Environment A to Z of Canal Wildlife
A to Z of Canal Wildlife: S - U

Slugs

Slug
Slugs are basically snails without shells, although a few do have tiny shells. The rest fall into two groups, the keeled slugs and the roundbacks. The common great grey slug, Limax maximus, is a keeled slug and has a keel or ridge along its back and will grow to 8 inches (200 mm) long. The common garden slug, Anon hortensis, just an inch (25 mm) long is a roundback. Both can be found alongside the canal. Slugs and snails are hermaphrodites and although they can fertilise themselves they rarely do. Being bi-sexual increases the chance of these slow-moving creatures meeting a suitable partner. Most have an elaborate courtship ritual that can last for some time. Eventually they mate and exchange their sperm and go their separate ways. They lay their eggs in suitable damp places; the soil, under stones, in rotting wood etc.. Eggs hatch out in three to four weeks but those laid late in the year do not hatch until the spring. Slugs and snails usually live only two or three years and that’s only if they avoid being eaten by birds, shrews, mice, foxes, hedgehogs, frogs, toads and even beetles.

Snails

Snail
Slugs and snails (such as the roman snail shown on the right) are so familiar that they need no description. They can be found almost everywhere in the countryside but as they favour damp or wet places are common along canals. After a warm summer evening’s shower the towpath and canal margins will be literally crawling with them as these generally nocturnal members of the mollusc family emerge early to take advantage of the favourable conditions to search for food. There are 23 species of slug and 120 species of snail in Britain, 80 land snails and 40 freshwater snails. (The water snails will be featured later).

Snails and slugs, are slow-moving creatures. They require moisture to produce the thin layer of mucus on which they slide along. Their diet varies according to species but being so slow they cannot catch prey so rasp their way through carrion, fungi, mosses and plant material.

The characteristic of all snails is the home or whorled shell they carry on their backs. It consists mainly of chalk and is a hollow cone coiled around a central column. Most snails have right-handed shells. If you hold the shell with the coil tip upwards, when the opening faces you it will be on the right. The body of the snail is coiled up within the shell and only the head and the bottom part known as the foot are visible. Movement is caused by muscular waves along the foot. The head, just a continuation of the foot, has two pairs of tentacles, the longer ones having eyes at the tips.

Spindles

The Spindle Tree, Euonymus europaeus can be found in canalside hedgerows, especially in southern England and where the soil is chalky. This rather small inconspicuous, shrubby tree, sometimes called pincushion shrub, got its name because its thin stems were used as spindles. Before the invention of the spinning wheel, women would spin wool into thread by twirling a weighted stick called a spindle between their fingers. The very straight hard smooth branches of this tree proved ideal for the purpose. The hard pale yellow wood was also used to make pegs, knitting needles, toothpicks and skewers and was also known as prickwood and skewerwood.

The young twigs are greyish-green and are square in cross section while the older stems are round with thin grey bark. The 1-to-4 inch (25-to-100 mm) long leaves in opposite pairs are elliptical to oval, toothed and slightly tapered towards the tip and deep green in summer, turning purplish-red in the autumn. Small greenish flowers which may be male, female or a combination of both appear in May and June. They have four tiny sepals, four narrow petals, four stamens and a central pistil. The flowers are pollinated by insects and an unusual bright pink fruit develops in the autumn. This small fruit is about a third to half-an-inch (10-to-15mm) in diameter and is a four-lobed capsule. Eventually, each lobe splits to reveal bright orange flesh surrounding a hard yellow seed. These are very attractive to birds and are soon eaten and spread along the hedgerow.

Toads & Tadpoles

The Common Toad, Bufo bufo, is indeed common and can be found throughout England, Scotland and parts of Wales and Ireland. Although it spends most of its life on land, it is a welcome visitor to canals during its breeding season. By day the squat, portly toad lives in holes and hollows, either natural or ones they scrape themselves, in banks oToadr beneath trees and hedgerows. They emerge at dusk to search for food. The toad feeds voraciously, eating hundreds of insects for a single meal.

In fact, it will eat almost anything that moves, slugs, snails, worms, beetles, caterpillars, woodlice and ants. Prey is studied with the creatures large, copper-red eyes for several seconds before it shoots out its tongue and catches it.

The toad is easily distinguished from its relative the frog, being much broader and shorter-limbed. It is also much more laboured in its movements slowly crawling or hopping awkwardly on all four feet. The warty skin is dull brown or olive-grey and, being earth-coloured, acts as camouflage, thus enabling it to squat motionless and unseen when a predator passes by. The toad sheds and eats its skin several times each summer.

