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Cucumerunio novaehollandiae (Gray, 1834)

Diagnostic features

Shell large sized (to about 200 mm in length), oblong to elongate (width/length ratio about 40%), equivalve, solid valves, sculpture of radial growth lines and coarse branching wrinkles on the posterior ridge and spreading dorsally and ventrally from it. Umbos with distinct ridges but are often eroded, thick black periostracum, interior of valves nacreous bluish to bronze to white.  Schizodont teeth – pseudocardinal teeth very erect, grooved, serrated and frequently denticulate. 'Lateral' teeth strong. Inhalant and exhalant siphons formed by the mantle edge which is open ventrally and fused posteriorly, branchial siphon larger than anal siphon bearing a variable number of prominent papillae, heavily pigmented and open below. Marsupium occupies middle four fifths of inner gill of females and possesses numerous incomplete septa.

Anatomy: The gills (ctenidia) are eulamellibranch and the foot is a compressed, tongue-shaped foot lacking a byssal groove. Larvae are brooded in a marsupium that occupies about four fifths of the inner pair of demibranchs. Inhalant and exhalant siphons large and prominent and formed by the mantle edge which is open ventrally, branchial 'siphon' larger than anal 'siphon' and with three rows of internal papillae; siphons heavily pigmented. Labial palps large, semilunar in shape.

The Australian genera of freshwater mussels are distinguished by the following shell characters (note that all are subject to erosion with age, depending on the local environment):-

Hyridella. Beak of young specimens at least sculptured with V-shaped ridges; shell quadrate to elongate (ratio of maximum height of shell to its length >50%), not markedly winged. Hinge strong with grooved pseudocardinal teeth and simple 'lateral' teeth. Shell surface (other than beaks) more-or-less smooth except for concentric growth lines.

Velesunio. Beaks smooth, shell can be rather thick, rounded in outline (ratio of maximum height of shell to its length >50%), often inflated, hinge lamellar, usually simple (rarely serrated). Shell surface with concentric growth lines only.

Alathyria. Shell typically large, elongate-ovate (ratio of maximum height of shell to its length >50%), often distinctly winged, thick, hinge usually with heavy, pseudocardinal teeth grooved, 'lateral' teeth smooth. Shell surface more-or-less smooth, with concentric growth lines only.

Cucumerunio. Shell very elongate (ratio of maximum height of shell to its length <40%), beaks sculptured with V-shaped ridges; rest of shell surface with conspicuous nodules or ridges. Hinge strong, pseudocardinal teeth grooved.

Lortiella. Shell elongate (ratio of maximum height of shell to its length <45%), winged posteriorly, hinge simple, not well developed. Beaks smooth and shell surface with concentric growth lines only. Found in NW Australia.

Westralunio. Shell more or less oblong (ratio of maximum height of shell to its length >50%). Pseudocardinal teeth erect, strongly serrated, shell small (less than 70 mm in length). Beaks smooth, shell rather thick, with concentric growth lines only. Restricted to SW Australia.

Classification

Cucumerunio novaehollandiae (Gray, 1834)

Common name: Freshwater Mussel; New Holland Mussel; Cucumber Mussel

Class Bivalvia

Subclass Heteroconchia

Superorder Palaeoheterodonta

Order Unionida

Superfamily Unionoidea

Family Hyriidae

Subfamily: Cucumerunioniae

Genus Cucumerunio Iredale, 1934 (Type species: Unio novaehollandiae Gray, 1834).

Original name: Unio novaehollandiae Gray, 1834. Gould, A. (1850). in Proceedings for May 18th, 1850. Proceedings of the Boston Society of Natural History 3: 292-296.

Type locality: Macquarie River (as Macquarrie), probably Port Macquarie, mouth of Hastings River, New South Wales.

Synonyms: Unio cucumoides Lea, 1840; Unio cumingianus Dunker, 1852; Unio navigioliformis Lea, 1859.

State of taxonomy

The last major taxonomic revision of Australian freshwater mussels was by McMichael and Hiscock (1958).

Based on the available molecular results, Walker et al. (2014) pointed out that a re-assessment of Australian hyriids is needed.

Biology and ecology

Shallow burrower in sand/gravel in actively flowing parts of rivers and large streams. Infaunal suspension feeder, living two thirds to almost fully buried in sand, sediment and amongst rocks. Sexes separate. Larvae (glochidia) are brooded by females in marsupia in the inner pair of demibranchs of the gill and, when released, become parasitic on fish gills before dropping to the sediment as young mussels. Thus, as with other unionoideans, the fish hosts serve as dispersal agents. Spawns in late summer and autumn and releases glochidia in winter (H. Jones, pers. comm.).

