Planorbella trivolvis

Taxobox
name = "Planorbella trivolvis"

---

image_caption = "Planorbella trivolvis"
image_width = 225px
status = NE
regnum = Animalia
phylum = Mollusca
classis = Gastropoda
subclassis = Orthogastropoda
superordo = Heterobranchia
ordo = Pulmonata
familia = Planorbidae
genus = "Planorbella"
species = "P. trivolvis"
binomial = "Planorbella trivolvis"
binomial_authority = (Linnaeus, 1758)

"Planorbella trivolvis" is a species of freshwater air-breathing snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails, or planorbids, which all have sinistral or left-coiling shells.

Planorbid gastropod shells are hard to make sense of in terms of their coiling and orientation. Most of the shells in this family are almost planispiral in coiling, and it is important to bear in mind the fact that all planorbids have sinistral shells. To complicate matters further however:

* In life, these pond snails often tend to hold their shells upside down, with the umbilicus facing upward
* The spire of the shell is quite sunken in many species
* The umbilicus of the shell is very wide, and it in some species the umbilicus is not as deeply "dished" as the sunken spire is.

Once you understand that the planorbid shell is sinistral, you can hold the shell with the aperture on the left and facing you, then the sunken spire side of the shell will be uppermost.

To repeat: the side of the shell which is in fact the spire (a sunken spire) often is carried facing down in the living animal, contrary to what is the case in almost all other shelled gastropods.

Description

The width of the shell of this species is up to 18 mm.

Distribution

This pond snail is native to North America, from the Arctic areas of Canada all the way south to Florida. It has also been introduced in other parts of the world.

* Not listed in IUCN red list - not evaluated (NE) [2006 IUCN Red List of Threatened Species. . Cited 5 March 2007.]

Habitat

This species prefers habitats with floating water weeds.

References

* [http://www.conchology.be/en/availableshells/searchresultsgallery.php?family=PLANORBIDAE]
* [http://www.cofc.edu/~fwgna/species/planorbidae/h_trivolvis.html]