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Alathyria Iredale, 1934

Diagnostic features

Alathyria is distinguished from other hyriid genera in lacking sculpture on the shell and beaks; ratio of maximum height to maximum length more than 50%; the shell thickened in the antero-dorsal region; maximum length 80-150mm; usually winged (dorsal margin extended as a posterior blade in some species, or there is at least a dorsal arch); some with a postero-ventral inflection.  The hinge is strong, unionid-type with the pseudocardinal teeth erect and denticulate, the lateral teeth smooth.

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 three quarters of the inner pair of demibranchs. Inhalant and exhalant siphons not protruding and formed by the mantle edge which is open ventrally, branchial 'siphon' larger than anal 'siphon' bearing two to three rows of papillae internally; heavily pigmented. Labial palps large, semilunar in shape.

Distinguishing the species of Alathyria:

A. pertexta: rather elongate (maximum height relative to maximum length about 50- 55%), has the dorsal margin excavated anterior to the beaks, reduced hinge teeth and a characteristic purple to pink internal nacre (other species white).

A. jacksoni: usually oval (maximum height relative to maximum length 55-60%), posterior dorsal margin winged in moderate currents and arched in strong currents; rarely with reduced teeth although they may be eroded.

A. condola (inland drainages): rounded and swollen shell (maximum height relative to maximum length 65%), posterior dorsal margin not winged, usually excavated anterior to the beaks; strong hinge teeth.

A. profuga (coastal drainages): more elongate (maximum height relative to maximum length less than 65%), dorsal margin winged; reduced hinge teeth.

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%), usually 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 or very weakly sculpured, 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. Beaks smooth to moderately sculpured. 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%), usually 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 sculptured in juveniles, shell rather thick, with concentric growth lines only. Restricted to SW Australia.

Classification

Class Bivalvia

Subclass Heteroconchia

Superorder Palaeoheterodonta

Superfamily Hyrioidea

Family Hyriidae

Subfamily: Velesunioninae

Genus Alathyria Iredale, 1934

Type species: Alathyria jacksoni Iredale, 1934

Original reference: Iredale, T. (1934). The Freshwater mussels of Australia. Australian Zoologist 8: 57-78.

Type locality: Barwon River, New South Wales.

Synonyms: Quaesithyria Iredale, 1943

State of taxonomy

The last major taxonomic revision of Australian freshwater mussels was by McMichael & 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

Rivers, streams and water catchment dams. Infaunal, living two thirds to almost fully buried in sand and sediment. Suspension feeder. Sexes separate. Females brood young in marsupia in the inner pair of demibranchs. Larvae (glochidia) parasitic, using fish as hosts and dispersal agents.

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

Distribution

Australian mainland between Gulf of Carpentaria drainage rivers, Northern Territory, through central and south-eastern Queensland, south to south-eastern New South Wales, Victoria and South Australia (in Tasmania as fossils). Also occurs in Papua New Guinea.

Notes

The variation in the posterior dorsal shape appears to be associated with water velocity. The changes apparently enhance the mussel's ability to maintain an anchorage in strong currents (Balla and Walker, 1991).

Further reading

Balla, S. A. & Walker, K. F. (1991). Shape variation in the Australian freshwater mussel Alathyria jacksoni Iredale (Bivalvia, Hyriidae). Hydrobiologia 220: 89-98.

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.

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. 

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.

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.

Zieritz, A., Sartori, A. F. & Klunzinger, M. W. (2013). Morphological evidence shows that not all Velesunioninae have smooth umbos. Journal of Molluscan Studies 79: 277–282.