Physella acuta

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Physella acuta
Physa acuta 001.JPG
A live individual of Physella acuta
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Superorder: Hygrophila
Family: Physidae
Genus: Physella
Species:
P. acuta
Binomial name
Physella acuta
(Draparnaud, 1805) [1]
Synonyms

Physa acuta Draparnaud, 1805
Physella heterostropha (Say, 1817) [2]
Physella integra (Haldeman, 1841) [2]
Physa globosa Haldeman, 1841
Haitia acuta

Contents

Physella acuta, also known as 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 is considered "the world's most cosmopolitan freshwater gastropod". [3] It is present as an invasive species across all continents except Antarctica, mainly due to the aquarium trade. [4]

Taxonomy and nomenclature

Physella acuta was first described in France in 1805 by Jacques Philippe Raymond Draparnaud, originally as Physa acuta. [5] The specific epithet acuta is Latin for "sharp". [6] In 1817, Thomas Say independently described the same species in Pennsylvania, naming it Physa heterostropha. [7] Until the beginning of the 21st century, these and several other species were treated as distinct until molecular and reproductive studies showed they represented a single species. [8]

Physella acuta was originally placed in the genus Physa but was later reassigned to Physella according to molecular phylogenetic studies. [9] This taxonomy is currently accepted by MolluscaBase, the mollusk-oriented branch of WoRMS. [10] However, species- and genus-level memberships within the subfamily Physinae remain debated. [11] [12] Taxonomic uncertainty is attributed partly to vague descriptions in early works, which could apply to multiple taxa, [13] and to the generalist nature of physids, including P. acuta, which lead to morphological plasticity and rapid evolution. [11]

Description

Shell

Shell of Physella acuta Physella acuta 01.JPG
Shell of Physella acuta

Like other members of the family Physidae, the shell lacks an operculum and is sinistral, meaning that when held with the aperture facing the observer and the spire pointing upward, the aperture is on the left-hand side. [14] The shell of P. acuta is thin and reaches up to 12 mm in length. [15] It forms a high spiral of four to five whorls, taking about two-thirds of the shell length and ending in a pointed apex. Sutures are impressed and clearly visible. The aperture is ear-shaped and occupies about three-quarters of the total shell height. Both the columella and the apertural lip are white. The shell surface is smooth and pale, slightly transparent due to its thinness. Colour is variable, typically ranging from fawn to yellowish. [16] [17]

According to the U.S. Geological Survey (USGS), the species presents a high diversity of shell shapes which led to numerous false species descriptions before the onset of genetic studies. [18] Shells of P. acuta can be especially difficult to distinguish from those of Physella gyrina if live specimens are not available. Typically, P. gyrina has a shorter spire with shallower sutures and a larger shell which can exceed 13 mm in length. [16] [19]

Soft parts

Physella acuta on an aquarium glass. Blasenschnecke 03.jpg
Physella acuta on an aquarium glass.

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, while the rest of the mantle follows the ground colour. [18] Digitated lobes extend from the mantle on both sides of the body. These lobes are touch-sensitive and help detect predators. [20] 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. The foot is narrow and terminates in a point at the rear 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, which has whitish spots over its whole dark grey body including the tentacles. [16]

Reproductive organs

P. acuta is a simultaneous hermaphrodite, meaning it has both male and female reproductive organs functioning at the same time. The male organs consist of prepuce, penis sheath, and penis. The prepuce a sac near the base of the penis including a small, lentil-shaped gland. It is wider and twice as long as the penis sheath. [21] The penis is 160 - 180 μm in length and white in colour. [22] It is elongated and narrow along most of its length but ends in a wider, rounded tip. [23] The penis musculature consists of circular muscles in both the outer and inner layers, while the intermediate layer has only longitudinal muscles. This musculature pattern can be used to distinguish major groups within the superorder Hygrophila along with molecular phylogenetic data. [22] 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 which opens to the outside. [21]

