The Freshwater Mussels (Hyriidae, Unionidae) of Australasia

KF Walker

Cooperative Research Centre for Freshwater Ecology, Department of Environmental Biology, University of Adelaide

Introduction

Most of the 29 known species of freshwater mussels in the Australian Zoogeographic Region (Australia, New Guinea, the Solomon Islands and New Zealand) are members of the Hyriidae, a family shared with South America and distantly related to the unionid mussels of other continents. The exceptions, two species of Haasodonta in New Guinea, are placed uncertainly with the Unionidae. Together, Hyriidae and Unionidae belong to the superfamily Unionoidea (Unionacea), in the order Unionoida.

Hyriid mussels for many years were in a thorough taxonomic muddle. McMichael and Hiscock (1958) published a monograph of the regional fauna, then considered part of the family Mutelidae. They did not include a key, but later McMichael (1967) provided a key to Australian genera, based on shell characters. Some problems identified in the review remain unresolved, and in some cases the diagnoses of species, genera and even subfamilies remain equivocal. As a result, it has been difficult to make confident identifications.

Shell shape and sculpture and hinge dentition presently are the basis of hyriid taxonomy, whereas anatomical and reproductive characters are more significant at family level and above. The difficulty here is that shapes vary with local environmental conditions, so that variation even within populations can be bewildering. The 1958 monograph uses characters interchangeably at different levels in the hierarchy, suggesting that the underlying concepts of genus and species are confused.

Although questions of hyriid phylogeny have languished for many years, they have been revitalised by advances in molecular cladistics, and new progress is being made. A review of the present state of knowledge, including a bibliography of ecological, physiological and taxonomic studies, is provided by Walker et'al . (1999).

It is not the business of a key to venture taxonomic opinions. The present one is faithful to the review by McMichael and Hiscock, with few exceptions. At a general level, it follows Parodiz and Bonetto (1963) in transferring the 'Mutelidae' sensu McMichael and Hiscock to the Hyriidae, and it follows Heard and Guckert's (1970) criteria for distinguishing the Hyriidae and Unionidae. The subspecies, but not the subgenera, recognised by McMichael and Hiscock are indicated. In some instances the couplets are not wholly satisfactory, but the author has not felt entitled to introduce better diagnoses.

There are good reasons, therefore, why this key lacks a precedent, and these might still be used to argue for postponement. However, if users maintain a generous regard for the uncertainties in freshwater mussel taxonomy, it should be a useful aid, particularly for field workers. There are traps for the unwary, and for critical work it is wise to refer to the literature and to compare shells with those in museum collections.

Guidelines

The key applies only to the Australasian fauna (the Australian Zoogeographic Region), and is intended as a companion to the review by McMichael and Hiscock.

The key should be used with a series of shells from any locality, one that reflects changes with age and habitat conditions. Ideally, the series should include live animals.

References to shapes or dimensions are averages and apply to adult shells unless qualified.

As this is a text-only format, it may be helpful to refer to the photographs of sample shells (often type specimens) provided by McMichael and Hiscock.

Distributions within Australia are indicated by Roman numerals that refer to the major drainage divisions (below). The letters NG, NZ and SI refer to New Guinea, New Zealand and the Solomon Islands respectively. Numerals or letters in parentheses indicate restricted occurrences (say, less than one-third of a drainage division). Further information is in the bibliography.

I North-East

II South-East

III Tasmania

IV Murray-Darling

V South Australian Gulf

VI South-West Coast

VII Indian Ocean

VIII Timor Sea

IX Gulf of Carpentaria

X Lake Eyre

XI Bulloo-Bancannia

XII Western Plateau

A full glossary would occupy more space than is available, but reference to McMichael and Hiscock (1958) should clarify any problems. Note that ?demibranch? is used here as a synonym for ?ctenidium? and ?gill?, and that ?pseudocardinal? is used in preference to ? cardinal?, in relation to hinge dentition.

