Invertebrates 101

Chapter 1

The Worms - Flatworms, Roundworms and Segmented Worms

OK, so maybe worms aren’t the sexiest place to start our overview of the Biology of Ponds, but they certainly have an impact. In fact, the Roundworms, or Nematodes, are the most abundant animals on Earth, accounting for 80% of all the animals on the planet, with an estimated 1 MILLION species, many parasitic. The Flatworms, Platyhelminthes, are mainly debris eaters and parasites, and include the tapeworms and flukes that plague millions across the globe. By far the most familiar are the Segmented Worms, Annelidae, made up of earthworms, marine worms and leeches.


Let’s start with the easy ones. The majority of Annelids are major contributors to the cleanliness of the pond, on the front line of decomposition as they ingest and digest organic wastes too tough for other critters to break down, like the cellulose in plant debris. Just as terrestrial earthworm action creates the rich soils of the forest floor, available to shelter and nourish countless other organisms great and small, so do the aquatic earthworms attack and break down the layer of muck and litter on the pond bottom, while they themselves become a valuable protein source for insects, amphibians and fish. And “aquatic earthworm” is not an oxymoron – these common worms are members of the same class, Oligochaeta, as their terrestrial relatives, just adapted to live a fully aquatic existence, absorbing oxygen through their skins, eating detritus and excreting ammonia just like fish do. Other annelids also thrive in the pond environment, like the Tubifex worms familiar to most aquarists, who value them as a tough live food for their tank fish. In fact, Tubifex are so efficient at surviving adverse conditions and polluted environments that they are also known as Sewage Worms, and are a major indicator of poor quality or oxygen deprived water. Typically found in the soft muck at the bottom of the pond, in the soils of aquatic plantings and the sediments in bogs, aquatic annelids help to turn over and aerate decomposing organic matter by burrowing through the muck.

Leeches on the shell of a turtle

Another major Annelid group is equally familiar but not nearly as well thought-of – the Leeches. Abundant in fresh waters all over the world, as well as some terrestrial and even marine environments, about a tenth of the 650 species of Leech are found in North America. Of those, only 6 feed mainly on vertebrate blood, and only one is likely to feed on us, the appropriately named American Medicinal Leech. (All the rest prefer to prey on other invertebrates.) Up to 2” long and really creepy looking (sorry, they are), these bloodsuckers feed by attaching to a host with either their front or rear suckers and then, thanks to an anesthetic and anticoagulant in their saliva, open a small, painless wound in the shape of an inverted Y inside a circle. After engorging themselves with up to 5 times their own weight in blood, they drop off and go their way, sated for months – they may only feed twice in a year. If you happen to find yourself feeding a Leech, don’t be too concerned. Slip a fingernail or credit card under the sucker to gently release the suction and they will let go – don’t squeeze or use salt or flame which may cause them to regurgitate ingested blood back into the wound. They do not carry disease, so simply wash the bite with soap and water and bandage until the bleeding stops.

Another type of worm you may encounter in the pond are the parasitic flatworms called flukes that occasionally parasitize fish. Although they are often present without apparent harm on healthy fish, a severe infestation usually manifests in the fish behavior known as “flashing”, where the fish will rub itself against objects to rub off attached worms. A 1.5% salt bath for 15 minutes is often effective against flukes, as are various commercial preparations, used as directed.