Environment A to Z of Canal Wildlife
A to Z of Canal Wildlife: G - I

Gastropods & Grass

Gastropods are snails. There are 52 species of freshwater snails found in Britain. They belong to the Mollusc family of which there are two types. The bivalves, the mussels and cockles, (more later in this series), and the gastropods, the snails and limpets. The obvious difference between the two is the form of the shell, in bivalves it consists of two parts, or valves, joined together and in the gastropods the shell is all in one piece and therefore the are referred to as unipods. In both, however they shell is not just a Snailshelter but an integral part of the mollusc and is attached to the body by powerful muscles.

The shell is made principally from calcium carbonate, chalk, so snails will only be found in hard water, though some have greater tolerance than others. The Great Pond Snail (left), Lymnaea stagnalis, (5cm.) is a hard water species and large numbers can be found in most canals especially the Wilts and Berks as it is watered by springs from the chalk Downs. Other common canal snails are the familiar and easily recognised Ramshorn snails; Planorbis planorbis (12mm.) and Great Ramshorn Planorbarius corneus (25mm.).In snails the foot which forms the flat sole on which they glide along also contains their mouth, hence gastro (mouth) pod (foot). There are two subclasses of freshwater gastropods, the prosbranchia, or gill breathers, and pulmonata which are able to breathe atmospheric air by means of a lung.

The first are truly aquatic and retain the method of respiration of their marine ancestors, gills. The freshwater pulmonates however are land snails that have taken to an aquatic life and need to come to the surface to take in air. Nevertheless, when the supply of dissolved air in the water is high, as it is in healthy cold water, then even these snails can obtain sufficient oxygen from the water and do not need to return to the surface. These snails are hermaphrodites, that is they have both male and female parts and all are capable of laying eggs. Although self fertilisation can take place in some species it is more usual that reproduction is through the union of two individuals. The eggs are laid in a jelly like substance on submerged stones and plants. Sometimes the eggs stick to water birds feet and can thus be carried to other distant waters. The food of water snails consists mainly of algae and decaying plants, although the Great Pond Snail will take small animals, Dead or Alive!

Many snails are to be found amongst the tangled vegetation, root and rhizomes of the grasses, sedges and reeds that grow in abundance along the banks of our canals. The Common Reed, Phragmites communis, the favourite reed for thatching and our tallest member of the grass family, reaches three metres in height and can be the dominant species, lording it over the others. However, often to be seen growing alongside is the easily recognised Reedmace, Typha latifolia, commonly but wrongly known as the Bulrush. The tall 2.5m. flower stems, which appear in June and July, bear a cylindrical mass of dark brown female flowers and above them a smaller mass of male flowers.

The smaller and rarer Lesser Bulrush T. Angustifolia is easily distinguished by a gap between the male and female flowers. After fertilisation there may be a quarter of a million seeds in a single head of the larger species but these are not shed until the following February. These are all typical members of the reedswamp community and it is characteristic that if they are left undisturbed they have a tendency to encroach into the water channel and in time completely block it.

The reedswamp plants tend to spread along the banks keeping to shallow water. They send out creeping roots and rhizomes, from which, at intervals, new plants arise. In time they can become established along great lengths of the canal bank providing wonderful cover for the large number of birds and insects. However, the tangle of roots and vegetation soon limits the water movement and encourages the accumulation of silt. Their dieback in winter and then decay, although feeding the snails, adds to the process and so year upon year their remains encroach into the water channel gradually making it shallower and so easier to spread into in the following season. Therefore a neglected canal will gradually become drier and drier; gradually the reedswamp will die and the wildlife haven it provided will be lost. The haunt of the snails, the nesting site of many birds, especially the Reed Warbler, Moorhen and Mallard will also be lost. So some dredging is definitely required to maintain a balance. There must be a partnership between Man and Mother Nature.


Although its often thought to be an urban myth, there are plenty of goldfish in stretches of the Wilts and Berks. At present, the areas where the canal is in water are self-contained and are often near towns (and in two cases near fairground sites).

However they got into the canal, in some areas they are doing very well and seem to be able to cope with both the winters and the indigenous predators. This example was taken by Gary Mason between Kingshill and Westlea in Swindon. When captive, some goldfish can live well into their late teens or twenties. Don't be too surprised if you see some older specimens in the Wilts and Berks.



The Grey Heron Ardea cinerea is a common and well known bird throughout Britain. It can be found hunting its prey in shallow water from estuary mud flats to rivers and canals, and even stealing goldfish from garden ponds. This large bird, three feet long, is often seen amongst the reeds along the banks of canals. They stand motionless, like grey statues, patiently waiting for a fish to swim by, which is instanly grabbed by the long dagger bill and swallowed. Also taken are frogs, small mammels, insects and worms.

