Known from southeastern Australia, with confirmed records from New South Wales and Victoria, and an unconfirmed report from Tasmania.
Upper Photo: Anderson Point, Portland, Victoria, Australia, 23 February 2007. Photo: Leon Altoff .
Lower Photo: Holotype of Hydatina exigua (Australian Museum collection) Scale = 1mm.
The true relationship of this species to other opisthobranchs is somewhat problematic because its anatomy has never been examined. It was originally describe on the basis of an empty shell (Hedley, 1912) and placed in the genus Hydatina [see H. physis], but Iredale (1936) transferred it to a new genus Noalda, with no further evidence than the original shell.
Recent drawings of the living animal (Burn & Thompson, 1998) and Leon Altoff's photos [see message #23262] certainly suggest it is not a hydatinid. Burn & Thompson suggest it is an aglajiid but we really need to examine its anatomy.
The partly enclosed shell, with two brown lines, the bifid 'tail', and bright yellow spots are all characteristic of this species.
I have copied Hedley's and Iredale's descriptions below:
"Shell minute, thin globular, truncate above, rounded below, imperforate. Colour: on a white ground are two narrow peripheral chcolate bands, twice thie breadth apart, connected with a wash of cream and visible internally. Surface smooth. Within the aperture a layer of dull callus is spread on the prededing whorl. Columella deeply, spirally inserted. Apex perforate, spire immersed. Height 1.3 mm breadth 1.5 mm. Middle head, Sydney, 15 may 1902. "
Description of Hydatina exigua Hedley 1912. p. 158-59 Pl XLV fig 46
"This curious little shell which Hedley called Hydatina exigua is obviously not congeneric with the preceding [Hydatina] species and is here distinguished generically with the new name Noalda. It is less than 2mm in length and breadth and the apex is quite different from that of Hydatina, the body whorl being also comparatively smaller, the mouth consequently more open."
Iredale's description of genus Noalda
- Hedley, C. 1912. Descriptions of some new or noteworthy shells. Records of the Australian Museum 8: 131-160.
- Iredale, T. 1936. Australian Molluscan Notes. No.2. Records of the Australian Museum 19: 267-340; pls 20-24.
- Burn, R. & Thompson, T.E. (1998). Order Cephalaspidea. Mollusca: The Southern Synthesis. Fauna of Australia. P. L. Beesley, G. J. B. Ross and A. Wells. Melbourne, CSIRO Publishing. 5, Part B.: 943-959.
- Burn, R., 2006. A checklist and bibliography of the Opisthobranchia (Mollusca: Gastropoda) of Victoria and the Bass Strait area, south-eastern Australia. Museum Victoria Science Reports 10: 1-42.
Rudman, W.B., 2010 (February 19) Noalda exigua (Hedley, 1912). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/noalexig