Lister's River Snail - Viviparus contectus
This remarkable freshwater snail is a slow and serene animal which may be easily identified thanks to its large size. One may also spot the species in dried-out bodies of water, since this snail can use its horny plate-like operculum to seal its own shell.
The Lister's river snail has a thick shell with a pointy spire; the shell may be up to 40mm high and 30mm wide. The whorls are clearly layered and the umbilicus is not sealed. Colouring is normally greenish-brown to black, with three reddish-brown bands. The operculum (corneous plate) is also used for hibernation in winter. The front part of the head protrudes like a trunk, with feelers located right and left. Males and females have varying characteristics: the right feeler of the male also serves as an insemination instrument and is shorter and fatter than the left feeler. While females have two feelers of the same size, overall they grow to be larger than the males.
Viviparus contectus is found across Central and Eastern Europe yet is rarely found in the Alps or low mountain ranges. It prefers moderately flowing waters in rivers and the shores of lakes. The Donau-Auen National Park is thus an ideal habitat for the Lister's river snail.
Endangerment and Conservation Status
This species is classified as endangered (EN) on the Austrian Red List and has suffered from human intervention in its habitat, for example through the building of dams and the drainage of wetlands and other bodies of water.
This species has a varied diet, feeding on decaying plants, grazing on carpets of algae and bacteria, consuming carrion and other decaying matter. The Lister's river snail feels at home in both muddy and sandy substrates as well as on gravel or stones. Normally the snail crawls along the ground on stones or wood; on sandy surfaces it tends to dig itself in, either to rest or to protect itself from predators. One interesting fact is that this mollusc is the only freshwater snail which can survive as either a filter feeder or a grazer. In filtration, the snail forms a mucous film which helps trap plankton and other food particles. Once this net is full it rips open, and its clumped contents are eaten in their entirety, including the filtered matter. If a spot is good for trapping food, the snail will often remain there for some time. During the winter, this species digs down sufficiently deep into the mud.
Unlike other snails, the Lister's river snail does not lay eggs, but instead gives birth to live young which already have a shell of around 5mm. This is in fact where the scientific name Viviparus comes from, e.g. "living birth". The egg capsules develop in the uterus of the mother and embryos are nourished by a protein-rich liquid. During the summer, the female snail carries up to 30 developed juveniles and gives birth to each individually, with the oldest always born first.
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