15-Mar Life Beneath the Surface

On the pond surface water skaters and springtails may also be seen. The Great pond snail Lymnaea stagnalis and Wandering pond snail L. peregra are often near the surface. Caddis flies got their name from the caddis men who were pedlars who carried all the wares for sale on their backs. Caddis fly larvae carry their homes or protective cases around with them at all times. After hatching from eggs they construct these cases from the materials available around them.

In London this tends to be small pieces of woody stems and leaves but elsewhere it can be coloured sand grains, water beetle cases or even tiny ramshorn shells. They can often be persuaded to abandon their cases with a little gentle prodding. What emerges is a surprisingly large caterpillar-like grub which, given the opportunity, readily returns to its home.

Any net sample taken from a pond, lake or river at this time of year is usually full of “worms” of all shapes and sizes. These are mainly oligochaetes, nematodes and chironomid larvae. Oligochaetes are true segmented worms which contract and extend like an earthworm and usually have bristles along their body. Smaller ones tend to be colourless and larger ones brown or red. They exist in substantial numbers in the mud which they eat, although larger species will take other animals such as waterfleas or rotifers. It is the bright red sludge worms Tubifex spp we tend to notice most and see in pet shops sold as live fish food. They can often be seen this month in stagnant pools on the surface of the mud waving their “tails” to collect as much oxygen as possible. In the 1960’s, when the Thames in central London was particularly malodorous, there was little else in the mud outside the House of Commons. Other common oligochaetes include Eiseniella tetraedra (30 mm), a square-tailed worm that looks a little like a small aquatic earthworm, Stylaria lacustris (10 mm) which has a long proboscis, long bristles and always seems to be swimming and various Chaetogaster spp such as C. limnaei which is often found hiding in the entrances of snail shells.

Nematodes are non-segmented worms that are usually very small, glass-like and seen wriggling in between mud particles. Their bodies often have tapered ends and they wriggle more like coiled springs. Chironomid larvae look like worms but have heads, eyes and mandibles and are actually the larvae of non-biting midges. They occur in truly vast numbers in the mud which accounts for the swarms of adults we will see later. There are over four hundred species in Britain. Although they come in a range of different colours it is the bright red larvae we call bloodworms that get noticed due to them constantly contorting their bodies into figures of eight. From egg to adult may be just a few weeks with midges. Fortunately they do all their eating now as larvae. Many of the adults we see and think may bite us haven’t even got the mouthparts to do so.

The suckers and looping motion of leeches makes them the easiest worms to recognise and there are only twelve harmless species in London. These are easy to identify due to their different sizes, colours and markings. Most spend their time among vegetation in shallow water waiting for prey to pass by and therefore are easily caught in nets. Among their prey are a wide variety of small invertebrates especially water snails, worms, water lice and midge larvae. Theromyzon tessulatum is a large leech (10 cms), usually dark olive in colour with tiny yellow spots. It prefers to feed on waterfowl which it enters through their mouths or nasal cavities before sucking blood from their gullets. An even larger leech, the horse leech Haemopis sanquinsorba (15 cms) which is also dark green but with a lighter coloured underside, is amphibious and quite capable of looping out of a pond to feed on earthworms.

The species seen the most are smaller leeches such as Helobdella stagnalis (10 mm), Glossiphonia complanata (30 mm), G. heteroclite (15 mm) and Erpobdella octoculata (60 mm), the latter two being usually found in organically polluted waters. Helobdella stagnalis is pale green with a small, dark, hardened scale just behind its eyes. It prefers sluggish rivers and can withstand some degree of pollution. Glossiphonia heteroclita is usually amber in colour whereas G. complanata is darker with black lines and yellow and grown spots. Both are best look for in sluggish rivers on plants and under stones. G. complanata rolls up rather like a pill woodlouse when caught. Erpobdella octoculata is usually brown with black spots. It is more slender and more active than other species and also very common in polluted ponds. A close relation E. testacea (50 mm) is almost transparent. Dutrochet’s leech Trocheta subviridis (10 cms) is also found in water contaminated with sewage. It is greyish green in colour but with a distinctly roughened surface. It was first found in London zoo in 1850. Since then it has bred in sewers and has even occasionally been found in lavatories. Hemiclepsis marginata (3 cms) and Piscicola geometra (8 cms) both prefer to feed on fish and have large suckers for the purpose. Hemiclepsis is usually greenish yellow with lemon spots and Piscicola attractively patterned in dark green and white.

The only British leech that is dangerous is the Medicinal leech Hirudo medicinalis (15 cms) which was thought to be extinct in London until there was a chance rediscovery on Bookham Common in 1947. This villain is the largest and most attractive of all being dark green with bright orange and brown longitudinal stripes. It is believed seven million were used by doctors to treat patients in 1867 alone, so much so that doctors themselves went on to be called “leechers”. Poor people collected them by wading out into ponds. The leeches quickly sense their presence and swim like snakes towards them before inflicting a Y-shaped bite with their three jaws. The anticoagulant they include in the bite then causes the wound to bleed for some time. They were even burnt to make a hair dye that had the unfortunate side effect of blackening teeth and were still being used to treat black eyes in the East End just over a century ago.

There are other creatures that we may notice due to the way they scurry across the surface of the mud in constant motion and always appearing to be very busy. These are ostracods, crustaceans made up of two shells a little like a mussel but only 2 or 3 mm long. They are often the colour of their surroundings so the ones we see are usually brown. They have antennae and limbs only visible under a microscope which are immediately retracted when they are disturbed. Two of their three pairs of limbs are used as legs and the other pair as jaws. Like so many other water creatures, in March they will be laying eggs.