On quiet stretches of canal the occassional walker may disturb a heron as it hunts. The heron usually spots the walker long before the walker sees the heron, which only reveals itself by flying off down the canal a 100 metres or so. In its slow leisurely flight it tucks back its head and trails its legs behind. Careful observation and stealth will enable the walker to approach and watch the heron hunting. Note precisely where it lands, then approach slowly, utilising the cover of reeds or hedgerow, and be patient. Should the heron spot you, and it probably will, it will just fly another 100 metres or so and you can try again. You may be lucky to get several goes before the heron tires of the game and flies in a wide loop back across the fields to its original position.

Herons usually build their nests in colonies in tall trees returning year after year to the same heronry. However some do nest among the reeds or in low shrubs. A dance ceremony is part of their courtship; the male stretches its long neck upwards then lowers it over his back with the yellow bill pointing skywards. Between three and five bluish green eggs are laid between February and May in a rough nest lined with roots, grass and reeds. The eggs are laid at intervals of about two days and the female sits as soon as the first egg is laid. So the young hatch successively and therefore vary considerably in size.

The heron is resident all the year in Britain, but some are visitors for the winter. These birds are very vulnerable in long cold winters because their shallow hunting grounds get frozen over.


Plants that will only grow submerged in water are called Hydrophites. If these true water plants are taken from the water it will be noticed that they are limp and weak and quite unable to support themselves. The leaves and stems are pale and translucent and lack the woody tissue and tough fibres of land plants. Hydrophites are supported by the water and thus have no need of strengthening, which would pondweedanyway prove to be a disadvantage preventing them from moving with water flow and changes of levels. In water plants each part of the plant absorbs directly the water that it needs together with a large part of the food it requires. Totally submerged plants have no breathing pores on their leaves, but in species that have floating leaves, such as water lilies, they are on the upper surface only. To prevent the breathing pores being choked by water the upper surface of the leaf is waxy so that any water soon rolls off.

Most of the true water plants are perennials and have evolved different ways of solving the problems of overwintering. The Water Starwort and Water Soldier just sink to the bottom and remain in the warmer water until spring. Some, including the Water Milfoil, Frogbit and Bladderwort, produce special tightly packed buds each autumn and when the plant dies the buds break loose to sink into the mud. When spring arrives they float back up to the surface and grow into new plants.

One of the best known totally submerged plants is Canadian Pondweed, Elodea canadensis. This plant was introduced from North America 150 years ago and has spread to most parts of Britain. This demonstrates, extremely well, a feature common in Hydrophites, that multiplication by vegetative means is more usual than by the setting of seed. Only the female plant was known here but its rapid spread was due to the ease with which the fragments broken off the brittle stems soon grew into new plants.

Hornwort, Ceratophyllum demersum, one of our most completely adapted aquatic plants actually flowers and pollinates under water. The inconspicious male and female flowers are bourne in the axils of the leaves. Sometimes pale shoots called rhizoids develop and anchor the plant in the mud but this plant has no true roots and usually floats freely in the water.

There are many Hydrophites; some will be featured later in this series.



Only two species of wild iris are native to Britain. Luckily, the most common is the beautiful yellow iris or flag, Iris pseudacorus, which flourishes in wet places like the banks of canals. Easily recognised, it grows up to 2 feet (600mm) high and has sharp edged, grey-green, sword-shaped leaves with a conspicuous, raised mid-rib. The stout many-branched flower stems each bear two or three bright yellow flowers, above the leaves, one at a time, from June to August. After the flowers fade, long pods develop which contain flattened, oblong pale brown seeds, the size of small peas. When the seeds are ripe the pod splits open and the seeds are shed. Some of the seeds inevitably fall into the water and, because they are very light, they float well and drift along carried by any current, even being blown by the slightest breeze. The seed usually comes to rest in the mud at the edge of the canal or between vegetation where it soon grows into a new plant.

The Iris is a perennial and its foliage dies back each winter. A rhizome, like a large, thick, tough root, stores sufficient nourishment to enable the plant to re-grow very quickly once warmer weather returns in the spring. The next time you spot this canal-side gem of a flower, stop, take a closer look and admire the most beautiful veining on its petals


More than three quarters of the known living species of animals are insects!. A million species have been named throughout the world. In Britain we have over 20,000 species, far too many to mention any here in detail.

Insects are powerful creatures. After all, it was insects that caused the demise of our canal in 1914. Because the residents were fed up with the annual summer onslaught of insects breeding in the derelict canal in Swindon, the borough council successfully petitioned parliament for an act to close it so that it could be filled in. Horrors!!

Most insects are at the bottom of the food chain and thus become an important food resource to a great number of other animals, birds and even other insects. Many species of British insects spend some, or all, of their life cycles in water. Thus the water that the canal brings to the countryside can be a huge bonus to a great many other of our native flora and fauna. The water supports the insects, insects pollinate flowers, insects are food, and so a much greater diversity of wildlife is brought into the surrounding countryside.

However, next time you are out strolling along the canal towpath admiring the irises and one of the little blighters bites you, feel free to swat it. There will plenty more where that one came from.!

Photographs in paragraphs marked * are provided by Gary Mason