Invertebrates in Canals

And what they can tell us about the health of the canal

By Frank Machin


Canals are more than just a means of transport, they are a complex ecosystem that is capable of supporting a diverse range of plant and animal life. In particular, the silty bottom and shallow edges of a canal play host to a wide range of invertebrate species.
Simply put, invertebrates are animals without backbones. This includes insects, crustaceans, snails and worms, to name a few.

The most common way for canals to become unhealthy is by pollution, whether this is by toxic chemicals, or fertilisers from nearby fields that cause overgrowth in the canal. 

Different species of invertebrates are sensitive to different levels of pollution, so that highly sensitive creatures are very unlikely to be found in a heavily polluted body of water. By looking at the number and types of these creatures in a canal, we can judge how healthy the ecosystem is.

How Do We Look at Invertebrates in Canals?

Many creatures can be easily spotted flying above the canal, on plants by the water's edge, or on the surface of the water. Some creatures spend most of their time underwater, and because of this, they make great indicators of the health of the water.

Canal Dipping

You will need:

  • A clean jar
  • A small net
  • A clean dark baking tray
  • Magnifying glass
  • Notebook


  1. Fill the jar to halfway and pour enough water into the baking tray to cover the bottom completely.
  2. Starting from the top and moving down towards the bottom of the canal, sweep the net slowly through the water sideways.
  3. If you catch anything, tip it from the net into the baking tray. The dark background of the tray will make it easier to see the creatures
  4. If you're near a shallow region of a canal, you may be able to reach down and scoop up some of the silt with the jar. This way, you may be able to spot some of the bottom-dwelling invertebrates
  5. Use your magnifying glass to study the creatures closely. This is especially important when it comes to invertebrate species that look very similar, like damselfly larvae and mayfly larvae.
  6. Remember to put every creature back exactly where you found it.


The Pondscope is a simple tool that's easy to make and fun to use. Often, the water can appear too reflective for us to see what lives underwater. The Pondscope creates an underwater window so that we can watch wildlife living under the water's surface. Unlike canal dipping, this is a great way to watch invertebrates undisturbed in their natural habitat.

You will need:

  • Pringles tube, cleaned
  • Cling film
  • Elastic bands


  1. Cut the bottom out of the Pringles tube so that you can put your arm through it

  2. Wrap cling film across the bottom of the tube, and secure in place with elastic bands

  3. Pour a very small amount of water into the tube, just enough to cover the cling film

  4. Push the tube into water and look through to see clearly underwater

Can You Spot These Common Invertebrates?

This is a guide to some of the more common creatures that you can expect to find in a healthy canal. These invertebrates spend most of their lives in or on the water, so their presence is a useful method to evaluate the health of a canal.

These creatures can be found most commonly in early to mid summer. There are often several varieties of each invertebrate, so don't worry if the creature you find doesn't exactly match the illustration. 

Caddisfly Larvae:

Expect to find these underwater, where they feed on algae or other creatures, depending on the species. Most of the species found in Stroudwater canals build themselves a case made from silk and bits of plant matter.

Fun Fact: Fossilised Caddisfly have been found dating from around 200 million years ago, making them very ancient creatures indeed!

Photo:Fig 1 Caddisfly Larvae

Fig 1 Caddisfly Larvae

Alderfly Larvae:

Alderfly larvae live under the water, where they breathe using the feathery filaments on their rear segments, much like fish breathe using their gills. They are carnivorous, and like to hunt for prey in the silt and mud on the bottom, especially under stones and plants.

Fun Fact: Alderfly larvae have strong, powerful jaws for catching and killing prey. If you catch one, try and hold a blade of grass in front of it, it will back away, snapping its jaws as a warning!

Photo:Fig 2 Alderfly larvae

Fig 2 Alderfly larvae

Damselfly Larvae:

Damselfly larvae live in the water, where they hunt small creatures for food near the bottom of the canal. They can also be spotted climbing plants at the canal's edge as they crawl out of the water to shed their skin and take on their adult form.

Damselfly larvae are easier to catch than alderfly larvae, as they are much slower swimmers.

Fun Fact: Damselfly larvae have a modified bottom lip, called a mask. The mask shoots out quickly to latch on to passing prey with its small teeth.

Photo:Fig 3 Damselfly Larvae

Fig 3 Damselfly Larvae

Dragonfly Larvae:

Much like damselflies, dragonfly larvae are carnivorous, hunting small creatures on the bottom of the canal. Like damselfly larvae, they also have a mask for grabbing at prey, and crawl out of the water to shed their skins. 

However, dragonfly larvae are much shorter and 'fatter', swimming much more slowly than damselfly larvae, so they are fairly easy to catch.

To tell the difference between an adult dragonfly and an adult damselfly, wait for them to land. If it folds its wings up, its a damselfly, if not, its a dragonfly.

Fun Fact: Dragonfly larvae are able to store air inside their bodies, and release it from their anus for a quick burst of speed!

Photo:Fig 4 Dragonfly Larvae

Fig 4 Dragonfly Larvae

Great Diving Beetles:

Great diving beetles are voracious predators that can be found in most any body of water, especially areas with plenty of plants. This is because the females lay their eggs inside the stems.
Despite their bulky appearance, they are able fliers, although they mostly fly at night. This is most commonly males, who frequently fly in search of females to breed with.

