| Physidae | |
|---|---|
| Physella acuta of Mediterranean area | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Gastropoda |
| (unranked): | clade Heterobranchia clade Euthyneura clade Panpulmonata clade Hygrophila |
| Superfamily: | Planorboidea |
| Family: | Physidae Fitzinger, 1833[1] |
| Diversity | |
| about 80 freshwater species[2] | |
Physidae, common name bladder snails, are a monophyletic taxonomic family of small freshwater snails, aquatic pulmonate gastropod molluscs in the clade Hygrophila.[3]
Contents |
The fresh water snails of Physa group, are present in aquariums and ponds, also of wild areas. These snails eat algae, diatoms and detritus. They eat algae and dead leaves and they are common freshwater snails in the North Temperate to Arctic Zones and throughout the Americas, in readily accessible habitats such as ditches, ponds, lakes, small streams, and rivers. The family has been recognized as such from XIX century. Yet there has been no classification in which relationships between genera are clarified, no agreement on what characters are primitive or advanced, and no consistent ranking. Scarcity of careful morphological studies is the principal cause. They alone are regulating poblations by the abundance of food and space. They are widespread, abundant, and tolerant of pollution.
The differences in the group make it more of 23 genera, four grades and four clades within the family. They are commonly referred to as tadpole snails or pouch snails. The two established subfamilies are divided into seven new tribes including 11 new genera.[4] Within this family, the shell is always sinistral, in other words it has left-handed coiling. Physidae has 23 genera, 17 occur in Pacific drainages of North and Central America, eight of these restricted to the region. Concentration of primitive genera along the Pacific coast from Mexico to Costa Rica conforms to previous observations that primitive pulmonate families are concentrated within, or along the continental margins of, the Pacific Ocean. An ancestral origin of Physidae along an ancient eastern Pacific coast is probable. From this region the several lineages have spread to north, south and east in the Americas, and through Siberia to Europe.
These small snails are quite distinctive, because they have shells that are sinistral, which means that if the shell is held such that the spire is pointing up and the aperture is facing the observer, then the aperture is on the left-hand side. The shells of Physidae species have a long and large aperture, a pointed spire, and no operculum. The shells are thin and corneous, and rather transparent. The Physidae, sinistral pond snails. Studies in 1982 indicate that they are most abundant in the New World. They have evidently found a shell morphology suitable for their life station, as he goes on to say "...the physids have undergone considerable diversification, much of which is not clearly exhibited in their shells. Many of the species, and genera, are not easy to identify on shell characters alone."
They have been used in studies of ecophenotypic plasticity, a so-called phenoplastic switch. Burt Vaughan of Washington State University indicates several studies in M. J. West-Eberhardt's recent compendium of research, "Developmental Plasticity & Evolution" (Oxford Press, 2003, pp. 307-362)[5]. A typical example involved rearing Physa gyrina, or Physa heterostropha in controlled pair groups in either water in which crayfish co-existed or water in which only fish co-existed. Within a month, differences in shell morphology appeared; i.e., snails exposed to shell-crushing fish predators showed wide apertures and very much strengthened, rotund shells. Snails exposed to crayfish only showed narrow-apertured, thin elongate shells, with barricading teeth.
In 1921 was first observed the strong reaction of Physa to contact with leeches, and later studies have been made. The observations are restricted to Physa fontinalis an indigenous species of an area with indigenous predatory leeches, and Haitia acuta introduced in Germany and the Netherlands. When Physa contacts another snail, either Physa or some other, the reaction is a rapid twisting of the shell back and forth to dislodge the other. The muscle used is the “physid muscle”, not found in other Hygrophila, which therefore do not show the reaction. The leech-avoidance reaction carries the action one step further: on contact with a leech the snail twists its shell violently and detaches its foot from the substratum as well. It was studied the reaction of two species of Physids to various species of leeches and to various salts. In Haitia acuta the avoidance reaction was much lower than in Physa fontinalis. The highest percentage of reactions in Physa were obtained with the two species of leeches that feed chiefly on snails. The nature of the substance that produces the reaction is undetermined, but presumably is a protein.[6]
According to the taxonomy by Bouchet & Rocroi (2005)[7] this family consists of the following subfamilies and tribes, which is based on classification by Taylor (2003):[8]
Genera in the family Physidae include:
subfamily Physinae
Haitiini
Physini
Physellini
subfamily Aplexinae
Aplexini
Amecanautini
Austrinautini
Stenophysini
Physidae can be pests to any tank with fish in it because they create waste, reproduce very often, and are very hard to remove. Still, many people put freshwater pond snails into their tank because they have the ability to eat uneaten fish food and waste. They can also eat unwanted fish carcasses.