Additional information on the biology and ecology of members of this family can be found in Fauna of Australia, vol. 5A, p. 296-298.

Distribution

Coastal rivers of mid Queensland to northern New South Wales as far south as the Hunter River. Only occurs at lower elevations (below 150 m - H. Jones, pers. comm.).

Notes

The Australian genera of freshwater mussels are distinguished by the following shell characters (note that all are subject to erosion with age, depending on the local environment):-

Hyridella. Beak of young specimens at least sculptured with V-shaped ridges; shell quadrate to elongate, not markedly winged. Hinge strong with grooved pseudocardinal teeth. Shell surface (other than beaks) more-or-less smooth except for concentric growth lines.

Velesunio. Beaks smooth, shell can be rather thick, rounded in outline, often inflated, hinge lamellar, usually simple (rarely serrated). Shell surface with concentric growth lines only.

Alathyria. Shell typically large, elongate-ovate, often distinctly winged, thick, hinge usually with heavy, teeth smooth. Shell surface more-or-less smooth, with concentric growth lines only.

Cucumerunio. Shell very elongate, beaks sculptured with V-shaped ridges; rest of shell surface with conspicuous nodules or ridges. Hinge strong, pseudocardinal teeth grooved.

Lortiella. Shell elongate, winged posteriorly, hinge simple, not well developed. Beaks smooth and shell surface with concentric growth lines only. Found in NW Australia.

Westralunio. Pseudocardinal teeth very strongly serrated, shell small (less than 70 mm in length). Beaks smooth, shell rather thick, with concentric growth lines only. Restricted to SW Australia.

The monotypic genus is unique in its characteristic shape, shell sculpture and hinge. Cucumerunio novaehollandiae is the largest freshwater mussel in Australia.

Further reading

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

Conrad, T.A. (1850). A synopsis of the family of naiades of North America with notes, etc. Proceedings of the Academy of Natural Sciences of Philadelphia 6: 243-269

Graf, D. L., Jones, H. A., Geneva, A. J., Pfeiffer, J. M. III & Klunzinger, M. W. (2015). Molecular phylogenetic analysis supports a Gondwanan origin of the Hyriidae (Mollusca: Bivalvia: Unionida) and the paraphyly of Australasian taxa. Molecular Phylogenetics and Evolution. 85: 1-9.

Haas, F. (1969). Superfamilia Unionacea. Das Terreich, 88 (1-10), 1-663.

Iredale, T. (1934). The freshwater mussels of Australia. Australian Zoologist 8: 57-78 pls 3-6.

Iredale, T. (1943). A basic list of the fresh water Mollusca of Australia. Australian Zoologist 10: 188-230.

Jones, H. A., Simpson, R. D. & Humphrey, C. L. (1986). The reproductive cycles and glochidia of Fresh-water mussels (Bivalvia: Hyriidae) of the Macleay river, Northern New South Wales, Australia. Malacologia 27: 185-202.

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

McMichael, D. F. & Hiscock, I. D. (1958). A monograph of the freshwater mussels (Mollusca: Pelecypoda) of the Australian region. Australian Journal of Marine and Freshwater Research 9: 372-508.

Walker, K. F. (1981). The distribution of freshwater mussels (Mollusca: Pelecypoda) in the Australian zoogeographic region. Pp. 1233-1249 in A. Keast. Ecological Biogeography of Australia. The Hague, Dr W. Junk.

Walker, K. F. (2004). A guide to the provisional identification of the freshwater mussels (Unionoida) of Australasia. Albury, Murray Darling Freshwater Research Centre.

Walker, K. F., Byrne, M., Hickey, C. W. & Roper, D. S. (2001). Freshwater Mussels (Hyriidae) of Australasia. Pp. 5-31 in G. Bauer & Wächtler, K. Ecology and Evolution of the Freshwater Mussels Unionoida. Ecological Studies. Berlin, Springer-Verlag.

Walker, K. F., Jones, H. A. &  Klunzinger, M. W. (2014). Bivalves in a bottleneck: taxonomy, phylogeography and conservation of freshwater mussels (Bivalvia: Unionoida) in Australasia. Hydrobiologia 735:61–79.

Woodward, F.R. (1984). What is Unio naviglioformis (Mollusca: Bivalvia: Unionacae)? Durban Museum Novitates 13:195-201.