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.

Description

Apical, apertural and umbilical view of the shell of Biomphalaria tenagophila. Scale bar is 3 mm.

The shells of most species in this family are disk-like or button-like, being coiled in one plane, although several groups have shells that are more higher-spired, and some are limpet-like.

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. Planorbids were once 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. Nearly all shells are composed of growth lines.

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.

Sinistral shells

Planorbarius corneus. View of the sunken spire (held facing downwards in life)
Planorbarius corneus. View of the umbilicus (held uppermost in life)

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. The shells of planorbids are sinistral on close inspection, despite the fact that most species carry the shell as if it were a normal dextral shell.

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 towards the spire, not away from it. and the umbilicus faces upwards. The spire of the shell is quite sunken in many species.

The umbilicus of the shell is very wide and shallow, and faces upwards. In some species the umbilicus is not as deeply "dished" as the sunken spire, so it may be hard to tell one from the other without close inspection.

Distribution and habitat

Species in this family occur worldwide. Most species of planorbids live only in fresh water, such as ponds, lakes, and slow moving rivers. However, some species are known to tolerate conditions such as brackish water or sewage.

Geological history

Ancestors of ramshorn snails are known with certainty since the Jurassic period, but there are a few possible earlier occurrences starting in the Late Devonian.

Taxonomy

Albino planorbid clearly shows the reddish-colored body tissues due to the pigment hemoglobin

The following genera are recognised in the family Planorbidae:

  • Subfamily Ancylinae Rafinesque, 1815 Genus †Ancylina Bandel & F. Riedel, 1994 Genus †Palaeancylus Yen, 1948 Tribe Ancylini Rafinesque, 1815 Genus Ancylus O. F. Müller, 1773 Genus Ferrissia B. Walker, 1903 Genus Pettancylus Iredale, 1943 Genus Rhodacmea B. Walker, 1917 Genus Stimulator Iredale, 1944 Tribe Laevapicini Hannibal, 1912 Genus Anisancylus Pilsbry, 1924 Genus Gundlachia L. Pfeiffer, 1849 Genus Hebetancylus Pilsbry, 1914 Genus Laevapex B. Walker, 1903 Genus Sineancylus Gutiérrez Gregoric, 2014 Genus Uncancylus Pilsbry, 1914
  • Subfamily Miratestinae P. Sarasin & F. Sarasin, 1897 Genus Amerianna Strand, 1928 Genus Ancylastrum Bourguignat, 1853 Genus Bayardella J. B. Burch, 1977 Genus Glyptophysa Crosse, 1872 Genus Isidorella Tate, 1896 Genus Kessneria J. C. Walker & Ponder, 2001 Genus Leichhardtia J. C. Walker, 1988 Genus Miratesta P. Sarasin & F. Sarasin, 1897 Genus Patelloplanorbis Hubendick, 1957 Genus Protancylus P. Sarasin & F. Sarasin, 1897
  • Subfamily Planorbinae Rafinesque, 1815 Tribe Camptoceratini Dall, 1870 Genus Camptoceras W. H. Benson, 1843 Genus Culmenella Clench, 1927 Tribe Coretini J. E. Gray, 1847 Genus Planorbarius Duméril, 1805 Tribe Drepanotrematini Zilch, 1959 Genus Antillorbis H. W. Harry & Hubendick, 1964 Genus Drepanotrema P. Fischer & Crosse, 1880 Tribe Helisomatini F. C. Baker, 1928 Genus Acrorbis Odhner, 1937 Genus Biomphalaria Preston, 1910 Genus Dilatata Clessin, 1884 Genus Helisoma Swainson, 1840 Genus Menetus H. Adams & A. Adams, 1855 Genus Pecosorbis D. W. Taylor, 1985 Genus Planorbella Haldeman, 1843 Genus Planorbula Haldeman, 1840 Genus Promenetus F. C. Baker, 1935 Genus Vorticifex Meek, 1870 Tribe Neoplanorbini Hannibal, 1912 Genus Amphigyra Pilsbry, 1906 Genus Neoplanorbis Pilsbry, 1906 Genus †Payettia Dall, 1924 Tribe Planorbini Rafinesque, 1815 Genus Afrogyrorbis Starobogatov, 1967 Genus Anisus S. Studer, 1820 Genus Armiger Hartmann, 1843 Genus Bathyomphalus Charpentier, 1837 Genus Choanomphalus Gerstfeldt, 1859 Genus Gyraulus Charpentier, 1837 Genus Hovorbis D. S. Brown & Mandahl-Barth, 1973 Genus †Orygoceras Brusina, 1882 Genus Planorbis O. F. Müller, 1773 Tribe Segmentinini F. C. Baker, 1945 Genus Helicorbis W. H. Benson, 1855 Genus Hippeutis Charpentier, 1837 Genus Intha Annandale, 1922 Genus Kolhymorbis Starobogatov & Streletzkaja, 1967 Genus Lentorbis Mandahl-Barth, 1954 Genus Polypylis Pilsbry, 1906 Genus Segmentina J. Fleming, 1818 Genus Segmentorbis Mandahl-Barth, 1954 Genus Trochorbis W. H. Benson, 1855

