Gastrotrichs are members of a very small phylum of worm-like animals. They are microscopic in size but relatively common in freshwater environments and sandy beaches. There are about 400 described species worldwide. The freshwater species plus a few marine types belong to the order Chaetonotoidea, while the marine sand dwellers are classified as order Macrodasyoidea. The phylum name, Gastrotrich, means ‘hairy stomach’.
A typical gastrotrich is a millimeter or two long and has a similar appearance to a rotifer but with bristles added to the body and no corona. Both have a forked tail like two toes, a transparent body and head with a dorsal mouth and a powerful pharynx. The gastrotrich replaces the ciliated crown of the rotifers with a lots of spiny bristles. Like Rotifers, Gastrotrichs can secrete glue from their tail to hold themselves to substrates. Gastrotrichs also have adhesive tubes on their bodies for gripping plants or rocks in moving water. Unlike rotifers, gastrotrichs usually move by gliding along worm-like from place to place instead of dancing about on those two toes.
So what are Gastrotrichs and rotifers related to? They were both once grouped with nematodes, kinorhynchs and nematomorphs in the old Phylum Aschelminthes. Gastrotrichs share some aschelminth characteristics: unsegmented body covered in epidermis and a harder cuticle, a simple one way gut that begins with mouth and muscular pharynx, and ending in an anus. But Gastrotrichs have no pseudocoelom, the body cavity that is found in the other aschelminthes, including rotifers. In this they show an intriguing link to the simpler flatworm phylum, Platyhelminthes. Gastrotrichs may be the modern versions of some long lost ancestral form that diverged from th flatworm to the roundworm line.
Gastrotrichs are filter feeders, sweeping up protozoans, algae and organic particles by cilia on their heads which carry their food into the central mouth. The cilia and bristles are sensitive to touch and water currents while there are pits on the head which may contain chemoreceptors. There is also a brain in the head to process this information. Gastrotrichs are part of the food chain, being prey for larger microorganisms such as crustaceans. When they are disturbed, gastrotrichs curl up like miniature porcupines with their bristles erected for protection.
Some gastrotrichs are hermaphroditic with both male and female sex organs in the same body. Other gastrotrich species are entirely made up of females who probably always reproduce hermaphroditically since no males have ever been found. There are no larval stages. The eggs hatch into young adults that mature in a few days, reproduce and then die. The eggs are tough and can last for months or years if conditions are unfavorable.
There isn’t a lot else to be said about the humble gastrotrichs. Pictures of these tiny creatures can be seen on
http://www.micrographia.com/specbiol/gastrot/gastro/gast0100.htm
References: Buchsbaum, R. 1968. Animals without Backbones. Penguin Books
Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press
http://www.earthlife.net/inverts/gastrotricha.html
