Category Archives: Management plan

Life below the surface – A stream teeming with life, by Bob Gomes

Life below the surface – A stream teeming with life

In August Anna, David and I undertook a preliminary survey to investigate the animal life living within the stream at three sites between the West Street bridge and Chart Mill.  A short report on our finds will be produced this Autumn but I thought members might be interested in seeing photographs of some of the creatures that were revealed by the sampling technique. At first sight, many of the small invertebrates look rather nondescript but close up views reveal their true intricate beauty. Even leeches and the flatworms, looking like aliens from outer space, are truly colourful.

The most abundant animals were small crustaceans, the amphipod (body flattened from side to side or laterally compressed) Freshwater Shrimp Gammarus pulex  and the isopod ( body flattened dorso ventrally ( body flattened from top to bottom)) Water Louse Asellus aquaticus. Water lice are very common in slow flowing waters where they live amongst the bottom debri or slowly crawl up water plants. The freshwater shrimp inhabit waters with abundant dissolved oxygen and a high lime content. They usually swim on their sides and are very active and commonly found on stones or bricks on the stream bottom. Both are abundant in the stream where they feed on organic matter within the silt. Plenty of that within the stream!

The tangle of watercress provides a refuge for small fish such as the Ten-spined Stickleback. The number of spines on its back varies from seven to twelve and it is sometimes also called the Nine-spined Stickleback. It prefers slow flowing or still water and can tolerate slightly brackish water. It is present in good numbers wherever there are beds of watercress. The Fish Leech illustrated lives free amongst water plants and is a common external parasite of fishes. It will prey on Sticklebacks by sucking their blood. The specimen photographed was just 15 mm in length but it can grow up to 25 mm. It has prominent suckers at both ends of its body and is a good swimmer. When not active it adopts a fishing posture while attached to a water plant. I was amazed by the pattern of the pigmented rays on the rear sucker when I looked and photographed it through the microscope.

The other leech illustrated (Glossiphonia complanata) is not so mobile as the Fish Leech and moves in a typical looping leech manner. It is rather rubbery in consistency and rolls into a lump when disturbed. The black dots at the front end are three pairs of eyes. It is sometimes called the snail leech because if feed on the body fluid of snails as well as other invertebrates. It grows up to 30 mm in length.

Looking a bit leech like are the flatworms but they glide over the surface rather than loop along. The two species photographed the milky  white-bodied Dendrocoelum lacteum with its digestive system showing through the body wall  and the darker Dugesia sp probably lugubris.
The former preys on water lice and the latter will feed on water snails. They are largely nocturnal and in the daytime are usually found under stones or on the underside of leaves looking like lumps of jelly. Their mouths are not at the front end but on the underside in the middle of the body!

Pond snails are abundant on the water cress. The Wandering Snail Radix (Lymnaea) peregra is probably the commonest water snail in Europe found in a wide variety of habitats from lowland ponds to mountain streams according to the Collins Field Guide. Note the eyes at the base of the tentacles. The other similarly shaped snail but coiling in the opposite direction (sinistrally) and with more pointed tentacles is the Bladder Snail Physa fontinalis. When expanded the fleshy body lobes – mantle – wrap around the outside of the snail. The flattened tightly amber snails with tight whorls belong to the genus Planorbis. The specimens photographed were just 5 – 8 mm in diameter, but the largest of the genus  can be up to 35 mm in diameter. The common name for the group is Ramshorn snails. I have tentatively identified three species living in the stream.

Many insects depend on the stream for various stages of their life cycle. A couple are illustrated. The larva of a small midge, the Meniscus midge, Dixa sp. with its body characteristically bent sideways into a U shape and the Black Fly larva Simulium. The adults of the Black Fly are notorius biters of humans and other animals. The Black fly larvae attach themselves by small hooks at the swollen rear end to  a web, woven onto stones. The front end – shown in close up – bears two moveable fans of bristles for catching food. They are common in flowing water and the specimens we found were at the Chart Mill site where the water is fast moving over a stony stream bed. We also found non case bearing Caddis fly larvae at this site.

The photographs illustrate just a few of the animals discovered in the stream, so give a thought to all that life when you are next moving the watercress! Pushing the cress to one side does give the majority of the animal life a chance of moving back into the water.

The full report can be found here:

Westbrook Aquatic Invertebrate Survey Gomes Summer 2020

All photos taken and supplied by Bob Gomes