Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Annelida
Class Clitellata
Order Tubificida
Families Phreodrilidae and Naididae

Common names: freshwater worms, sludge worms, microdriles

Overview

Tubificida is a group of small worms found in salt and freshwater with a few terrestrial representatives. Their bodies are elongate and many-segmented with a thin cuticle (body wall) through which water diffuses in and out. In contrast to earthworms, their clitellum is only one cell thick. Most segments, except for the first, bear bundles of small chitinous setae (hairs). There are usually more than two setae per bundle and may be simple in structure, either straight or sigmoid (s-shaped) or with forked or comb-like tips. Some also have other long thin hairs in addition to those setae in bundles. Some species have posterior lateral gills, a proboscis on the prostomium (first body segment) and/or eye spots. Tubificida range from being pale in colour to red, brown or black and are small, slender worms (usually <30mm), with some species only being a few millimetres long.

Distribution and diversity

Tubificida occur worldwide across a range of marine, fresh and brackish water and some terrestrial habitats. Two families occur in Australian freshwater, the Phreodrilidae and Naididae (Naididae and Tubificidae have been shown to form a monophyletic grouping, hence Tubificidae has been merged with Naididae). Two other families (the Capilloventridae and Enchytraeidae) are sometimes included in the Tubificida, but genetic evidence points to them being separate groups, which is how they are treated in this key. Most are marine, however around 70 freshwater (and a few terrestrial) species occur across Australia in a diverse range of habitats, from sub-Antarctic islands to groundwater and caves in semi-arid regions. Phreodrilidae has a Gondwanan distribution. Some species are cosmopolitan while others are much more restricted in their distributions. Many are awaiting description and it is thought that further surveying of freshwater habitats will reveal many more Australian species.

Life cycle

Depending on the family and/or species, Tubificida are either sexually reproducing hermaphrodites or reproduce by asexual fragmentation (new heads and tails formed after the body splits into fragments) or by fission (regeneration by budding of new head and tail ends before separation of two new individuals). Self-fertilisation is also suspected in some taxa. When reproducing sexually, the eggs are laid and sperm deposited into a cocoon produced by the worm. The fertilised eggs develop directly into small, but fully formed worms that emerge and grow continuously until adulthood. Some species can survive adverse conditions such as drought or food shortages by forming a protective cyst around the body and lowering their metabolic rate. Encystment may also function in the dispersal of the worm.

Feeding

Most Tubificida are detritivores, feeding on organic detritus and its associated microflora (bacteria, fungi, microalgae and protists) attached to the sediment particles they ingest. The family Naididae includes species with more diverse feeding habits such as filter feeders with very long hairs on one segment that are used to sweep food particles towards the mouth; predators with modified mouth, pharynx and chaetae to prey on on invertebrates, including other worms and molluscs; while others have been recorded as parasites and/or symbionts on snails, clams and frogs.

Ecology

Phreodrilidae are mostly aquatic and are common inhabitants of caves, springs and groundwater aquifers in southern continents, with some overlap in the species of surface and underground habitats. Species have also been recorded from wet moss on the banks of streams and granite outcrops ranging from subantarctic islands to semi-arid regions. Two species in New Zealand inhabit damp leaf litter of beech forests and three eastern Australian species are commensal detritivores on the carapace of freshwater crayfish. Naididae are common in both running and standing waters, occurring in a diverse range of habitats including lakes (mostly the littoral margins but some occur in deeper regions), swamps and flowing water (especially stony or vegetated reaches), ephemeral arid-zone rivers, groundwater, streams in caves, water-filled tree hollows, mildly saline inland wetlands and estuaries. Like most Tubificida, many naidids occur in soft sediments but other species are found among aquatic plants or even swimming above the sediment. There are also a few semi-terrestrial species that inhabit rainforest floors in moist litter in New Zealand. An endemic Australian species bizarrely inhabits the ureters of the green tree frog (Litoria caerulea).

Freshwater species are often called �sludge worms� for the tendency of some widespread species, such as Limnodrilus hoffmeisteri and Tubifex tubifex, to occur in organically polluted waters, including sewage sludge. However, many other species are sensitive to organic enrichment and Tubificida as a whole show a wide range of sensitivities to pollutants such as pesticides and heavy metals making them valuable as biological indicators in freshwater systems. Tubificida are important in the breakdown of organic materials and nutrient cycling and their feeding behaviour also mixes the sediment, keeping it aerated and providing oxygen for other organisms. They are also an important part of many aquatic food webs being preyed upon by fish, reptiles, amphibians and other aquatic invertebrates.

The cosmopolitan species T. tubifex was one of the first aquatic Oligochaeta described and is cultured as a live fish food for the aquarium industry. This species is also the subject of intensive research in the Northern Hemisphere because of its role as the intermediate host of the myxozoan parasite (Myxobolus cerebralis) that causes whirling disease in salmonid fish. The parasite has not yet been recorded in Australia but the occurrence of T. tubifex is monitored by the fishing industry.