Annelida: segmental worms

The annelids form the phylum of segmented worms. Latin annellus is a ring — think "earthworm" for a sample. Not many annelids are purely aquatic in fresh water; the ones you'll find are all oligochaetes ("few-bristled") and much less important in the freshwater ecology than their multifarious marine cousins, the polychaetes ("multi-bristled"), are in marine environments. All the oligochaete worms are hermaphrodites. Some are on the edge of microscopic, as small as 1 mm. At that scale you might mistake them for nematodes.
 
Tubificids and lumbriculids. You're already familiar with some of the four families of freshwater annelids: the tubificids, which include the many species of the genus Tubifex and its close relatives, and the lumbriculids or blackworms. Though there are only three species of lumbriculids in North America, and only two of those at all common, one of them is Lumbriculus variegatus, the familiar California blackworm, which you'll mostly think of in terms of live food. The other, Stylodrilus heringianus, is an indicator species for unpolluted water.
 
In marginal mudbanks you'd expect to find a third family of annelid worms, the enchytraeids, familiar to fishkeepers both as whiteworms and those miniature whiteworms called Grindal worms, all cultivated for live food.
 
A fourth family, the naidid worms noted below, are probably less familiar to the average aquarist, but one web biology page remarks that Naididae and Tubificidae form 80% to 100% of the segmented worm communities in the sediments of most streams and lakes. Their major predators are fish and insect larvae.
 
All these segmented worms respire through their skin, yet some of them, tubificids and lumbriculids for example, can tolerate anoxic conditions for periods of time. As oxygen levels drop in natural waters, tubificids come to predominate in the worm community, but they don't live where toxic byproducts of anaerobic bacteria accumulate.  Instead of suffocating and dying in your gravel, the tubificids stand on their heads to swallow organic detritus and digest its microflora, while they wave their rear segments in the open water and get their oxygen by simple diffusion. Their reddish color derives from a red blood pigment that acts like hemoglobin to carry oxygen extracted from even quite low levels in the water.
 
 
The lumbriculids won't suffocate either, as long as they are in water shallow enough that they can extend their rear segments along the water surface to get their oxygen directly from the atmosphere. Lumbriculids tend to predominate in green algal detritus along marshy shores.
 
An overview of the roles of annelid worms in freshwater, perhaps all you'd need to know, is at the freshwater benthos site 
 
Leeches. You may be unlucky enough to encounter the occasional leech, which is an oligochaete segmented worm specialized as a blood-sucking parasite. Though leeches commonly prey on invertebrates, the authors of The Ecology and Classification of North American Freshwater Invertebrates report that leeches from every family have been found on fish catches. Small leeches that may attack fish come in with pond-grown aquarium plants that haven't been cleaned in an alum bath, or sometimes you'll see tiny leeches lurking amongpond-farmed California blackworms. When you gently slide the blackworms to one end of the dish, the leeches will reveal themselves by clinging to the dish with the suckers they have at either end. In the aquarium, you may identify a leech by its habit of rearing up its front end, the better to test the water for chemical cues. So leeches are the exception to my rule, "If you can see it, it's not a parasite." You'll want to eliminate any leeches before they get into the aquarium or filtration; even when they're too small to threaten fishes, leeches can carry fish parasites.
 
Naidids. Though Tubificids or blackworms might establish themselves in your aquarium for a time, the only other annelid worms likely to turn up unexpectedly in a planted aquarium are Naidids — "like Nais," for Nais communis is the species that serves as a prototype. The name derives from the naiads, pre-Greek nymphs of springs and wells, literally the "sisterhood of the water's flow." Naid worms and the minute Aeolosomatids, together called Naidids ("like Naids"), are common enough in running and standing waters, and in any planted aquarium.
 
Most of the Naididae are sediment and biofilm dwellers, where many build tubes in mud, as does Tubifex, the "tube-builder", but some can be found among plants or swim freely in midwater, where you might first have noticed them. In biofilms, many Naidids prey upon bacteria and algae in detritus or bruised vegetable matter, so if there were any damaged leaves on the plants you put in the tank, it would be very possible to introduce them. They are recycling nutrients in the detritus, and some of the largest, such as Chætogaster spp, eat ciliates and minute crustacea, like copepods, so they are helping to control the plankton, too. Others actively swim with an S-curve motion that could remind you of nematodes, but the head of the worm stays in a straight travel line. Snakelike.
 
Dero spp, not much longer than half a centimeter, live among the hanging roots of duckweed. One maverick even parasitizes snails, and Dero digitata is the alternate host for a parasite of channel catfish, Ictalurus. Purpose-bred, they're safe: in the UK, some advanced aquarists are culturing Dero digitata as food for guppy fry; in the US they sometimes appear unasked for in Daphnia cultures, but I've never seen them.
 
Biologists who cultivate Naidids in the lab feed them on algae and diatoms. Perhaps a stabilized population of rapidly multiplying Naidids in a maturing aquarium is one of the buffering mechanisms that reduce the early bloom of diatoms so often noticed in a raw new system.
 
Naids are colorless, sometimes all-but-transparent oligochaetes, so glassy and delicate that one common species is named Pristina. Though they are very thin, they can get to be longer than ¾ inch, about as big around as a toothbrush bristle, recent reporters in Natural History pointed out. Naids don't form massed colonies the way tubificids do, but if you find really long worms and put them under the microscope, you may see that you have a "chain" of individuals connected head-to-tail. This is an asexual method of reproduction. Naids reproduce mainly by developing a new tail and head in a regenerating body segment and then breaking apart, something they can do in a couple of days. Each segment will be viable: paratomic fission or "paratomy" are the words for it.
 
Under a low-power microscope, you may see the sparse long straight hairs along their sides: they're oligochaetes, right? and Chaetogaster, "hairy-belly" is the typical genus of one group of Naidids. Often there are two tufts of longer hairs on opposite sides near the head. Any of several similar North American naid worm species would fit this description as well as Nais communis.
 
Naidids as fry food. Though naidids are widespread and very abundant, a major part of the food web and an important food for small native fish and fry, McDaphnia is the only fishkeeper I know of who's actually cultured Naidids, enough to raise Kribensis fry on them, according to his post in the Live-Foods List, Dec 1998. According to McDaphnia, they can double their number in a few days.
 
Æolosomatids ("ee-OH-luh-SO-matids") are very small naidids, about 1 to 2 mm, but they form chains of semi-independent immature worms up to 10 mm long. The juveniles drop away as they mature. With a 10x loupe, you might detect their smooth, gliding locomotion, operated by invisibly small moving hair-like cilia. These all-but-microscopic worms are so small that they slip through most nets and are largely ignored by biologists doing ecological studies, and by the fish, but they are part of the natural diet of fry. These worms work their way through detritus, especially in stagnant water. They're tolerant of low oxygen levels.
 
Their Greek scientific name is a mouthful and a mystery. They were named (by Christian Gottfried Ehrenberg) in 1828, in a time when German biologists had rich and deep Classical educations. Aeolus, the guardian of the winds, gets a bare passing mention in the Odyssey. In Greek he is "Aiolos," the "shifter." So translucent, flexible, wriggling Æolosoma is "the shape-shifter."