Toads hibernate in winter choosing old animal burrows, preferably under trees and hedgerows, but, being excellent climbers, even abandoned sand martins’ holes high up in banks are sought out. They emerge from hibernation in late March and April and instinctively make straight for their breeding pond. They are very selective where they breed and will pass many suitable ponds, ditches or canals in the search for their chosen location. It is thought that, like migratory birds and fish, they always return to the same place as they were originally spawned. They prefer deep water and will travel many miles in their week-to-ten-day-long search. This is a dangerous time for them and many fall prey to animals and birds. However, when they are attacked, a protective gland located behind the eye emits a thick white poisonous liquid which oozes from their warty skin. This protects the toad from many would-be predators but some still manage to kill and eat them. Some birds, such as herons and crows disembowel them and some mammals such as rats and shrews skin them before eating them.

Those that do make it to their favoured breeding-place may have already found a partner on the journey. Should the much smaller male come across a female on the way he will hitch a ride, firmly grasping her with his limbs. The other, less fortunate, males will emit high-pitched croaks in their attempts to attract a mate. There are usually more males than females, thus competition is intense. Very often, several males will cling tightly on to the back a single female, forming a ‘knot of toads’. The paired toads swim around until they find a suitable underwater plant. She then lays her twin string of jelly-like eggs, at the same time the male releases his sperm into the water to fertilise them. There may be as many as 7000 eggs in the 7-to-10 feet (2-to-3-metre)-long strings entwined amongst the plants. That night, the female leaves the pond, her mission over, but the male may stay a while just in case another unattached female comes along. A week later, the round eggs begin to turn oval and a few days later faint traces of the tadpoles head, body and tail can be seen.

Then just two weeks later, the black tadpole emerges from the protective jelly and swims away. Its mouth and gills soon begin to develop. Initially it feeds on algae, later progressing to animal food. The time taken for it to develop into a miniature toad depends on the weather. In a warm summer, it can be as little as two months, but if it is cooler it may take three. The little toads spend the rest of the summer by the waterside, hiding under stones and vegetation, before moving away to hibernate for the winter. It takes about five years for the toad to mature to a breeding adult, and the usual life-span is up to ten years.

Travellers Joy

Travellers JoyThe quaintly named Travellers Joy, Clematis vitalba, is common on chalk downland and limestone hills, and thus can be found clambering along the towpath hedges of the Wilts & Berks Canal. This wild clematis was given that name by John Gerard in his ‘Great Herbal’ of 1597 because of the beauty of its massed, fragrant, greenish white flowers, ‘which greets one along every track and by-way’. Also called virgin’s bower, after its interlaced stems and old man’s beard after its white seed hairs this member of the buttercup family starts life as a two-leafed seedling. It develops a slender shoot bearing deeply lobed, glossy, pale green, leaves.

If a leaf-stalk touches a hedgerow stem it quickly becomes a tendril, twisting around it. Thus it climbs upwards, winding around any support it touches, quickly growing into the crowns of small trees and shrubs and will clamber along many yards of hedgerow. The originally weak stems soon thicken to become tough and woody, with pale brown fibrous bark. Masses of small flowers with five petals, many stamens and a cluster of separate carpels appear in May, and continue into the summer. By October the seeds form, bearing long silver hairs thus draping the hedgerows in wreaths of silken plumes. The shaggy seedheads survive the winter to be blown far and wide by the winds of spring.

Underwater


The water environment of a canal can usually be considered to have four zones of life; the surface film, a region of submerged and floating vegetation, open water and bottom mud. Each zone will have its peculiarities with advantages favouring certain flora and fauna.

The surface film is the frontier between air and water and is a habitat with adequate oxygen and lots of food. Over and underwater, this film supports many creatures that are superbly adapted to live there. Light, long legged, water bugs like the pond skater and the water measurer are supported above the film whilst on the underside hang larvae and pupae of flies and gnats. Swimming just below will be whirligig beetles and waterboatmen. A zone of vegetation of varying width, depending upon the depth and condition of the water, will stretch from the bank to the open water. In a clean shallow canal, during the summer, the vegetation may well extend right across the canal and thus there will be no open water. On a deep well-used canal, with murky water where the plants are starved of light, the vegetation may hardly extend away from the banks so, although it may be of poor quality, there will be plenty of open water. Generally something between the two conditions exists on most canals and here amongst the vegetation will live the greatest variety of species, including water bugs, beetles, nymphs, snails, mites, leeches and flatworms.

The open water in the middle supports the largest, the fish, and the smallest, the planktonic animals, of the free swimming or floating fauna.

Lastly, the most unpromising zone, the bottom mud, supports surprisingly large numbers of, albeit a few, species of creatures. Although oxygen may be scarce food is usually plentiful in the form of decaying plant and animal remains and some creatures such as the larvae of midges, annelid worms and bivalve molluscs thrive here.

So the water, whatever its condition, will support an abundance of life. Vast numbers of insects and other small animals such as the larvae of dragonflies and caddisflies, waterboatmen and water stick beetles. Unimaginable numbers of planktonic water fleas and copepods which in turn feed on microscopic algae. Among the weeds live hydras, shrimps and water lice and on lock walls grow sponges and bryozoans. The canal underwater thus provides a rich and varied haven for life.