Planorbidae, common name the ramshorn snails or ram's horn snails, is a family of air-breathing freshwater snails, aquatic pulmonate gastropod molluscs. Unlike most molluscs, the blood of ram's horn snails contains iron-based hemoglobin instead of copper-based hemocyanin. As a result, planorbids are able to breathe oxygen more efficiently than other molluscs. The presence of hemoglobin gives the body a reddish colour. This is especially apparent in albino animals.
Being air breathers like other Panpulmonata, planorbids do not have gills, but instead have a lung. The foot and head of planorbids are rather small, while their thread-like tentacles are relatively long. Many of the species in this family have coiled shells that are planispiral, in other words, the shells are more or less coiled flat, rather than having an elevated spire as is the case in most gastropod shells. Although they carry their shell in a way that makes it appear to be dextral, the shell of coiled planorbids is in fact sinistral in coiling, but is carried upside down, which makes it appear to be dextral.
General taxonomic context
For several taxa, no consensus exists as to whether the taxa should even be assigned to the family Planorbidae. This is certainly the case with the freshwater limpets Ferrissia, and Ancylus. Both of these genera have sometimes been assigned to the family Lymnaeidae. Alternatively sometimes each one of them is raised to the level of a family. However, according to the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005), these genera are currently placed in the tribe Ancylini within the family Planorbidae, and that is the taxonomic system that is followed here.
According to the taxonomy of the Gastropoda (Bouchet & Rocroi, 2005), this family consists of the following subfamilies:
- subfamily Planorbinae Rafinesque, 1815 tribe Planorbini Rafinesque, 1815 - synonyms: Choanomphalinae P. Fisher & Crosse, 1880; Orygoceratidae Brusina, 1882 tribe Ancylini Rafinesque, 1815 - synonym: Pseudancylinae Walker, 1923 (inv.) tribe Biomphalariini H. Watson, 1954 - synonyms: Acrorbini Starobogatov, 1958; Drepanothrematini Zilch, 1959; Taphiinae Harry & Hubendick, 1964 tribe Planorbulini Pilsbry, 1934 tribe Segmentinini F.C. Baker, 1945
- subfamily Bulininae P. Fischer & Crosse, 1880 tribe Bulinini P. Fischer & Crosse, 1880 - synonyms: Laevapicinae Hannibal, 1912; Isidorinae Annandale, 1922; Gundlachiinae Starobogatov, 1967 tribe Coretini Gray, 1847 - synonyms: Pompholicinae Dall, 1866 (inv.); Camptoceratinae Dall, 1870; Megasystrophinae Tryon, 1871 (inv.); Pompholycodeinae Lindholm, 1927; Helisomatinae F. C. Baker, 1928; Bayardellini Starobogatov & Prozorova, 1990; Planorbariini Starobogatov, 1990 tribe Miratestini P. & F. Sarasin, 1897 - synonyms: Ferrissiinae Walker, 1917; Ancylastrinae Walker, 1923; Protancylinae Walker, 1923; Physastrinae Starobogatov, 1958; Ameriannini Zilch, 1959; Patelloplanorbidae Franc, 1968 tribe Plesiophysini Bequaert & Clench, 1939
- subfamily Neoplanorbinae Hannibal, 1912 - synonym: Payettiinae Dall, 1924
- subfamily Rhodacmeinae Walker, 1917
2007 taxonomy for part of the family
Albrecht et al. (2007) analyzed a limited number of genera of Planorbidae, based on sequences of mitochondrial 18S ribosomal DNA and cytochrome-c oxidase I (COI) genes, and on the basis of the results, they rearranged the taxonomy like this:
"A-clade" sensu Albrecht et al. (2007)
- Burnupia Walker, 1912
Tribus Ancylini Rafinesque-Schmaltz, 1815
- Ancylus O. F. Müller, 1774
- Ferrissia Walker, 1903
- Gundlachia Pfeiffer, 1849
- Laevapex Walker, 1903
- Hebetancylus Pilsbry, 1914
"B-clade" sensu Albrecht et al. (2007)
- Glyptophysa Crosse, 1872
- Protancylus Sarasin, 1897
- Kessneria Walker & Ponder 2001
- Leichhardtia Walker, 1988
- Planorbarius Duméril 1806
- Anisus Studer, 1820
- Bathyomphalus Charpentier, 1837
