays constant changes of water supply, temperature and currents, as a result the habitats there are a mosaic of small ecological spaces and so oblige their inhabitants to adapt to the ecological conditions they provide.
In the course of evolution groups related to each other may be isolated by ecological adaptation. The isolation may even go so far that mating is not possible any more – a new species has evolved. It is because of this that fresh water molluscs, mussels and snails, are much richer in species than are their marine relatives.
So, contrary to the other, exclusively marine, molluscs, the number of species in snails and mussels is much larger: Of the 55,400 mollusc species known to science, about 53,000 are gastropods or bivalves, of these about 10,000 bivalves and 43,000 gastropods - over three quarters of all mollusc species.
Common river nerite (Theodoxus fluviatilis).
Picture: Vollrath Wiese (Source).
Comparing some European species of fresh water gastropods, it becomes obvious, that relationship is closer between some freshwater groups and certain marine gastropod groups, than to other groups of fresh water gastropods. For this reason, it is assumed today that the transition of gastropods from sea into fresh water has happened several times independently during evolution, thus creating different groups of fresh water snails, which are no more closely related to each other, than all being gastropods.
Spring Snails, Mud Snails, Bithynias and their Relatives (Rissooidea)
Valve snails (Valvatidae)
Melanopsidae (e. g. pitch snails)
The common river nerite, (Theodoxus fluviatilis L. 1758) is a good example to see how the transition into fresh water could have looked like in earth history. There is one subspecies of river nerite, Theodoxus fluviatilis littoralis, that lives in Pomerania on the Baltic Sea coast. The astonishing thing is that this snail species lives in brackish water, where fresh water and sea water mix in a river mouth, so this snail species' adaptation to salinity is very sophisticated. This is very energy intensive, which is why this snail species has got a very thin shell, compared to its relatives in the sea, in fresh water and on land. Concerning the current assessment of the subspecies' status see: Common river nerite (Theodoxus fluviatilis).
Nerites are primordial snails with a thick-walled half-oval shell, whose few whorls expand fast to the final diameter of the aperture. Additionally they tend to melt with growing age, so the animal then is hardly recognisable as a snail. The pattern of dots and stripes is so variable that species from different regions can hardly be determined except by their characteristic operculum, the shell lid.
The preferred habitats of the fresh water nerite are large flowing waters with hard underground. Here the nerite lives on diatoms - silicate algae. To eat the contents, the nerite has to crush the diatom's hard silicate armour on the ground. Nerites breathe using gills. They have separate sexes and lay egg capsules on the ground, as well as on other gastropods' shells.
Concrete constructions on water flows, as well as water pollution have deprived the nerite of its feeding sources, so that today it is found on the red lists of endangered species. For that reason the common fresh water nerite has been selected "Mollusc of the Year" in 2004. Other species of fresh water nerites are the Danube nerite (Theodoxus danubialis) and the striped nerite (Theodoxus transversalis).
More about nerites (Neritidae).
Acute mud snail (Viviparus contectus). [RN]
A juvenile Viviparus contectus. Note the long hair on the shell!
Picture: Micaela Brugsch.
In contrary to the nerite snail the mud snail has got a bulbous shell coiled in a classical snail's spiral, whose aperture, like a nerite's, is closed, if need be, by an apertural lid (operculum). Mud snails belong to the Ampullariacea superfamily, they are related to the tropic apple snails (Ampullaria) also often kept in aquariums.
In Europe there are different Viviparus species, most of which live in the flowing waters of lowland rivers and on the banks (littoral) of large lakes. Only the acute mud snail (Viviparus contectus) also lives on the bottom of large lakes rich in plant growth, in ponds, backwaters and in swamps.
Mud snails are herbivores; they eat plants from the lake's floor. Mud snails breathe using gills. From their breathing water they can also filter nutrients, like a mussel does. This is why at a place rich in nutrients, they sometimes stop and rest for a longer time.
On the search for food juvenile acute mud snails (Viviparus contectus) crawl hanging upside down from the water surface, like pond snails (Lymnaeidae) do.
Acute mud snail (Viviparus contectus). The lobe-like organ in
the centre is the ingestive siphon. The right tentacle has de-
veloped into a copulation organ - this is a male snail.
Picture: © Alexander Mrkvicka, Vienna (mrkvicka.at).
frontal part of a mud snail's head is elongated to a prominent proboscis, left and right of which are the two tentacles. To feed, the mud snail, like other snails, uses its rasp tongue (radula) to rasp food from the ground.
J. Ramsauer: Radulae - Electron microscope picture from the Salzburg University, among others also of Viviparus contectus.
Mud snails have separate sexes – among some species there even is a clearly visible sexual dimorphism, the female being much larger and their shell more bulbous than the male's.
The male mud snail's right tentacle is modified to a copulation organ. The mud snail's scientific name, Viviparus, is due to their bearing live young: Mud snails are ovoviviparous: Their young hatch from the eggs in the female's body and are born alive (see picture on the right). Juvenile acute mud snails (Viviparus contectus) have got long hair on their shells, which enhance camouflage by binding earth particles to the shell and thus concealing it from its environment.
Ovovivipary is a phenomenon that also occurs among certain land snail families (for example door snails, Clausiliidae). The difference to real vivipary, as found among mammals, is that the eggs are built, the young hatch, but all that happens in the female's body and the young are born only when the conditions are favourable.
Picture: Prof. G. Ribi, Zurich University
Wolfgang Fischer: Checklist of the European Viviparidae, recent and fossil species.
Common Bithynia (Bithynia tentaculata). Picture: Lars Peters.
The country of Bithynia (Greek: Βιθυνία, Bithynia) in Antiquity was found on the peninsula of Asia Minor. It is located in the Northwest of today's Turkey, on the south coast of the Marmara Sea, between the Bosporus and the Black Sea.
Bithynias (Bithyniidae), owing their name to this ancient landscape, are well distributed on all continents, except on the Americas, where only Bithynia tentaculata has been introduced. While in the Mediterranean there are further species of Bithynias, only three are native to Central Europe: The common Bithynia (Bithynia tentaculata), Leach's Bithynia (Bithynia leachi) and the eastern Bithynia, concerning which there is still dissent, whether it should be called correctly Bithynia troschelii or Bithynia transsilvanica.
The oval or conical shell of a Bithynia is closed by an operculum. Bithynias breathe using gills. Like mud snails, they as well are able to filter their breathing water for nutrients. Additionally they feed on decaying plant matter and other organic detritus.
Before laying its 20 to 40 single eggs, the female Bithynia cleans the underground from stones, mussel shells and algae. Then with its foot, it arranges the eggs to a band of spawn.
Bithynias are a part of the Rissooidea, a superfamily especially rich in species, so they are related with spring snails (Amnicolidae), mud snails in the Wadden Sea and the dwarf fresh water mud snails (Hydrobiidae), and finally the gravel snails (Lithoglyphidae).
More about Bithynias.
Comparing the three species of fresh water gastropods, it is obvious, that they all breathe by gills and have an operculum at their foot tip. They only have two tentacles, at the base of which the eyes are placed. Their head is elongated to a prominent snout or proboscis. All described snail species have separate sexes – there are sometimes very different males and females. As a special thing the young of a mud snail are born alive, though they hatch inside the female mud snail's body (ovovivipary).