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Fluviolanatus subtortus (Dunker, 1857)

Diagnostic features

Shell medium-sized, thin, brown with brown colour rays internally, fragile elongate/oblong, inequivalve overlapping valves, slight posterior gape, umbones at anterior end, ligament internal, hinge teeth absent, adductor and pedal retractor muscle scars unequal. Anatomy: Eulamellibranch gills, foot small with byssal gland, small siphons formed by mantle folds, extensive ventral mantle fusion, unusually large non-plicate gills, labial palps quite small, expanded pallial folds of the stomach house algal cells (zooxanthellae), the mid gut is not extensively joined to the style sac. Heart traversed by rectum.


Fluviolanatus subtortus (Dunker, 1857)

Class Bivalvia

Subclass Heterodonta

Superfamily Arcticoidea

Family Trapezidae

Genus Fluviolanatus Iredale, 1924 (Type species: Modiolarca subtorta Dunker, 1857).

Original reference: Iredale T. (1924). Results from Roy Bell's molluscan collections. Proceedings of the Linnean Society of New South Wales, 49: 179-278, pl. 31-36.

Original name: Modiolarca (Volsella) subtorta Dunker, 1857. Dunker, W. 1857. Mytilacea nova collectionis Cumingianae. Proceedings of the Zoological Society, London 1856: 358-366.

Type locality: North Australia.

Synonyms: Fluviolanatus amarus Laseron, 1956

Biology and ecology

Lives intertidally and subtidally attached to logs and reeds with a fine byssus in lower reaches brackish estuaries and coastal lagoons at or near the junction with freshwater. Also lives in brackish environments in coastal lakes, upper reaches of harbours, bays and inlets. Fluviolanatus appears to possess zooxanthellae in the mantle folds. It ranges into freshwater in southwest Western Australia.


Australian endemic. Disjunct - eastern and northern Australia from Victoria to Northern Territory and SW Western Australia. Absent from southern Tasmania.


This mainly brackish-water species has an unusual disjunct geographical range. The isolated southwest Western Australian population contains some populations ranging into pure freshwater, and it is commonly found alongside freshwater planorbids and lymnaeids.

Note that there is a Trapeziidae (with two i's) in Crustacea.

Further reading

Beesley, P. L., Ross, G. J. B. & Wells, A., Eds. (1998). Mollusca: The Southern Synthesis. Parts A & B. Melbourne, CSIRO Publishing.

Lamprell, K. & Healy, J. (1998). Bivalves of Australia, volume 2. Leiden, Backhuys Publishers.

Huber, M. (2010). Compendium of bivalves. A full-color guide to 3,300 of the World’s Marine Bivalves. A status on Bivalvia after 250 years of research. Hackenheim, Germany, ConchBooks.

Morton, B. (1982). The biology, functional morphology and taxonomic status of Fluviolanatus subtorta (Bivalvia: Trapeziidae), a heteromyarian bivalve possessing "zooxanthellae". Journal of the Malacological Society of Australia 5: 113-140.

Ponder, W. F., Clark, S. A. & Dallwitz, M. J. (2000). Freshwater and estuarine molluscs: an interactive, illustrated key for New South Wales. Melbourne, CSIRO Publishing.