Planorbidae incertae sedis:

  • Genus †Ampullariella Raspail, 1909
  • Genus †Archaeoplanorbis W. Yu, 1982
  • Genus †Atopippeutis Kadolsky, 2015
  • Genus †Bicarinopsis H.-Z. Pan, 1979
  • Genus †Brannerillus Hannibal, 1912
  • Genus †Fuchsogyra Bandel, 2010
  • Genus †Graptophysa Yen & Reeside, 1946
  • Genus †Headonia Harzhauser & Neubauer, 2020
  • Genus †Idahoella Yen, 1948
  • Genus †Jiangyouspira H.-Z. Pan, 1984
  • Genus †Liangulorbis Wenz, 1947
  • Genus †Marinescugyra Bandel, 2010
  • Genus †Mioplatytaphius Čtyroký, 1972
  • Genus †Nihewanspira S.-Y. Guo, 1983
  • Genus †Omalodiscus W. H. Benson, 1855
  • Genus †Paraplanorbis G. D. Hanna, 1922
  • Genus †Pentagoniostoma C. C. Branson, 1935
  • Genus †Physoides H.-Z. Pan, 1982
  • Genus †Planorbifex Pilsbry, 1935
  • Genus †Pompholopsis Call, 1888
  • Genus †Proplanorbarius Bandel, 1991
  • Genus †Proplanorbis Starobogatov, 1967
  • Genus †Pseudophysa Yen, 1938
  • Genus †Rostroapertura Firby, 1963
  • Genus †Sinoplanorbis W. Yü, 1965
  • Genus †Sinosegmentina Y.-T. Li, 1986
  • Genus †Steklovidiscus Gozhik & Prysjazhnjuk, 1980
  • (in Dutch) Gittenberger, E., Janssen, A.W., Kuijper, W.J., Kuiper, J.G.J., Meijer, T., Velde, G. van der & Vries, J.N. de (1998) De Nederlandse zoetwatermollusken. Recente en fossiele weekdieren uit zoet en brak water Nederlandse Fauna 2. Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij & EIS-Nederland, Leiden, 288 pp.
  • (in French) Fischer, P.H. (1880-1887) Manuel de Conchyliologie et de Paléontologie conchyliologique ou histoire naturelle des Mollusques vivants et fossiles suivi d'un appendice sur les Brachiopodes par D. Oehlert. – XXIV + 1369 pp.
  • (German) Thiele, J. (1929-1935) Handbuch der Systematischen Weichtierkunde. Jena, (1), 1–376, 1929; (2), 377–778, 1931; 779–1022, 1934; 1023–1134, 1935.
  • (in English) Vaught, K.C. (1989) A classification of living mollusca. American Malacologists Inc., Melbourne USA, ISBN 0-915826-22-4 & 0-915826-22-6, 195 pp.
  • (German) Wenz, W. (1923-1930) Gastropoda extramarina tertiaria. Fossilium Catalogus I. (4 vols.), 3387 pp.
  • (German) Zilch, A. (1959-1960) Euthyneura. In: H. Schindewolf (ed.), Handbuch der Paläozoologie, 6(2): pp. I-XII + 1–834.

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