Freshwater Worms

The title of this page might be taken to cover a whole 'can of worms' - for the word 'worm' is used to cover any long, thin, small animal and many of these are not worms at all but are insect larvae. This page deals only with the true worms. For these other larvae, try looking for some of the following:

There are also the flatworms or Planarians, not covered on this page.

There are lots of true worms that live in fresh water - one book I have says around 200 species. Some are completely aquatic and some simply like boggy conditions. However even garden worms live by transpiration through a layer of slime, they are in fact transpiring 'under water' if you will! Indeed the red worm that is very common in compost heaps will live indefinitely in the gravel of an aquarium - though, not being adapted for it, they have no evolved mechanism for escaping the larger fish. They find burrowing difficult under water.

Most aquarists will only be aware of the tubifex worms, which used to be commonly sold as live fish food. However tubificidae are only one family of worms!

Just as in garden soil, or the garden compost heap, worms are a necessary part of a balanced ecosystem. Most aquaria are not balanced ecosystems - but worms are still useful and will often find their way in on plants or other items introduced into the tank. If you find worms in your tank, it is extremely unlikely that these are harmful: treat them as just another interesting inhabitant!

The following notes are compiled from various books I own and may give you clues as to what to search for on the www to find better identification..


All true worms are members of the phyllum Annelida - the most familiar of which is the common earthworm. All annelids are segmented: you can easily see the rings separating the segments of an earthworm.

Freshwater annelids belong to two classes:

Chaetopoda From the greek chaitos (bristle) and pod (foot). These all have (usually microscopic) bristles on the segments to help them move
From the latin hirudo - a leech. These, the leeches, have no bristles!


Each segment in a true worm in fact contains an almost complete set of internal organs - which accounts for the fact that, if you cut a worm in two, both halves heal up and you get two worms! The Chaetopoda are divided into two orders:

Greek = many bristles. These are exclusively marine, so we will consider only the Oligochaetes
Greek = few bristles.


These are very common in both still and running water, even though they are not always evident. Often they live in the mud at the bottom where some of them make burrows. The best way to get these is to scoop up the mud and spread it in a thin layer in a shallow dish. If you sieve it, the worms you are looking for can often wriggle through the sieve and escape.

Other species live amongst dense vegetation (the ones in my tank were in hornwort in a garden pond) or inside the stems of such plants as reed-mace, bur reed and other rushes.

Many of these worms are transparent and nearly colourless, so they are not easy to see, even in a dish. The transparent ones are interesting studies under the microscope as you can see their internal organs.

Most of these worms eat decaying organic matter in the mud, just as does the common earthworm.

There are eight families of freshwater Oligochaetes which ae relatively easy to identify - the arrangement of the bristles is the best indication. The following five families are the best known:-

These are relatives of the common earthworm and the aquatic members, of the genus Eiseniella are like small, pink earthworms - up to 50mm long. They occur in still and running water. There are 5 species according to John Clegg's 'Observer's book of Pond Life'.
These are probably the commonest of the freshwater annelids. They are small, rarely more than 25mm long (though the Ophiodonais are up to 75mm). Some species are only 2mm or so.
Five species of colourless worms, about 10mm long.
Four species. Colourless or pale pink. Usually found in mud where they make loose tubes. Size 15mm.
Two species. They have a long narrow proboscis which makes then easily recognizable. They can be found among vegetation. Size 15mm.
Seven species. Pinkish in colour, they have 6 retractable gills at the tail end of the body. They live in mud tubes. Size 15mm.
Two species which live in mud. Size up to about 75mm.
These will doubtless be the most familiar: they are red worms which live in mud in pond margins and slow streams and some species are particularly capable of thriving amongst high pollution as in sewage polluted waters. As sewage discharges have been reduced, so the very heavy densities of these tubifex have decreased.
Tubifex species inhabit the mud with their heads on the mud and their sensitive tails waving around in the water - almost like red grass. They are very sensitive to vibration and will withdraw with lighting speed at the first disturbance. But if you wait, slowly the 'red grass' will grow again as they extend their bodies again. Their movement circulates the water, and their red colur is die to haemoglobin. This movement means they can survive in very foul mud with little oxygen.

The photo shows a colony of these worms in my aquarium. They are useful there as they do the same task that earthworms do in a garden. However, lots of fish will eat them, so geting a colony established depends on what fish you have.

Only one species, but a very common worm which lives in mud tubes in still or slow moving waters. Length can be up to 80mm. They are red or brown but have a greenish coloration in part of their body. They multiply by division.
These are about 10mm long and are usually found amongst the roots of water plants. They are white so can be easily mistaken for rootlets. They are very common either in the water or in damp conditions.


The leeches have bodies that are not simple tubes like the Chaetopoda worms. Their organs do not correspond with the segments - in fact the segments of a leech are more a series of rings marking their bodies.

The leeches are a large family. The easiest way of distinguishing them from the Chaetopoda is by their means of locomotion, for leeches move by using their suckers, in a looping action. They will adhere by the front sucker, loop so that the rear sucker is close to the front one, then adhere by the rear sucker and release te front sucker. Then they extend to maximum length and re-attach the front sucker as far as possible from the first.

During this looping movement there are times when they will wave about frantically in the water, as they search by feel for another front anchorage point.

Leeches also swim well, by a rapid snake-like waving motion. For this reason most leeches have flattened bodies.

Leeches are very common and there are a large number of different types. Few of them are harmful to fish - indeed many fish will eat them with relish.

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Last modified: Fri, 24 Nov 2017 12:10:02 GMT
Page first published 18th December 2002.
Page written and © by Richard Torrens