Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Mollusca
Class Bivalvia
Order Veneroida

Common names: freshwater clams, pea shells, pea clams, fingernail clams, basket clams

Overview

Veneroida is a large order of mostly marine clams, which also includes a number of freshwater species. Two families occur in Australasian freshwaters. Corbiculidae or �basket clams� are small to relatively large (10-100 mm in length) bivalves with rounded, strong, thick shells that often are brightly coloured such as orange, yellow, pink or purple. A brown to blackish cuticular layer (periostracum) may be present over parts of the outer shell layer. The foot is sub-triangular and usually retracted within the shell. Sphaeriidae, or �pea or fingernail clams� are small to minute clams (1-25 mm) with thin, fragile, semi-transparent, greyish shells with often with one or more very fine, irregular concentric lines of growth ridges and darker concentric zones. Recent molecular systematic research suggests that this order is not monophyletic, and hence may be divided in the future.

Distribution and diversity

Veneroida is a large order of mostly marine clams, however freshwater species are known from three families: Corbiculidae and Sphaeriidae (Corbiculoidea) and Dreissenidae (Dreissenoidea), which does not occur in Australasian freshwaters. Corbiculidae is a tropical to subtropical family of around 12 genera found in Asia, Australia, Oceania, Africa and South America. Several species have been introduced to other parts of the world. The Australian fauna comprises three genera, with five species, however the number of species is uncertain as the family needs revision. Batissa and Polymesoda inhabit fresh and brackish coastal streams of northern Australia, and are closely related to species found in south-east Asia. Corbicula (endemic subgenus Corbiculina) are found in freshwater throughout Australia except for Tasmania and the southern part of Western Australia. Sphaeriidae occur worldwide and are found on every continent except Antarctica in almost all kinds of freshwater habitats. They are represented in Australia by 17 species and subspecies belonging to the genera Musculium and Pisidium. Some species are widespread throughout Australia while others are confined to small regions or, in some cases, to single bodies of water. Most are endemic, but one species found near Adelaide is probably the introduced Musculium lacustre.

Life cycle

In Corbiculidae the sexes are separate but in Sphaeriidae, individuals are hermaphrodites and self-fertilise. Males release sperm into water that enter the female via the inhalant siphon and are passed to the gills where unfertilised eggs are held. In both families, eggs are brooded in a gill pouch (marsupia), where early larval developmental takes place. Completely developed miniature clams are released into the water.

Feeding

Veneroida are opportunistic filter feeders on zooplankton, phytoplankton, and organic detritus suspended in the water. These are are extracted from the water that is pumped into the shell by being trapped in mucous then passed by cilia on their gill filaments to the mouth.

Ecology

Corbiculidae are typically found flowing water among dense aquatic plants or in sandy sediments. In large numbers corbiculids (especially Corbicula australis) may become pests of town water supplies and irrigation systems, where they block pipes and metering equipment. The Asiatic clam (C. fluminea) is an invasive species in many parts of the world and is a serious pest of hydro installations in North America. The species is currently spreading through Europe but is not recorded yet from Australia. Another significant widespread invasive species not yet present in Australasian waters is the zebra mussel (Dreissena polymorpha), which belongs to the other freshwater family of Veneroida, Dreissenidae. Sphaeriidae occur in mud or sand in almost any freshwater habitat, including ephemeral and stagnant water bodies. Sphaeriids often constitute a large proportion of the benthic biomass of freshwater habitats, where they may exceed 100,000 individuals per square metre. Accordingly, they play a key role in energy and nutrient cycling and are important components in the diets of some fish, aquatic insects and water birds. Many sphaeriid species are known as the first intermediate hosts for several digenetic trematodes. Bivalves accumulate toxic chemicals in their tissues and both corbiculids and sphaeriids have been used to monitor various chemical contaminants such as copper, zinc and ammonia pollution.