The following shell characters are cited:

Using the beaks (umbos) as a reference, the shorter distance to the end of the shell is the anterior region, and the longer distance is the posterior region.

The dorsal margin is hinged; the ventral margin is not.

The posterior dorsal margin may be extended as a blade or 'wing', or it may be rounded.

There are conspicuous adductor (opening) and retractor (closing) muscle scars (points of muscle attachment) in both left and right valves, in both anterior and posterior regions of the shell. Adductor scars generally are largest.

The pallial line , on the inner shell surface slightly in from the edge of the shell, marks the line of contact between mantle tissue and shell.

Inhalant and exhalant siphons are formed by the mantle margins at the posterior end of the shell.

The Maximum Height Index (MHI) is the ratio of maximum height of the shell to its length.

The Beak Height Index (BHI) is the ratio of 'beak height' (the perpendicular height of the shell at the beak) to the maximum height.

The Beak Length Index (BLI) is the ratio of 'beak length' (the distance from the beak to the anterior shell margin) to shell length. In other words, it is the ratio of the anterior to posterior axes.

These ratios generally are expressed as percentages.

1a. Marsupium present in each of the two innermost demibranchs of the female; bronchial and anal siphons distinct but without a separate supra-anal opening; diaphragm formed anteriorly by gills, and posteriorly by union of mantle margins; glochidia hooked(Family Hyriidae) .......................... 2

lb. Marsupium present in each of the two outermost demi-branchs of the female; bronchial and anal siphons distinct but anal siphon closed above, forming a distinct supra-anal opening; diaphragm formed entirely by gills; posterior mantle margins not joined; glochidia hooked (Family Unionidae) ........................ 28

2a. In young specimens, beaks and shell surface without sculpture ..........3

2b. In young specimens (and well-preserved older specimens), beaks with sculpture and shell surface with or without sculpture................17

3a. Shell markedly elongate: MHI <45% (sub-family Lortiellinae, genus Lortiella ) (Footnote1)..4

3b. Shell not markedly elongate: MHI >50% (sub-family Velesunioninae) .........5

4a. Hinge reduced; shell posterior squarely truncate

Lortiella rugata (Sowerby) : IX

4b. Hinge strong; shell posterior rounded, obliquely truncate

Lortiella froggatti Iredale : IX

5a. Shell material usually thickened in antero-ventral region; maximum length 80-150 mm; usually winged; strong unionid hinge, pseudocardinals erect and denticulate .......................... 6

5b. Shell material usually thin; maximum length 40-80 mm, rarely to 120 mm; rarely winged; hinge and pseudocardinals variously developed .........10

6a. Shell posterior rostrate

Microdontia anodontaeformis (Tapparone Canefri) : (NG)

6b. Shell posterior not rostrate (genus Alathyria ) ....... 7

7a. Shell elongate: MHI <55%

Alathyria pertexta Iredale : I, (II), IX, (X) (NG)

(Footnote 2)

7b. Shell less elongate: MHI >55% ................ 8

8a. Shell oval

Alathyria jacksoni Iredale : IV (Footnote 3)

8b. Shell rounded ..... 9

9a. Shell not winged but subcircular and swollen: MHI near 65%; teeth strong

Alathvria condola Iredale : (IV)

9b. Shell winged: MHI <65%; teeth reduced

Alathyria profuga (Gould): (II)

10a. Shape oval to subcircular; pseudocardinals simple lamellae (genus Velesunio )........... 11

10b. Shape more-or-less oblong; pseudocardinals erect and serrated (genus Westralunio ) .... 15

11a. Adult length <50 mm

Velesunio sentaniensis (Haas) : (NG)

11b. Adult length >50 mm .......................... 12

12a. Shell relatively elongate: MHI <60% ......13

12b. Shell less elongate: MHI >60% ..... 14

13a. Tapered posterior, rarely more than slightly winged; MHI <55%, siphons little pigmented

Velesunio wilsonii (Lea) : I, (II), (VIl), VIll, (IX), X

13b. Posterior expanded, usually winged; MHI >55%; siphons brick red with dark blotches

