The phylum Gastrotricha is moderately small, but that’s OK because the animals themselves are small. There are around 450 species of gastrotrichs, and they are generally around 1mm long (Brusca and Brusca 2003). Their small size means that they live in interstitial environments of marine and freshwater benthos – i.e., they are meiofauna. Gastrotrichs are surprisingly abundant as constituents of marine meiofauna, being third most abundant invertebrate group behind nematodes and copepods (Todaro et al. 2006).
Gastrotrichs are characterised by a well-developed external cuticle, which can be armed with many spines and/or plates, along with cilia. This gives them a hairy appearance – hence their name is Greek for ‘hairy belly’. Despite their size, gastrotrichs are triploblasts, have a throughput gut, and they move by gliding on cilia on their ventral surfaces (Brusca and Brusca 2003). Most species of gastrotrichs are hermaphrodites. From an evolutionary point of view, gastrotrichs fit within the Lophotrochozoa, and are most closely related to other small meiofaunal groups like the Gnathostomula, but also to the Mollusca (Todaro et al. 2006).
Brusca & Brusca 2003. Invertebrates. 2nd ed. Sinauer Associates, Sunderland, USA.
Todaro et al. 2006. Interrelationships of the Gastrotricha and their place among the Metazoa inferred from 18S rRNA genes. Zoologica Scripta 35, 251-259.