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Hyridella Swainson, 1840

Diagnostic features

Shell medium-sized, oblong to oval to elongate, not winged; solid to thin valves, sculpture of growth lines; umbos with distinct chevron ridges but commonly eroded; thick black periostracum; interior of valves nacreous bluish to bronze to white; schizodont hinge teeth where the central pseudocardinal teeth tend to become fissured radially.  

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 a third of the inner pair of demibranchs. Inhalant and exhalant siphons short but prominent and formed by the mantle edge which is open ventrally, branchial 'siphon' larger than anal 'siphon' bearing a variable number of prominent papillae and heavily pigmented. 

The SE Australian species of Hyridella can be separated as follows:

H. australis. With heavy beak sculpture, restricted to umbos.  Shell oblong-ovateposterior end strongly ridged and rounded, reaches 90 mm in length, height/length ratio about 60%.

H. drapeta. With fine beak sculpture, restricted to umbos. Shell moderately elongate, oval, posteriorly rather rounded and with weak flattened ridge, dorsal margin weakly winged; reaches about 90 mm in length, height/length ratio less than 50%.

H. depressa. With fine beak sculpture, restricted to umbos. Shell moderately elongate, oval, posteriorly rather rounded and with weak ridge, postero-dorsal margin angled and dorsal margin anterior to beaks slopes away markedly (flatter in the other species); reaches about 90 mm in length, height/length ratio about 50%.

H. glenelgensis. Strong umbonal sculpture extends onto surface of adult. Shell almond-shaped, posterior end ridged and pointed, reaches 40 mm in length.

H. narracanensis. With heavy beak sculpture, restricted to umbos. Shell almond-shaped, posteriorly ridged and pointed, reaches 60 mm in length.

 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

Class Bivalvia

Subclass Heteroconchia

Superorder Palaeoheterodonta

Order Unionida

Superfamily Unionoidea

Family Hyriidae

Subfamily: Hyridellinae

Genus Hyridella Swainson, 1840

Subgenus Hyridella Swainson, 1840

Type species: Unio australis Lamarck, 1819

Original reference: Swainson, W. (1840) A treatise on Malacology or the natural classification of shells and shellfish, London. Longman, Brown, Green and Longmans. 419 pp.

Type locality: Nepean River, New South Wales.

Synonyms: Leiovirgus, Haas, 1912; Propehyridella Cotton & Gabriel, 1932; Hyridunio Iredale, 1934; Rugoshyria Iredale 1934.

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

This genus favours flowing freshwater rivers and streams, but also occurs in water supply dams. Infaunal, living two thirds to almost fully buried in sand and sediment. Suspension feeders. Sexes separate; females brood young in marsupia in the inner pair of demibranchs. Larvae (glochidia) are parasitic, using fish as hosts and dispersal agents.

Distribution

Eastern Australian coastal rivers, including Tasmania. The genus also occurs in New Zealand and Papua New Guinea.

Notes

Rugoshyria’ interserta (Iredale, 1934) and ‘Rugoshyria’ aquilonalis (Iredale,1934) are both known only from single specimens and have never been recollected. They were placed in the synonymy of Hyridella by McMichael and Hiscock (1958), as we have done here, but the validity and relationships of both are uncertain.

Further reading

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

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.

Jones, H. A. & Byrne, M. (2014). Changes in the distributions of freshwater mussels (Unionoida: Hyriidae) in coastal southeastern Australia and implications for their conservation status. Aquatic Conservation: Marine and Freshwater Ecosystems 24: 203-217.

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.

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.

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.

Smith, B. J. (1992). Non-marine Mollusca. Pp. i-xii, 1-408 in W. W. K. Houston. Zoological Catalogue of Australia, 8. Canberra, Australian Government Publishing Service.

Smith, B. J. & Kershaw, R. C. (1979). Field guide to the non-marine Molluscs of South-eastern Australia. Canberra, A.N.U. Press.

Smith, B. J. & Kershaw, R. C. (1981). Tasmanian Land and Freshwater Molluscs. Hobart, University of Tasmania.

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.