Key to Families of Australian Aquatic Molluscs
This key is apart of the Keys to Australian Aquatic Macro-Invertebrates.
This large phylum is divided into several classes of which only the Bivalvia (bivalved molluscs) and Gastropoda (snails and slugs) are found in inland waters.
Bivalved molluscs occur commonly in marine and estuarine environments. Four families are known from Australian inland waters. Identification to Bivalvia is relatively easy based on the bi-valved shell which covers the whole body, but care should be taken, especially with small specimens, to check the animal is not a crustacean of class Conchostraca (clam shrimp) Ostracoda (mussel-shrimp or seed-shrimp), or Cladocera (water flea). These animals have a bi-valved carapace and may superficially resemble bivalved molluscs, but within the carapace is a shrimp-like body with several pairs of jointed legs.
The four freshwater bivalve families are:
1 Hyriidae (Freshwater mussels)
A family restricted to South America and Australasia. Found throughout mainland Australia and central-north Tasmania, absent from southern Tasmania. Some species are endemic to particular catchments whilst others are cosmopolitan. The first larval stage is the minute, bivalved glochidium, ectoparasitic on the gills, fins, or general body surface of various fish.
2 Corbiculidae (Orb-shell mussels)
Found in Asia, Australia and Oceania, with isolated introductions in other parts of the world. A tropical to subtropical group. There are two distinct species groups in Australia. The first (genera Batissa and Polymesoda) inhabit fresh and estuarine coastal streams of northern Australia, and are closely related to forms found in south-east Asia. The second (genus Corbicula, endemic subgenus Corbiculina) are found throughout Australia except for Tasmania and the southern part of Western Australia.
3 Sphaeriidae (Pea shell mussels, pea shells, fingernail clams)
A little studied but cosmopolitan family of small bivalves. Previously, many names were based on minute differences in the shell and on geographic distribution. Some species are widespread throughout Australia but others are confined to small regions or in some cases to a single body of water.
4 Mytilidae (Mussels)
The cosmopolitan family which includes the marine mussels. Several species are estuarine and some may be found in near-fresh water, but never entirely away from marine influence.
Identification to family can be based on the shell. The Hydriidae and Mytilidae are elongate mussels with the beak of the shell at one end. Corbiculidae and Sphaeriidae are sub-circular. Hydriid larvae (glochidium larvae), parasitic on freshwater fish, are keyed separately in this key
The aquatic gastropod families divide into two subclasses, the gill-breathing Prosobranchia are related to marine snails, the lung-breathing (or secondarily gilled) Pulmonata are related to land snails. Eleven prosobranch families and nine pulmonate families are recorded from Australian inland waters.
Almost all species are shelled and recognisably snail-like, but one family (Onchidiidae) of marine, air-breathing slugs extends into estuaries and damp terrestrial situations near the sea.
Identification to class Gastropoda is relatively easy based on the snail-like shell and/or the presence of a pulmonate lung in the mantle cavity. Larvae of Helicopsychidae, a family of caddis-fly (Insecta: Trichoptera) construct a helical case which resembles a snail shell, and could be mistaken for molluscs. The trichopteran case is composed of sand grains and the animal inside is a typical insect larva with segmented body and three pairs of legs.
The identification of gastropods to family level can be difficult because the families are defined on internal anatomical structures. There are few readily observable characters, other than those of the shell, but shells vary as much within as between families. Adult whole animals can be identified using a combination of shell (and operculum) plus external body characters, but juveniles and shells alone are not always identifiable to the family level.
Character systems useful for identifying gastropod families include:
- Shell coiling (dextral, sinistral, flat, or not-coiled)
- Operculum presence/absence
- Shell shape (spired, globose, turbinate, etc.) and spire height
- Shell patterning (ridges, striations, colour patterns, etc.)
- Tentacle and eye position
Some species will key easily to family level, but others may be difficult.