Nematomorpha (Gordian worms)

Nematomorpha. Rarely you might see extremely thin wiry worms up to many inches long, called "horsehair worms" or "Gordian worms" because they can tangle in knots. They would likely be secretively wound among the plants, including floating plants. These representatives of a small and obscure phylum, Nemata or Nematomorpha ("threads" or "thread-forms"), would have come into your aquarium among pond-raised plants, for at one life-stage they are parasites of insects, such as grasshoppers or crickets, or water-loving flies like mayflies. You might rarely find them as parasites in bloodworms (Chironomid midge larvae). They do not trouble fish.
 
These Gordiids don't feed as adults, but sustain themselves on stored nutrients, for up to a month sometimes. The fertilized females lay minute eggs in long gelatinous strings that get wound round plants. The eggs will hatch into microscopic larvae that drift to the bottom. Gordian worms aren't well-documented, so they're a bit mysterious.
 
Dr Mark Wetzel and Dreux Watermolen's description of the lifestyles of Gordian worms is also trustworthy. They offer a long list of potential hosts for the immature worms: "As larvae, horsehair worms are parasites of insects and other aquatic and terrestrial invertebrates, most notably grasshoppers, crickets, locusts, katydids, and beetles. Other hosts include caddisflies, dragonflies, spiders, millipedes, centipedes, crustaceans, and leeches. Host specificity has not yet been well documented. Other hosts include representatives of vertebrate groups." Their 1995 article, "Horsehair worms in Illinois," is archived at the Illinois Natural History Survey site. (The crustaceans mentioned as hosts refer to the very few nemata that have colonized marine environments, and don't concern us freshwater types.)
 
Apparently fishes aren't normally implicated in this convoluted parasitic lifestyle. Nevertheless, if you have fertilized adult female gordiids laying egg-strings in your tank, a bottom-feeding fish might ingest a larva, which might then succeed in encysting in the fish. Ben Hanelt has e-mailed me reassuringly, "the cysts produced by these worms will not harm the fish. If a very small fish gets infected with thousands, the sheer number of cysts in the gut 'could' lead to digestive problems. However this scenario is unlikely." Hanelt did suggest another way an adult could get into an aquarium: if an infested cricket found its way indoors, the ready-to-exit gordian worm is able to make its doomed host thirsty and the cricket might seek out a fish tank.
 
Links. The most detailed sketch of gordian worms is at the website of Ben Hanelt, University of New Mexico, who's been studying them. Nematomorphs take their place among the "lower" metazoa at UC Davis. Nemaplex ("Nematode-Plant Expert Information System") concentrates on the terrestrial nematodes in soil and plants, of concern in agriculture, so there's lots of material on nematode ecology.