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Family  Omalogyridae





The Omalogyridae is a small family of minute, almost planar shells which are some of the smallest molluscs known. They are usually less than 2 mm in diameter, but one NSW species is merely 0.6 mm! Omalogyrids have thin reddish or brown translucent shells. They occur worldwide in the lower intertidal and subtidal zones, living on or in association with algae.

Omalogyrids feed on the contents of alga cells, such as Ulva, by puncturing the cells with radular teeth and sucking out the contents. The NSW species have been found abundantly on Ulva intertidally, and on other algae. They have also been reported from under stones, where they are probably feeding on minute algae.

Omalogyrids are hermaphrodite with separate male and female gonads within the same individual. The European species Omalogyra atomus, which is the best studied species, is protandrous, having separate ovary and testis, being at first male then changing to female. Egg capsules are attached to algal fronds.

There are only three species of omalogyrids known from NSW, all described by Charles Laseron in 1954. He placed them in the family Liotiidae, where they remained until Ponder & de Keyzer (1998) investigated the animal's external morphology and transferred them to the Omalogyridae.

Family References

Laseron, C. 1954. Revision of the Liotiidae of New South Wales. The Australian Zoologist 12:1-25.

Ponder, W.F. & de Keyzer, R.G. 1998. Superfamily Omalogyroidea Pp. 864-865 in Beesley, P.L., Ross, G.J.B. & Wells, A. (eds) Mollusca: The Southern Synthesis. Fauna of Australia Vol. 5. CSIRO Publishing: Melbourne, part B viii 565-1234 pp.


The three named species from NSW are detailed here, but it is likely that there are further undescribed species.

Identification Notes

The shells of this family are recognised by

  • Small size, less than 2 mm in diameter.

  • Almost planar, translucent reddish brown shell

  • Spire depressed, umbilicus widely open, making the dorsal and ventral aspects of the shell almost identical.

Families that have similar shells are:

  • Skeneidae, genera Liotella and Lodderina - the spire is not depressed.

  • Orbitestellidae, genura Orbitestella and Microdiscula - These are very similar to omalogyrids, with sunken spire and open umbilicus. Species-by-species comparison is necessary to see the differences.

  • Architectonicidae - although these have a wide umbilicus, all are much larger than the omalogyrids.


Copyright Des Beechey 2016