About Physella acuta (Draparnaud, 1805)
Physella acuta, commonly known as the European physa, tadpole snail, sewage snail, bladder snail, or acute bladder snail, is a species of small, air-breathing freshwater snail in the family Physidae. It is native to North America, and was first formally described in 1805 by Jacques Philippe Raymond Draparnaud from a specimen collected in France. Like all other members of the Physidae family, P. acuta has a sinistral (left-coiling) shell, and a distinct muscle arrangement called physid musculature that lets it twist its shell quickly as a defense mechanism. P. acuta is invasive on every continent except Antarctica, and was described by Dillon and colleagues in 2002 as "the world's most cosmopolitan freshwater gastropod". Its first introduction outside of North America to Europe was most likely through the 18th century cotton trade, while most later spread has occurred via the aquarium trade. The species can live in a wide range of freshwater habitats, and tolerates both polluted and low-oxygen environments. It can reproduce with other individuals, and is also capable of self-fertilisation. Thanks to its high reproductive rate and tolerance of habitat degradation, it frequently outcompetes native snail species. Parasitic infection prevalence is often low in invasive P. acuta populations, but a 2024 study detected the human parasite Echinostoma, which causes the gastrointestinal disease echinostomiasis, in an individual collected from Rio de Janeiro, Brazil. In aquariums, P. acuta is usually accidentally introduced with ornamental plants, and can become a "nuisance snail" because of its rapid reproduction. However, a controlled P. acuta population in an aquarium can help clean up organic waste and control algal growth. P. acuta is widely distributed across the globe, largely as a result of the aquarium trade. Originally, it was thought to be a European species, because the first record of P. acuta in North America after Draparnaud's initial description was only published in 1997. However, reproductive isolation experiments and molecular genetic studies have shown that it is the same species as the North American taxa Physella heterostropha and Physa integra. Comparative anatomy, fossil evidence, and phylogenetic data also support a North American origin, specifically within the United States. Its exact native range within the US is still debated, with competing hypotheses for an eastern origin and a western origin. The timing and pathway of P. acuta's first arrival in Europe also remain uncertain. Anderson (2003) connected the first introduction to 18th century cotton trade from eastern US populations; this hypothesis is based on the fact that Draparnaud described the species at a time when there was active trade between the US and France, with shipping between the Mississippi port and the port of Bordeaux. Other hypotheses have been proposed for the first introduction to Europe, including natural dispersal by birds. Once P. acuta became established in Europe, its spread was likely aided by man-made canals and waterbirds. In recent decades, the spread of P. acuta is still monitored and modelled to support population control efforts. Pesticides may harm non-target organisms that share P. acuta's habitat, but P. acuta predators show potential as biological control agents that can reduce snail population numbers. Laboratory studies have found that water bugs (Diplonychus rusticus) and glossiphoniid leeches prefer to prey on P. acuta even when other prey snails are available, though as of 2023 no studies of this preference in natural environments have been completed. The Black Carp (Mylopharyngodon piceus) can consume more than 250 P. acuta snails per day under laboratory conditions, including snails buried in substrate. However, as of 2001 the potential risks of introducing this fish, including pathogen transmission and unintended population growth, still require assessment. P. acuta can live in a wide variety of freshwater habitats, as well as habitats that vary widely in water availability. As a pulmonate snail, it can tolerate harsh environmental conditions such as polluted and eutrophic (oxygen-depleted) water, because it is able to surface to breathe air. P. acuta has been recorded in lakes, ponds, streams, ditches, and artificial sites including reservoirs, sewage drains, and irrigation systems. It is a scraper feeder that uses its radula, a tongue-like structure covered in small chitinous teeth, to scrape green algae, diatoms, and aquatic plants off surfaces. P. acuta is a simultaneous hermaphrodite that is self-compatible, meaning it is capable of both outcrossing (sexual reproduction with another individual) and self-fertilisation. In natural populations, reproduction happens mainly through outcrossing, but self-fertilisation still makes up 10–30% of reproduction, and self-fertilisation rates can increase as an adaptation when mates are rare. During reproductive development, snails first reach maturity for outcrossing after approximately 5–7 weeks, while maturity for self-fertilisation occurs at around 14 weeks. Sexually mature adults breed year-round and lay 50–100 eggs per week. Eggs are laid in elongated gelatinous sacs, which the snails attach to rocks, compacted mud, or the shells of other P. acuta. Eggs hatch after 15–20 days. Under laboratory conditions, individuals live for approximately 22–30 weeks, though Núñez (2010) recorded a maximum lifespan of 88 weeks.
