Hairybacks (Phylum Gastrotricha)
Hairybacks or gastrotrichs are small but abundant worm-like invertebrates found in detritus and sediments in estuaries, on the seafloor, and in small bodies of still water such as puddles, ponds, and bogs. A few are more or less terrestrial, living in films of water in moist soil. They vary from <1 mm to 3 mm in length.
Gastrotrichs are bilaterally symmetric and have a head, a trunk which bears rows or patches of cilia and is forked at the end, and a simple gut. They have no respiratory or circulatory systems, absorbing oxygen directly from the water in which they live. They can be identified by their flattened bodies, spiny or hairy backs, and forked trunk, the branches of which produce sticky substances that they use to adhere to moist surfaces.
Using the cilia on their undersurface, gastrotrichs glide through the water and slide over sediment, feeding on detritus, algae, single-celled protozoans, and bacteria. They help maintain the health of ecosystems by recycling decaying matter and detritus.
An individual gastrotrich produces both sperm and eggs, but sometimes at different times in its life. Some species have a pair of gonads, parts of which produce sperm while other parts produce eggs. Fertilised eggs are released through a rupture in the body wall, which then repairs itself.
Species that occupy ephemeral terrestrial habitats such as small puddles often lay two kinds of eggs. The first kind hatches quickly, and is laid when environmental conditions are favourable. The second kind is thick-shelled and can remain dormant when conditions are unfavourable, such as when a puddle dries out. These may hatch years after laying, once conditions are favourable again.
The marine gastrotrich Thaumastoderma ramuliferum. Image: M. Antonio Todaro, Tobias Kånneby, Matteo Dal Zotto, Ulf Jondelius CC BY 2.5. Source: https://commons.wikimedia.org/wiki/File:Thaumastoderma_ramuliferum.jpg
Some 45 species of gastrotrichs are recorded from Australia. However, despite their abundance, Australian gastrotrichs are very poorly studied.
Thanks to Robin Hare for helping prepare this article.