Distribution and habitat

Origin and introduction history

P. acuta was originally thought to be native to the Mediterranean region. However, reproductive isolation experiments [3] and molecular genetic studies [12] revealed it to be the same species as the North American Physella heterostropha and Physa integra. [3] Comparative anatomy, fossil evidence, and phylogenetic data also support a North American origin. It is currently accepted that P. acuta is native to the United States, although its exact native range within the country remains debated. [3] [24] Hypotheses propose both eastern and western origins or a single, historically isolated population that only recently resumed genetic exchange. [8]

The timing and pathway of P. acuta's arrival in Europe also remain uncertain. One hypothesis links its first introduction in Europe to eastern U.S. populations via the cotton trade in the 18th century, [8] although earlier or natural introductions have also been proposed. [13] Its initial spread within Europe was likely facilitated by man-made canals. [13] [25]

Current distribution and habitat

Today, P. acuta is widely dispersed across the globe, largely due to the aquarium trade. [26] It is an invasive species which can be found on all continents except Antarctica. [18] P. acuta can occupy a variety of freshwater habitats as well as habitats varying widely in water availability. [18] It tolerates a variety of harsh environmental conditions, such as eutrophic and polluted waters. [27] [28] [29] It has been reported in lakes, reservoirs, ponds, streams, ditches, as well as artificial sites such as sewage drains and irrigation systems. [30] [31]

Ecology

Feeding habits

P. acuta is a scraper feeder. It uses its radula to scrape green algae, diatoms, and aquatic plants from the surface. [32]

Reproduction

P. acuta is a self-compatible hermaphrodite capable of both outcrossing and self-fertilisation. [33] 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. [34] [35]

Adults lay 50 - 100 eggs per week for up to a year after reaching sexual maturity. [18] Eggs are deposited in elongate gelatinous sacs and hatch after 15 - 20 days. Individuals reach sexual maturity after 17 - 18 months. [17]

Ecological interactions

Coexistence and competition

P. acuta can coexist with other non-native gastropods such as Stenophysa marmorata, [36] Potamopyrgus antipodarum, [37] Lithoglyphus naticoides, and Radix auricularia. [38] In these cases, competition may not be strong enough to cause exclusion, and species can differ in their competition strategies or life-history traits. [36] More often, however, its presence leads to the decline of native gastropod populations by competition. [39] [8] It outcompetes Glyptophysa gibbosa in Australia [40] , Physafontinalis in Italy, [41] as well as Racesina luteola and Filopaludina bengalensis in India. [42] [43] In Mozambique, it displaced Bulinus forskalii to become the dominant gastropod. [44] Like many invasive freshwater snails, the competitive success of P. acuta can be explained by its higher fecundity, shorter egg development time, and broader tolerance to habitat degradation and salinity. [41] It also shows a stronger growth response to rising temperatures than some native species. These traits make P. acuta more efficient in competing for food when diets overlap. [44] The presence of P. acuta can also inhibit the growth of other species, while its own growth is stimulated by them, although exact mechanisms are unknown. [39]

Predation

P. acuta is predated by a variety of animals, including water bugs, [45] marsh fly larvae, [46] crayfish, [47] leeches, [48] and various fish species. [49] [50] To a lesser degree, it is also prey to the invasive carnivorous snail Anentome helena. [43] Anti-predator behaviours include shell-shaking (rapid shell movements) and crawling to or above the waterline, [48] [20] as well as burying into the sediment, [50] leaping (shell-shaking combined with detachment from the surface, causing the snail to jerk away), [48] [51] clamping to the substrate, and detatching to float up to the surface. [52]

Aquarium trade

Physella acuta is often called a "pest snail" in freshwater fishkeeping. It usually spreads by laying its eggs on ornamental plants which are then bought and taken home. Physella acuta can reproduce quickly compared to other snails, in part to its ability to self-fertilise. It is generally non-harmful to the aquarium, but often found to be unsightly as well as adding to the aquarium's waste production. Others will, however, intentionally keep bladder snails, as their diet and ease of care can prove to make them a useful part of a tank's clean-up crew.

References

This article incorporates CC-BY-2.0 text (but not under GFDL) from reference.

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Further reading