Bladder Snails (Physidae)

 
Acute bladder snail (Physella acuta).
Picture: Alexander Mrkvicka, Vienna.

Systematics

Class: Gastropoda
Subclass: Pulmonata
Superorder: Basommatophora
Order: Hygrophila
Suborder: Branchiopulmonata
Infraorder: Planorboinei
Superfamily: Physoidea
Family: Physidae Fitzinger 1830

Source: Mollbase on http://www.mollbase.de/list/.

Systematics of Gastropoda: Informal Group Pulmonata: Physidae.

Bladder snails (Physidae) and ram's horn snails (Planorbidae) have in common long and thin pointed tentacles. Recent discoveries in molecular genetics support a close relationship of those two families. They are different, though, in spite of the external resemblance, e.g. between bladder snails and the African planorbid genus Bulinus, in that bladder snails always are coiled to the left (sinistral), which is quite rare in snails. Besides, they lack the secondary gill visible as a lobe-like organ protruding from the shell aperture. Bladder snails are very fast, taking in account that they are snails, as one can clearly see in an aquarium, when they crawl through the water at an amazing speed. The shell of a bladder snail is oval to turricular, shiny and smooth.


Bladder snail (Physidae family) floating at the water surface's
underside. Picture: Vollrath Wiese.
 

Bladder snails mainly live in stagnant or slowly moving waters. Some species can even tolerate less than optimal water quality due to Eutrophication, resulting in increasing nitrate concentration or acidity (pH value). Bladder snails feed on devaying plant matter and detritus, but also on algae which are grazed off water plants and stones. Like pond snails (Lymnaeidae), bladder snails can also crawl hanging from the water (floating) to feed on algae growing there.

Eutrophication.

Originally, bladder snails were distributed over the Holarctic (see Faunal provinces of the Earth), appearing especially far south on the American continents, as far south as central and south America. In the meantime, many species have been introduced to other parts of the world by human interference, such as the species introduced to central Europe from America, Physella heterostropha and Physella gyrina, the latter of which is often confounded with the native acute bladder snail (Physella acuta).

Today's assumption is that bladder snails have been present at least since the Triassic (210 to 250 mio. years ago, see Geological Timeline), but those results are not certain. What is certain, are fossil bladder snails from the Miocene, the lower Pannonium (10 to 11 mio. years ago) of the Vienna basin.

Harzhauser, M.; Binder, H.: Synopsis of the Late Miocene mollusc fauna of the classical sections Richardhof and Eichkogel in the Vienna Basin. Arch. Moll. 133: 1-57 (2004)

Moss Bladder Snail - Aplexa hypnorum (Linnaeus 1758)

 
Moss bladder snail (Aplexa hypnorum).
Picture: Alexander Mrkvicka, Vienna (mrkvicka.at).

Description: The moss bladder snail has a slender, thin-walled and shining shell of yellowish to reddish brown colour. The whorls are slightly rounded with a visible suture. The snail itself is black, but the mantle does not cover outside parts of the shell, as in other bladder snail species.

Dimensions: H: 9 - 15 mm; W: 4 - 6 mm; N: 4 - 6. (Abbreviations).


Moss bladder snail (Aplexa hypnorum).
Picture: Alexander Mrkvicka, Vienna (mrkvicka.at).
 

Habitat and Distribution: The moss bladder snail lives in small lowland waters with lots of vegetation, as well a in swampy lake banks neighboured by trees, in swampy alluvial forests and very humid meadows. In Schleswig-Holstein, Aplexa hypnorum regularly inhabit ponds in beech forests. The moss bladder snail can tolerate temporary falling dry of its home water, the species even prefers pond rich in vegetation temporarily falling dry. Moss bladder snails live on detritus and decaying leaves fallen into the water.

Moss bladder snails can leave their lake to reach plants standing near the water (hence their name). Of all water snails, moss bladder snails are the fastest. They show in astonishing tolerance for water acidity and can live with pH values between 5.8 and 8.5.

The moss bladder snail is a palaearctic species distributed through north Africa and Eurasia. In southern Europe it becomes rare, in Norway it appears as far north as the 61 latitude.

Threat Situation: In many parts of their distribution area, moss bladder snails are threatened by habitat destruction, many of which were lost in Great Britain, for example, after being filled up. While it is already extinct in Sweden, in Bavaria, southern Switzerland, Rhineland-Palatinate and Vorarlberg, Aplexa hypnorum is critically endangered, in Poland, the rest of Germany, Switzerland and Austria generally the species is classified vulnerable (see also: IUCN Threat Categories).

Francisco Welter-Schultes: Aplexa hypnorum species homepage.

Acute Bladder Snail - Physella acuta (Draparnaud 1805)


Acute bladder snail (Physella acuta). Picture: Lars Peters.
 

Description: The acute bladder snail has a yellowish horn coloured shell, which may be translucent and has a pointed apex, to which it has to thank its name. The aperture takes of the overall shell height; in Physella acuta the aperture is relatively higher than in Physella heterostropha, with which it is often confounded. shell spindle and apertural lip in fully grown specimens are white. The snail itself is blackish or dark grey violet. The mantle shows golden yellow spots that can be seen through the translucent shell wall. Only the right mantle lobe reaches the outer shell wall.

Dimensions: H: 8 - 16 mm; W: 5 - 9 mm; N: 4 - 5. (Abbreviations).

 
Right mantle lobe of an acute bladder snail (Physella acuta).
Picture: Alexander Mrkvicka, Vienna (mrkvicka.at).

Habitat and Distribution: Physella acuta lives in warm stagnant and slow streaming waters, also near the banks of lakes. The species also tolerates eutrophic conditions. At the central Portuguese coast, the acute bladder snail also lives in irrigated rice fields. The species mainly feeds on detritus and algae.

The acute bladder snail today is distributed worldwide, it remains unclear if it originally came from south-west Europe or from America. In Switzerland, where it has been found since the 1940s, it appears to altitudes of up to 800 m MSL. Frequent it is only in warm springs. In Germany, the acute bladder snail is a lowland snail, it only lives in altitudes up to 280 mm MSL. In south Germany, it is, however, more frequent.

Threat Situation: While in Spain Physella acuta in many regions is the most frequent water snail species, in Switzerland it is classified as vulnerable (VU) (see also: IUCN Threat Categories).

Relatives: The only other Physella species present in Europe, are Physella heterostropha (often confounded with Physella acuta, but with a larger shell and a relatively smaller aperture), as well as Physella gyrina, which has a dark coloured body with whitish spots, but whose shell is quite similar.

Conchological Society of Great Britain and Ireland: Physidae, Identification Aid.
Francisco Welter-Schultes: Physella acuta species homepage.