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Family  Cystiscidae

Rice snails

The Cystiscidae is a family of minute to small gastropods with shells that are usually smooth and white, occasionally coloured, with strong plaits on the columella.  Most species of the family are less than six millimeters in length, although there are some larger, attractive tropical species.  The Marginellidae shares general shell appearance and features with the Cystiscidae, and it is only recently that the two families have been distinguished.

The Cystiscidae was separated from the Marginellidae on shell and anatomical characters, and is considered to be more closely related to the Olividae, while the remaining Marginellidae have more in common with the Volutidae. There are obvious differences in the shape of the radula teeth and the structure of the radula cartilage.

There are three major shell differences between the Cystiscidae and Marginellidae:

• In the cystiscids the internal whorls of the shell are resorbed by the animal as the shell grows, leaving the shell interior hollowed out, as shown in Fig. 1.

• The columellar plaits differ in structure.  In the marginellids, the plaits that are visible externally on the columella have been the same in number since the larval shell commenced growth.  In the Cystiscidae only three or four of the plaits visible on the adult columella have been produced continuously as the shell grew, the remainder, the upper ones, only appearing in the last quarter or eight of a whorl.  Those present since the juvenile shell are termed ‘plaits’, while the shorter upper ones are termed ‘lirae’.  It is usually not possible from an external view of an adult shell to tell the difference, so descriptions give the total of plaits/lirae.

• The Cystiscidae usually lack an external thickening of the outer lip, while this is usually present in the Marginellidae

Family References

Coovert, G.A. & Coovert, H.K. 1995. Revision of the supraspecific classification of marginelliform gastropods.  The Nautilus 109(2&3): 43-110.

Hewish, D.R. in Wilson, B.R. 1994. Australian Marine Shells. Prosobranch Gastropods (Part 2 Neogastropoda). Odyssey Publishing: Kallaroo, Western Australia. pp. 139-143

Laseron, C.F. 1948. New South Wales Marginellidae.  Records of the Australian Museum 22(1): 35-48

Laseron, C.F. 1957. A new classification of the Australian Marginellidae  (Mollusca), with a review of the species from the Solanderian and Dampierian zoogeographical regions.  Australian Journal of Marine and Freshwater Research 8(3): 274-311

Coverage

All the named species of the family known to occur in NSW are detailed here.  In some cases, such as Cystiscus angasi, it is obvious that there is more than one species lumped under the name currently in use.  There are a few known undescribed species in Australian museum collection which are not treated here.

Identification Notes

The terminology used here largely follows that established by Coovert & Coovert (1995), and is shown in Fig. 2.  For further illustrations of terminology, see the identification notes in the introduction to the Marginellidae. 

Size: minute for  shell length up to 2.4 mm, small for shell length 2.4 – 6 mm, medium for shell length 6 – 13 mm

Spire: may be low or immersed; immersed means the apex is below the level of the latter whorls.

Outer lip:  The outer lip is thickened internally in mature shells in this family. On this internal thickening there may be denticles – more or less circular projections – or lirae, which are elongate ridges.  A thickening on the outside of the outer lip is called an external varix, and usually absent in this family, but usually present in the Marginellidae.

Siphonal notch: a notch for the siphon at the anterior of the shell, absent in many cystiscids.

Posterior notch: a notch at the posterior end of the aperture.  It is sometimes difficult to distinguish a weak siphonal notch from the normal junction of shell lip with the shell, but a well defined  posterior notch is obvious.

Columellar plaits and lirae: true columella plaits are formed continuously on the columella as the shell grows; lirae are short elongated ridges that only extend a short way into the aperture.  It is difficult from external examination to tell which are plaits and which are lirae, so shell descriptions give a total of plaits/lirae.  For NSW species, all marginellids have four plaits except one that has three; cystiscids vary from two to eight plaits/lirae.

Colour: in at least some species, the shell is completely transparent and colourless when the animal is alive, with the animal’s body being plainly visible through the shell.  In a few species opaque spots or bands occur on the transparent shell.  As the shell ages it becomes opaque and white.