Velesunio angasi (Sowerby) : (VII), (VIII), IX

14a. Pseudocardinals smooth, rarely grooved; BHI usually >90% but not <85%; anterior adductor scar weak except in old specimens; ventral margin at least slightly rounded

Velesunio ambiguus (Philippi) : I, II, IV, V, (X), XI

14b. Pseudocardinals grooved; BHI usually <85%, rarely to 90%; anterior adductor scar usually deeply impressed; ventral margin straight over mid-section

Velesunio moretonicus (Reeve) (III) (Footnote 4)

15a. Shell subcircular

Westralunio albertisi Clench : (NG)

15b. Shell more or less oblong ....... 16

16a. Shell posterior squarely truncate

Westralunio carteri Iredale : VI

16b. Shell posterior obliquely truncate

Westralunio flyensis (Tapparone Canefri) : (NG)

17a. Female marsupium in middle third of two inner demibranchs (subfamily Cucumerunioninae) ............................................... 18

17b. Female marsupium in middle four-fifths of two inner demibranchs (subfamily Hyridellinae) .. 20

18a. Anterior region relatively long: BLI about 20%; length to 200 mm; lateral teeth long, heavy; anterior retractor and adductor fused (genus Cucumerunio ) ........................ 19

18b. Anterior region short: BLI 10-15%; length to 100 mm; lateral teeth slender, serrated posteriorly; anterior retractor separate from adductor, scar forming separate pit under pseudocardinals

Virgus beccarianus (Tapparone Canefri) : (NG)

19a. Elongate, length to 200 mm; often linguiform; conspicuous, regular shell sculpture; BHI about 80%; MHI about 40%

Cucumerunio novaehollandiae (Gray) : (I), (II)

19b. Elongate, length to 90 mm; BHI >85%; MHI >45%

Cucumerunio websteri (Simpson) : (NZ) (Footnote 5)

20a. Sculpture on beak and shell surface ......... 21

20b. Sculpture on beak only ........................... 22

21a. Bold sculpture, posterior border pointed; amygdaloid shape; length to 40 mm; MHI 55-60%

Hyridella glenelgensis (Dennant) : (II) (Footnote 6)

21b. Fine sculpture; posterior border rounded; elongate shape; length to 80 mm; MHI about 50%

Hyridella guppyi (Smith) : (NG), (SI) (Footnote 7)

22a. Anterior adductor and retractor muscle scars fused; usually winged, hence BHI about 75%, otherwise to 95%; hinge often reduced; siphons thick and fleshy

Hyridella menziesi (Gray) : (NZ) (Footnote 8)

22b. Anterior adductor and retractor muscle scars distinct; usually not winged, BHI>80%; hinge rarely reduced; siphons prominent but not fleshy ................................................ 23

23a. Shape amygdaloid; length to 60 mm

Hyridella narracanensis (Cotton & Gabriel) : (II), (III)

23b. Shape elongate oval; length to 90 mm ......... 24

24a. Shell elongate: MHI 45-50% ....... 25

24b. Shell less elongate: MHI >50%. ............... 26

25a. Ventral margin sinuate; posterior ridge without sulcation

Hyridella aucklandica (Gray) : (NZ)

25b. Ventral margin not sinuate but may be slightly curved; posterior ridge with shallow sulcation

Hyridella misoolensis (Schepman) : (NG)

26a. With delicate beak sculpture

Hyridella drapeta (Iredale) : II

26b. With heavy beak, sculpture .......... 27

27a. Prominent posterior ridge; MHI about 60%

Hyridella australis (Lamarck) : II

27b. Flattened posterior ridge; MHI about 50%

Hyridella depressa (Lamarck) : II

28a. Hinge-line equal to or slightly less than maximum length of shell; dorsal margin squarely truncate or at less than 90 deg to posterior margin; length to 90 mm

Haasodonta fannyae (Johnson) : (NG)

28b. Hinge-line less than maximum length of shell; dorsal margin obliquely truncate posteriorly; length to 112 mm

Haasodonta vanheurni McMichael & Hiscock : (NG)

Footnotes

I. McMichael and Hiscock (1958) doubted the affinities of Lortiellinae, but later work (Hiscock in McMichael 1967, Walker et'al . 1999) supports membership of Hyriidae.

