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Family  Marginellide

Margin Shells

The Marginellidae is a family of minute to moderately sized gastropods with shells that are usually smooth and white, occasionally coloured, with strong plaits on the columella and a thickened margin on the outer lip.  Most species of the family are less than about 15 mm in length, although there are some larger, attractive tropical species.  The Marginellidae shares shell appearance and features with the Cystiscidae, and it is only recently that the two families have been separated.

A major advance in the understanding the Marginellidae and Cystiscidae was achieved in 1995, with the publication by G.A. Coovert and H.K. Coovert of a revision of the families (See Family References, below).  They presented much new data on the anatomy of the animals, particularly radulae and foregut structure, which made it clear that two families were involved.  They were able to provide a concise generic classification based on phylogeny of the animals rather than the previous shell-based schemes. Dean Hewish of Melbourne, working with Coovert & Coovert, produced a new list of the Australian Marginellidae and Cystiscidae, which was  published by Wilson (1994, pp. 139-143).

There are substantial differences in shell form between the two families. In the cystiscids the internal whorls of the shell are resorbed by the animal as the shell grows, leaving the shell interior hollowed out.  In marginellids, the early whorls remain intact, covered by later whorls as the shell grows, as shown in the illustration. The structure of the columellar plaits differs markedly between the two families.  The number of plaits is constant within a species, but variable between species, and is an important character for defining species.  In the marginellids, species have 2-6 plaits on the columella (4 in all NSW species except one), which are all about the same size.  In the cystiscids, species have  3-10 plaits, generally becoming smaller progressively up the columella. Another shell feature is that the marginellids usually have an external thickening of the outer lip, (the “margin” from which the family name is derived), but this is usually absent in the cystiscids.

The Marginellidae of NSW, including those now recognised as the Cystiscidae, was revised in two papers by Charles Laseron, in 1948 and 1957.  In the first paper he itemised the NSW species, and in the second, he proposed a generic arrangement.  Laseron was working with only a few specimens of some species, without the benefit of modern microscopes, and without access to types of some of the species. Consequently, some of the specimens he illustrated were not of the correct species, and he erected new species duplicating some already existing.

In NSW, the marginellids occur from intertidal to at least 1100 metres depth.  Of the 26 species recognized from NSW, only one,  Serrata mustelina, occurs intertidally, where it lives on exposed rocky shores under rocks and in crevices.  About twelve species occur in sufficiently shallow water to be common as beach shells, but the remainder must be obtained by dredging or trawling. The precise habitat and food of the species is generally unknown.  Two Austroginella species living in sand in shallow water drill holes in minute bivalves.

Our current state of knowledge of the NSW marginellids is still very imperfect.  In some cases, such as Mesoginella olivella, it is impossible to determine if there is one variable species, or if there are groups of closely related species with overlapping characters.  This question will not be resolved until research is done into the anatomy, genetics, ecology and reproduction of the family.  Consequently, in this work a firm position is often not taken on delineation of the species.

Family References

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

• 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.  The Australian Museum collection contains a few unnamed NSW species, usually only a few lots of each, which await a complete revision of the family.

Identification Notes

The treatment here largely follows the terminology established by Coovert & Coovert. 

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 high, or immersed where the apex is below the level of the latter whorls.

Outer lip:  The outer lip is more or less thickened internally in mature shells in this family. On this internal thickening there may be denticles, which are small circular projections.  Some species are quite variable in the presence or absence of lip denticles, and in others they are only present when the lip is very mature.

External varix: A thickening on the outside of the outer lip is called an external varix, which is present in all the NSW Marginellidae.

Siphonal notch: a notch for the siphon at the anterior of the shell.  This is characteristic of a genus; present in some genera, absent in others.

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

Columellar plaits: all NSW marginellids have four plaits on the columella, except Balanetta baylei which has only three.

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.  As the shell ages it becomes opaque and white.  Some species have pale brown bands, but they are usually present in only a proportion of specimens, and are not a useful feature for identification.