- Gyraulus Charpentier, 1837
- Hovorbis D.S. Brown & Mandahl-Barth, 1973
- Choanomphalus Gerstfeldt, 1859
- Planorbis O. F. Müller 1774
"C-Clade" sensu Albrecht et al. (2007)
- Biomphalaria Preston, 1910
- Menetus H. & A. Adams, 1855
- Planorbella Haldeman, 1843
- Planorbula Haldeman, 1843
The type genus of this family is Planorbis Müller. The following list of genera is organized according to the 2005 taxonomy, because Albrecht's 2007 taxonomy is not available for all genera of Planorbidae. Genera in the family Planorbidae include (subgenera listed according to Glöer (2002):
- subfamily Planorbinae Rafinesque, 1815
- Anisus S. Studer, 1820 subgenus Disculifer C. Boettger, 1944 Bathyomphalus Charpentier, 1837 Gyraulus Charpentier, 1837 subgenus Torquis Dall, 1905 subgenus Lamorbis Starobogatov, 1967 subgenus Armiger Hartmann, 1843 Hippeutis Charpentier, 1837 tribe Ancylini Rafinesque, 1815 Ancylus O. F. Müller, 1773 - type genus of tribe Ancylini tribe Biomphalariini H. Watson, 1954 Biomphalaria Preston, 1910 - type genus of tribe Biomphalariini Drepanotrema Crosse & P. Fischer, 1880 tribe Planorbini Rafinesque, 1815 Afrogyrorbis Starobogatov, 1967 Planorbis Müller, 1773 tribe Planorbulini Pilsbry, 1934 Planorbula Haldeman, 1840 - type genus of tribe Planorbulini tribe Segmentinini F.C. Baker, 1945 Segmentina Fleming, 1818 - type genus of tribe Segmentinini
- subfamily Bulininae P. Fischer & Crosse, 1880 Indoplanorbis Annandale & Prashad, 1920 - contains one species Indoplanorbis exustus Planorbarius Duméril, 1806 Planorbella Haldeman, 1842 Menetus H. Adams & A. Adams, 1855 subgenus Dilatata Clessin, 1885 tribe Bulinini P. Fischer & Crosse, 1880 Bulinus O. F. Müller, 1781 - type genus of subfamily Bulininae Gundlachia Pfeiffer, 1849 tribe Coretini Gray, 1847 Coretus Gray, 1847 - type genus of tribe Coretini tribe Miratestini P. & F. Sarasin, 1897 Miratesta P. & F. Sarasin, 1897 - type genus of tribe Miratestini Amerianna Strand, 1928 Ferrissia Walker, 1903 Pettancyclus Iredale, 1943 tribe Plesiophysini Bequaert & Clench, 1939 Plesiophysa P. Fischer, 1883 - type genus of tribe Plesiophysini
- subfamily Neoplanorbinae Hannibal, 1912 Neoplanorbis Pilsbry, 1906 - type genus of subfamily Neoplanorbinae
- subfamily Rhodacmeinae Walker, 1917 Rhodacmea Walker, 1917 - type genus of subfamily Rhodacmeinae
Subfamily = ? (other genera that are not yet sorted are listed here)
- Acrorbis Odhner, 1937
- Africanogyrus Özdikmen & Darilmaz, 2007 - synonym: Afrogyrus Brown & Mandahl-Barth, 1973
- Afroplanorbis Thiele, 1931
- Amphigyra Pilsbry, 1906
- Anisopsis Sandberger, 1875
- Antillorbis Harry & Hubendick, 1964
- Armigerus Clessin, 1884
- Australorbis Pilsbry, 1934
- Bayardella Burch, 1977
- Berellaia Laubrière & Carez, 1880
- Camptoceras Benson, 1843
- Camptoceratops Wenz, 1923
- Carinifex W.G. Binney, 1865
- Carinogyraulis Polinski, 1929
- Ceratophallus Brown & Mandahl-Barth, 1973
- Choanomphalus Gerstfeldt, 1859
- Culmenella Clench, 1927
- Fossulorbis Pilsbry, 1934
- Glyptophysa Crosse, 1872
- Helicorbis Benson, 1855
- Helisoma Swainson, 1840
- Intha Annandale, 1922
- Isidorella Tate, 1896
- Kessneria Walker & Ponder, 2001
- Leichhardtia Walker, 1988
- Lentorbis Mandahl-Barth, 1954
- Macrophysa (Meek) Dall, 1870
- Paraplanorbis Hanna, 1922
- Patelloplanorbis Hubendick, 1957
- Pecosorbis D.W. Taylor, 1985
- Pentagoniostoma Branson, 1935
- Perrinilla Hannibal, 1912
- Physastra Tapparone-Canefri, 1883
- Physopsis Krauss, 1848
- Pingiella F.C. Baker, 1945
- Pitharella F. Edwards, 1860
- Planorbifex Pilsbry, 1935
- Planorbina Haldeman, 1842
- Platyphysa P. Fischer, 1883
- Platytaphius Pilsbry, 1924
- Polypylis Pilsbry, 1906
- Promenetus F.C. Baker, 1935 Promenetus exacuous Henderson-Daniels, 1917 - Sharp sprite
- Protancylus P. & F. Sarasin, 1897
- Pygmanisus Iredale, 1943
