Key to Australian Freshwater and Terrestrial Invertebrates

Phylum Gastrotricha, Order Chaetonotida

Common name: gastrotrichs

Overview

Gastrotrichs are a small phylum of small, pale, translucent or transparent elongate, usually flattened animals, often with a swollen head and forked posterior. Gastrotrichs range in size from 0.05 mm to 1 mm long and 5 �m to 0.5 mm in diameter. Gastrotrichs are a well-defined phylum quite unlike any other animal, but their evolutionary relationship to other phyla remains unclear: they have been thought related to nematodes, rotifers, nematomorphs and other small phyla in a group known as aschelminths, but some evidence points to a relationship with tardigrades, insects and crustacea in a group known as ecdyzozoans.

Distribution and diversity

There is around 700 known species of gastrotrichs worldwide, which represents a moderate proportion of the estimated total number of species. Less than 20 species have been described from Australia, and many of these species are cosmopolitan. The only order with freshwater groups known from Australia is Chaetonotida.

Life cycle

Gastrotrichs have a direct life cycle with several immature stages. Between each stage a new cuticle is grown underneath the old cuticle, which is then moulted and lost. Immature stages are mostly morphologically similar to the adult stage, but may be shorter and have fewer structures. All species are primitively hermaphroditic, but most terrestrial and freshwater species are parthenogenetic. A relatively small number of eggs are laid, mostly singly. Some eggs hatch within a few days, while others form a dormant stage which is viable almost indefinitely until an appropriate hatching stimulus occurs. The total life cycle varies in duration from a few days to several weeks.

Feeding

Gastrotrichs feed on small particles, either live or dead, for example plant debris, bacteria, diatoms or protozoa.

Ecology

Gastrotrichs are ubiquitous but patchy, being found in relatively small numbers in the interstitial spaces of marine and freshwater sediments, detritus, the surfaces of submerged plants, and very damp terrestrial substrates.