Physella acuta
Physella acuta, also known as the European physa, tadpole 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 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 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 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.
Taxonomy and nomenclature
Physella acuta 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. 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, as well as Physella cubensis, 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 have reported stronger evidence for moving the species from its original genus Physa to Physella.the World Register of Marine Species accepts the taxonomic classification proposed by Albrecht and colleagues. The authors place P. acuta in the genus Physella, with Physa 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
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, and is sinistral. Sinistral shells are left-coiling, meaning that when held with the aperture facing the observer and the spire pointing upward, the aperture is on the left-hand side. The shell is thin and small, reaching up to length and in width. It forms a high spiral of five to six whorls which take about two-thirds of the shell length and end in a pointed apex. Sutures are impressed and clearly visible. The aperture is ear-shaped and takes about three-quarters of the total shell height. Both the columella and the apertural lip are white. The shell surface is smooth, thin to the point of being slightly transparent, and ranges in colour from pale horn to brownish yellow.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 onset 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 length. The shell of S. marmorata is longer and narrower.
Soft parts
The body of P. acuta is very variable in colour, which can range from blue to dark grey. The top mantle has golden spots often visible under the thin shell. Finger-like lobes extend from the mantle on both sides of the body, smaller than in other physids such as Physa fontinalis or Stenophysa marmorata. They can act as an accessory gill by extracting additional oxygen from the environment and help detect predators through their touch-sensitive tissue. The tentacles are grey and follow the build of other members within the family Physidae: cylindrical and slender, almost transparent, with small black eyes at the base. These pit eyes only distinguish between light and dark. The foot is narrow and ends in a pointed tail, as is also typical of the family. The mouth edge is large and flared. The body is an important point of distinction from P. gyrina and S. marmorata: ''P. gyrina has whitish spots over its whole dark grey body including the tentacles. The mantle extensions of S. marmorata'' are not digitated and extend broadly over the sides of the shell and this species has a black stripe running through the middle of the tail.Internal anatomy
Like all members of the family Physidae, P. acuta has a pulmonary cavity within the mantle which enables it to take oxygen from the water or from the air. It also has a specialised set of muscles called the physid musculature, unique among gastropods. Based on dissections, Naranjo-García and Appleton proposed that these muscles allow the snail to rapidly twist its shell in clockwise rotations of up to 120°. This shell-shaking movement is an important defence against slow-moving predators like leeches and fly larvae, which cannot properly attach to the moving shell. However, it is not effective when predators are fast-moving and actively pursue the snails, as is the case with water bugs.The physid musculature has two main parts. The first is the physid muscle sensu stricto, meaning the muscle "in the strict sense" because it is the major component of the musculature complex. It is a branched muscle that anchors the side of the snail's body to its neck, head, foot, and mantle. The second part is the fan muscle, a group of thin, broad fibres that fan out from the base of the muscle sensu stricto toward the mantle roof. According to Naranjo-García and Appleton, the configuration and attachment points of these muscles are consistent with a shell-twisting role, allowing them to provide the twisting force, serve as a base for the movement, anchor the shell to the body, and help return the shell to its original position afterwards.
P. acuta is a simultaneous hermaphrodite, meaning it has both male and female reproductive organs functioning at the same time. In Physidae, the male organs are important for taxonomic identification and consist of prepuce, penis sheath, and penis. The prepuce is a tube that releases or receives sperm from outside and, in the case of P. acuta, includes a small, lentil-shaped gland. It is wider and twice as long as the muscular penis sheath. The penis is 160–180 μm in length, elongated and narrow along most of its length but ends in a wider, rounded tip. The penis musculature consists of circular muscles in both the outer and inner layers, while the intermediate layer has only longitudinal muscles. Together with molecular phylogenetic data, this musculature pattern can be used to distinguish major groups within the superorder Hygrophila. The female organs are less frequently described in detail. They consist of a convoluted oviduct, a nidamental gland, and a vagina connected to the spermathecal duct.
Distribution
P. acuta is widely dispersed across the globe, largely due to the aquarium trade. It is an invasive species which can be found on all continents except Antarctica and is considered by Dillon and colleagues as "the world's most cosmopolitan freshwater gastropod".P. acuta was originally thought to be a European species, as its first record in North America following Draparnaud's initial description was only published in 1997. However, reproductive isolation experiments and molecular genetic studies revealed it to be the same species as the North American Physella heterostropha and Physa integra. Comparative anatomy, fossil evidence, and phylogenetic data also support a North American origin, specifically within the United States. However, its exact native range within the country remains debated, with hypotheses for both eastern and western origins. The timing and pathway of P. acutas first arrival in Europe are also uncertain. The most cited hypothesis, proposed by Anderson, links it back to eastern U.S. populations via the 18th century cotton trade. This view is based on the fact that Draparnaud's description happened during a time when USA and France had trade relations, with intense traffic between the ports of Mississippi and Bordeaux. After the Napoleonic wars, the American cotton trade switched to Britain, and British records of P. acuta only emerged after this point in time. This hypothesis is based on circumstantial evidence and earlier as well as natural introductions have also been proposed. Once within Europe, P. acutas spread was likely facilitated by man-made canals and waterbirds.
Ecology and behaviour
P. acuta can occupy a variety of freshwater habitats as well as habitats varying widely in water availability. As a pulmonate snail, it tolerates harsh environmental conditions such as polluted and eutrophic waters, since it is able to come to the surface to breathe air. P. acuta has been reported in lakes, reservoirs, ponds, streams, ditches, as well as artificial sites such as sewage drains and irrigation systems. It is a scraper feeder and uses its radula to scrape green algae, diatoms, and aquatic plants from the surface.P. acuta is a simultaneous hermaphrodite which is self-compatible, meaning it is capable of both outcrossing and self-fertilisation. In natural populations, reproduction occurs mainly by outcrossing, but self-fertilisation rates still remain between 10–30% and can increase as an adaptation strategy when mates are scarce. Adults lay 50–100 eggs per week for up to a year after reaching sexual maturity. Eggs are deposited in elongate gelatinous sacs and hatch after 15–20 days. Individuals reach sexual maturity after 17–18 months.