- Segmentorbis Mandahl-Barth, 1954
- Sineancylus Gutiérrez Gregoric, 2014
- Syrioplanorbis F.C. Baker, 1945
- Trochorbis Benson, 1855
- Vorticifex Meek in Dall, 1870
The genus Camptoceratops Wenz, 1923 is no longer considered to be a planorbid. It was recognised by Curry (1965, p. 360) as a euthecosomatous pteropod (Heterobranchia) (note by Arie W. Janssen, 092507).
All coiled shell Planorbidae are sinistral in their shell coiling, as is proved by their internal anatomy (the respiratory and the genital orifice are situated on the left side), however the animals carry their shells with what would normally be the ventral (i.e. umbilical) surface uppermost, and because of this, the shells appear to be dextral.
Indeed, formerly planorbids were thought to have dextral shells, and so species of this family were figured as if they had dextral shells. Although it is now understood that these species are sinistral in shell coiling, disk-like Planorbid shells are often still shown in illustrations oriented as if they were dextral.
Most species of coiled planorbids have a rather thin and moderately smooth shell, although more distinct sculpture such as a keel occurs in, and is diagnostic of, certain species. In the flat, keeled species, the whorls tend to overlap.
The aperture has a sharp outer lip. A peristome can be present, but often the lip is not thickened nor reflected. Those planorbid species which have a high-spired shell may have a narrow umbilicus, but frequently this is covered by callus.
In height most species vary between 6 mm and 6 cm, however, disk-like shells are usually less than about 2 cm in maximum dimension.
Like all pulmonate aquatic snails, ramshorn shells do not have an operculum to close the shell aperture.
Flat-coiled planorbid gastropod shells are hard to understand in terms of their coiling and orientation. Many of the shells of species in this family are almost planispiral in coiling such that one side of the shell often looks rather like the other side, but it is important to bear in mind that nonetheless there is an umbilical side and a spire side of the shell. In addition these are in fact sinistral shells, despite the fact that the snail carries its shell as if it were a normal dextral shell. To make sense of the shell coiling, the following facts are useful:
- In life, these pond snails hold their shells upside down compared to the normal gastropod shell orientation, with the umbilicus facing upwards
- The spire of the shell is quite sunken in many species, in addition it is carried facing downwards
- The umbilicus of the shell is very wide and shallow
- In some species the umbilicus is not as deeply "dished" as the sunken spire is, so superficially it can be hard to tell one from the other
However, once it is understood that the planorbid shell is sinistral, if the shell is held with the aperture on the left and facing the observer, then the sunken spire side of the shell is uppermost. This is a convenience for understanding the shell, but is the opposite of the way the shell is actually carried in life.
The side of the shell which is in fact the spire (a sunken spire) faces down in the living animal, contrary to what is the case in almost all other shelled gastropods. Because the shell is carried "upside down" like this, the aperture of the shell is angled to face downwards also, so the aperture faces a little towards the spire, not away from it, as is usually the case in other shelled gastropods.
Most species of planorbids live only in fresh water, such as ponds, lakes, and slow moving rivers. A minority of species are able to survive in brackish water.
Species in this family occur worldwide. In Northwest Europe about 20 species are known (including non-indigenous species). In this region, various extinct taxa are known to have occurred, starting in the Jurassic period.