Physella acuta, also known as the European physa, tadpole snail, sewage snail, bladder snail, or acute bladder snail, is a species of small, air-breathing freshwater snail of the family Physidae. It originates from North America and was first described in 1805 by Jacques Philippe Raymond Draparnaud based on a specimen found in France. Like other physids, P. acuta presents a sinistral (left-coiling) shell as well as a unique set of muscles called the physid musculature that allows it to rapidly twist the shell as a defence mechanism. P. acuta is invasive on all continents except Antarctica and is considered by Dillon and colleagues (2002) as "the world's most cosmopolitan freshwater gastropod". Its first introduction outside North America likely occurred through the 18th-century cotton trade to Europe, while later spread mainly happened through the aquarium trade. The species can occupy diverse freshwater habitats and tolerates polluted as well as oxygen-poor environments. It can reproduce with other individuals and also self-fertilise. Due to its high reproductive rate and tolerance to habitat degradation, it frequently outcompetes native snail species. Prevalence of parasitic infections within invasive P. acuta populations is often low, but a 2024 study detected the human parasite Echinostoma (which causes a disease known as echinostomiasis upon infecting the gastrointestinal tract) in an individual from Rio de Janeiro, Brazil. In aquariums, P. acuta is usually introduced through ornamental plants and can become a "nuisance snail" due to its rapid reproduction. However, a controlled population in an aquarium can help clean up organic leftovers and control algal growth.
Etymology and taxonomy Physella acuta is commonly known as European physa, tadpole snail, sewage snail, bladder snail, or acute bladder snail. The species was first described as Physa acuta in 1805 by Jacques Philippe Raymond Draparnaud, based on a specimen found in the Garonne catchment in France. The specific epithet acuta is Latin for 'sharp'. In 1817 Thomas Say independently described the same species in Pennsylvania, naming it Physa heterostropha. This and several other newly described species, such as Physella virgata and Physella integra (described from North America), as well as Physella cubensis (described from Central America), were considered distinct from each other until molecular and reproductive studies at the beginning of the 21st century revealed them to be synonyms of P. acuta. Since 2021, molecular phylogenetic studies (which investigate evolutionary relationships using genetic data) have reported stronger evidence for moving the species from its original genus Physa to Physella.
As of 2025 the World Register of Marine Species (WoRMS) accepts the taxonomic classification proposed by Albrecht and colleagues (2025). The authors place P. acuta in the genus Physella, with Physa (the genus to which it was previously assigned) and Stenophysa as sister groups. Together, these three genera form the subfamily named Physinae. The family Physidae consists of Physinae and the genus Aplexa. P. acuta's current classification is shown in the simplified cladogram below.While the family Physidae is well established, the structure of its subfamily Physinae remains unclear as of 2021. Debates include the taxonomic relationships between Physinae members as well as the definition and number of physinine genera and species, which in turn affect how individual taxa, including P. acuta, are defined and assigned. Consequently, P. acuta's classification as Physella is not universally accepted, and other authors and databases continue to use synonyms like Physa acuta or Haitia acuta. This taxonomic uncertainty is partly due to vague descriptions in early works that could apply to multiple taxa, and to the generalist nature of physids, including P. acuta, which lead to morphological plasticity (variation in morphology in response to environmental conditions) and rapid evolution.
Description
External anatomy
Shell
Like other members of the family Physidae, the shell of P. acuta lacks an operculum, a "trapdoor" that closes the aperture (shell opening), and is sinistral. Sinistral shells are left-coiling, meaning that when held with the aperture facing the observer and the spire (coiled part of the shell) pointing upward, the aperture is on the left-hand side. The shell is small, reaching up to 16 mm (0.63 in) length and 9 mm (0.35 in) in width. It forms a high spiral of five to six whorls (complete revolutions) which take about two-thirds of the shell length and end in a pointed apex (tip). Sutures (grooves between the whorls) are impressed and clearly visible. The aperture is ear-shaped and extends about three-quarters of the total shell height. Both the columella (central pillar within the shell) and the apertural lip (the opening's margin) are white. The shell surface is smooth and ranges in colour from pale horn to brownish yellow. P. acuta's shell is thin, but shell thickness increases in the presence of predators, a response known as "adaptive plasticity". This response is weaker in individuals born through self-fertilisation (when the snail reproduces with itself), which also develop thinner shells. In 2010 Auld and Relyea reported shell thickness in P. acuta ranging from 0.15 mm (0.0059 in) to 0.35 mm (0.014 in), depending on predator cues in the environment and self-fertilisation rates. The species presents a high diversity of shell shapes which led to numerous false species descriptions before the onset of molecular phylogenetic studies at the beginning of the 21st century. Shells of P. acuta can be especially difficult to distinguish from those of Physella gyrina and Stenophysa marmorata if live specimens are not available, since body morphology provides key distinguishing features. Typically, P. gyrina has a shorter spire with shallower sutures and a larger shell which can exceed 13 millimetres (0.51 in) length. The shell of S. marmorata is longer and narrower.
Soft parts
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