Gastrotrich

I've been struggling to capture a decent image of a gastrotrich for quite a while, but this afternoon this obliging example (Chaetonotus sp., I think) paused just long enough in my field of view for me to record it. Gastrotrichs are miniature aquatic equivalents of the roadrunner, always on the move - although in this case propelled by a layer of beating cilia on their underside (gastrotrich means 'hairy stomach') rather than legs. This one seemed to be feeding. That circular mouth, surrounded by a ring of tiny teeth, and the muscular pharynx that it leads to ingest just about anything that the animal collides with and that's small enough to enter the gap. The outer covering of cuticular spines give this gastrotrich a fearsome appearence and if you are smaller that it is (i.e. less than about a quarter of a millimetre) the sight of one of these high-speed hunters hurtling towards you must be a nightmare - not that pond life suffers from nightmares; sorry, lapsed into anthropomorphism there. Been watching too many horror movies.  

This specimen came from the edge of a Phragmites reed swarm, amongst the rotting debris in shallow water - a favourite habitat for gastrotrichs. You can see here how agile they are, capable of turning in their own length through 180 degrees. Those two appendages on the tail are adhesive organs. Gastrotrichs do sometimes rest and when they do they glue themselves to something convenient with adhesive secreted from the tips of those appendages. When they are ready to go again they secrete a releasing agent - that, and the glue, which both work under water, must be very interesting substances.

Hairy Stomachs

TV wildlife documentaries tend to focus on large animals with fur or feathers, but most of the animal world is tiny and many of the microscopic life forms that live in the water or in soil, that play a vital role in the functioning of ecosystems, are poorly understood. This animal is a gastrotrich – a name that literally means ‘hairy stomach’, on account of the tiny beating hairs on its underside that propel it through the water. These restless little animals, none longer than half a millimetre, are in every pond and also occur in marine environments, but compared with larger life forms we know very little about the lives of the 400 or so species that have been discovered so far. If you have a garden pond, or any water in the garden that contains decaying vegetation, there’ll be gastrotrichs in it, along with the heliozoans and rotifers that I mentioned in earlier postings. Gastrotrichs are fast moving, endlessly exploring detritus in the water in search of a meal, and defend themselves with tiny spines on the body surface. I used DIC optics again for these pictures, to highlight the animal’s spiny covering.