2. Three subspecies of Alathyria pertexta :

MHI near 43%; slight posterior ventral sinuation; no anterior dorsal excavation

A. p. magnifica McMichael & Hiscock

MHI 50-55%; no posterior ventral sinuation; marked anterior dorsal excavation

A. p. pertexta Iredale

MHI 55%; marked posterior ventral sinuation; slight anterior dorsal excavation

A. p. wardi (Iredale)

3. Two growth forms of Alathyria jacksoni occur. One, found in strong currents, has a pronounced dorsal arch (and in larger specimens a ventral sinuation); the other, found in moderate currents, has a straight dorsal margin, with variable wing development. Intergrades are common. In addition, stunted specimens are common in some reaches. See Walker (1981b).

4. Velesunio moretonicus (Reeve) is V. legrandi (Petterd) sensu McMichael and Hiscock (1958): Hiscock (1960).

5. Two subspecies of Cucumerunio websteri :

Shell sculpture present

C. w. websteri (Simpson)

Shell sculpture absent

C. w. delli McMichael & Hiscock

6. Hyridella glenelgensis is restricted to the Glenelg-Wannon river system of SW Victoria. It is exceedingly rare, if not extinct (Walker et'al . 1999).

7. Two subspecies of Hyridella guppyi: :

BLI 17-18%

H. g. quppvi (Smith)

BLI 23%-26%

H. g. aipiana (McMichael)

8. Hyridella menziesi is highly variable: Roper and Hickey (1994).

References

See Walker et'al. (1999) for a more comprehensive, current list.

Heard WH, Guckert RH. 1970. A re-evaluation of the Recent Unionacea (Pelecypoda) of North America. Malacologia 10: 333-355.

Hiscock ID. 1960. Supplementary data on freshwater mussels (Mollusca: Pelecypoda) of the Australian Region. Australian Journal of Marine and Freshwater Research 11: 123-126.

McMichael DF. 1967. In Weatherly AH (ed.), Australian Inland Waters and Their Fauna: Eleven Studies. Australian National University Press, Canberra.

McMichael DF, Hiscock ID. 1958. A monograph of the freshwater mussels (Mollusca: Pelecypoda) of the Australian Region. Australian Journal of Marine and Freshwater Research 9: 372-508.

Parodiz JJ, Bonetto AA. 1963. Taxonomy and zoogeographic relationships of the South American naiades (Pelecypoda: Unionacea and Mutelacea). Malacologia 1: 179-213.

Roper DS, Hickey CW. 1994. Population structure, shell morphology, age and condition of the freshwater mussel Hyridella menziesi (Unionacea: Hyriidae): implications for bioaccumulation. Hydrobiologia 312: 17-25.

Walker KF. 1981a. The distribution of freshwater mussels (Mollusca: Pelecypoda: Hyriidae, Unionidae) in the Australian Zoogeographic Region. In: Keast A (ed.), Ecological Biogeography in Australia. Dr W. Junk, The Hague: 1233-1249.

Walker KF. 1981b. Ecology of freshwater mussels in the River Murray. Australian Water Resources Council Technical Paper 61, 119p.

Walker KF. 1998. Molluscs of inland waters. In Beesley PL, Ross GJB, Well A (eds), Mollusca: The Southern Synthesis. CSIRO, Melbourne: 59-65.

Walker KF, Byrne M, Hickey CW, Roper DS. 1999. Freshwater mussels (Hyriidae, Unionidae) of Australasia. In Bauer G, W'chtel K (eds), Ecology and Evolutionary Biology of Freshwater Mussels (Unionoidea). Springer. In press.