The male has a shiny appearance, while the female appears more dull with a grooved carapace.

Be careful, these creatures may bite.

Fun Fact: Before they dive, they collect air bubbles in their wing cases which they can use to breathe underwater.

Photo:Fig 5 Great Diving Beetle

Fig 5 Great Diving Beetle

Water Boatmen:

Water boatmen are so-called because their powerful legs resemble the oars of a rowing boat. They are also known as backswimmers because they spend a large amount of their time swimming on their backs just under the surface of the water.

They are vicious predators, preferring tadpoles, but sometimes feasting upon small fish fry!

Fun Fact: Water boatmen use sunlight to tell them which way is up. If they are placed in a tank that is lit from below, they will swim on their fronts!

Photo:Fig 6 Water Boatman

Fig 6 Water Boatman

Pond Skaters:

Pond skaters are probably the easiest to spot of all the invertebrates listed here, as they skate across the surface of the water. Pond skaters are predators that mainly feed on dead and dying insects that have fallen onto the surface of the water.

Fun Fact: Pond skaters don't always stay on the surface, they swim about a centimetre underwater to lay their eggs.

Photo:Fig 7 PondSkater

Fig 7 PondSkater

Mayfly Larvae:

Mayfly larvae are found underwater, where they breathe through the gill-like appendages that stick out of the sides of their abdomen. Unlike many of the larvae in this list, mayfly larvae are herbivorous, feeding on algae and aquatic plants.

Fun Fact: Their adult life is notoriously short, with some species having less than an hour to reproduce before they die!

Photo:Fig 8 Mayfly Larvae

Fig 8 Mayfly Larvae

Water Snails:

Snails need very little introduction, and their large shell makes them easy to spot. They can be found anywhere from the bottom of the canal to plants at the edge and even on the bank of the canal. Snails cannot swim, so their movements are limited by available surfaces and their slow speed.
Snails feed on aquatic plants and rotting plant matter, and are very much at home in a polluted canal.

Fun Fact: Many species of water snail breathe through gills like the majority of aquatic invertebrates, but some species such as the Great Pond Snail breathe through a set of lungs!

Photo:Fig 9 Water Snail

Fig 9 Water Snail


Bloodworms are the larval stage of midges, the small flies that buzz around in summer. 

Bloodworms are often found in small tubes made out of the mud they find on the bottom of the canal, where they feed on small amounts of detritus. Sometimes, however, they leave their tubes and float to the surface. Fun Fact: Bloodworms are named for their red colour, caused by a protein called haemoglobin. Just like in human blood, haemoglobin acts as an oxygen store, allowing the worms to survive underwater. 

Photo:Fig 10 Bloodworm

Fig 10 Bloodworm


Leeches are remarkably good swimmers, and are most likely found near the bottom of the canal. The key distinguishing feature is the large sucker around the mouth.

Most leeches found in Britain feed upon snails, worms and frogs. Only one species actually sucks blood, in particular the medicinal leech, which can grow up to 20cm in length. Thankfully, however, the monstrous medicinal leech has become rare in Britain since the widespread building of bridges that led to fewer victims travelling through streams.

Fun Fact: Leeches have two brains, one at the head, the other at the tail. These are not really brains like the ones in higher animals, these are clusters of nerves called 'ganglia'.

Photo:Fig 11 Leech

Fig 11 Leech

How Do We Use This to Evaluate the Health of a Canal?

The invertebrates mentioned in the spotter's guide above all have different sensitivities to pollution, and by counting the numbers of different invertebrates that we find, we can effectively measure the health of the canal.

A biotic index is a scale for measuring how tolerant a species of invertebrate is to pollution. We can then do a survey of invertebrates in a section of the canals, and use this information to evaluate the health of the canal.

Biotic indices can be very complex, but we're going to use the well researched yet simple method developed by Their excellent methodology can be found in full here.

The first step is to get outside, go down to a canal (take an adult if you are a child) and get dipping! Use the spotter's guide, and the advice written above to try and find as many invertebrates as you can. Don't forget to write down how many you find in your notebook, and if you can, take a photograph of them as well.

The next step requires a little maths.

First, count all of the worms, snails and leeches that you found. Add up the total, and this is the Number of Tolerant Invertebrates.

Next, count up all of the great diving beetles, water boatmen, pond skaters and mayfly larvae. Add up the total, and multiply it by 5. This is the Number of Semi-Tolerant Invertebrates.

Thirdly, count up all of the caddisfly larvae, damselfly larvae, dragonfly larvae and alderfly larvae. Add up the total, and multiply it by 10. This is the Number of Intolerant Invertebrates.

Lastly, add together the Number of Tolerant Invertebrates, the Number of Semi-Tolerant Invertebrates and the Number of Intolerant Invertebrates.

This number that you worked out is your Canal Score. Now let's see how your canal fared on the Canal Score Chart:

Photo:Fig 12 Canal Chart

Fig 12 Canal Chart

How healthy are the canals near you?

This page was added by Frank Machin on 10/10/2012.

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