- Ferrissia fragilis (Tryon 1863)
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A tiny freshwater limpet with ovoid, flattened shell which is easily mistaken for a juvenile Acroloxus lacustris. Unlike Acroloxus, however, the apex of the shell is evenly rounded in profile and lacks the distinctive sharply pointed apex of that species. The apex of Ferrissia is blunt and arches backwards and to the right (not to the left as in Acroloxus). Found in aquaria and introduced widely in Europe, in ponds, lakes and larger rivers.
Key characteristics
- Very small, ovoid, limpet-like shell, clinging to aquatic plants
- Shell rounded in profile, apex blunt and twisted to the right
- Shell nearly smooth but showing radial ridges near the apex
Size
3-4 mm (often smaller).
World Distribution
Native to northern North America, but present in many areas of Europe south to the Dunube and the Mediterranean. It probably occurs in other parts of the Palaearctic as well, as a cryptic invader.
Irish Distribution
A common inhabitant of tropical and coldwater aquaria which is almost certainly how it has gained entrance to Irish fresh waters. A single site is currently known: Black Lough, Corkeenagh, Co. Roscommon where it was found on 30 October 2006. There are no further reports.
Ecology
- Primarily found on floating leaves of aquatic macrophytes
- Also on submerged stems of reeds and grasses
- In aquaria may be seen adhering to the glass sides
- Mainly found in artificial ponds but also in other small areas of still or gently flowing water
Taxonomy
The name to be applied to this taxon is contentious. In the Clecom List (Falkner et al., 2001) it is considered synonymous with F. clessiniana (Jickeli 1882), which was described from Alexandria in Egypt. Glöer (2002) retains the name F. wautieri (Mirolli 1960) on the basis that its identity and internal morphology are well established whereas some of the alternative contenders are poorly characterized. More recently the molecular and other evidence provided by Walther et al. (2006) and Beran & Horsàk (2007) confirms the suspicion that this is a North American taxon introduced to Europe for which the correct name is F. fragilis (Tryon 1863).
Red List status
- Not applicable