Freshwater Invertebrates

Freshwater invertebrates play a vital role in maintaining clean water – helping to break down and filter organic matter. They are also an important food source for fish, birds and mammals. Their presence is the standard indicator of the health of the habitat they live in. However, many of our freshwater invertebrates are declining in the face of pollution, invasive species, abstraction and development, and are already being affected by climate change with an increase in water temperature.

Dragonflies and damselflies (Order Odonata)

We are probably more familiar with the adult stage that are often seen whizzing by in the search of prey or a mate. There are 57 species in the UK that includes 36 species of dragonfly and 21 species of damselfly. The overall figure includes 2 species that are extinct and a further 12 migratory species.

Dragonflies and damselflies spend most of their life in their larval stage taking months to years to develop, depending on the species and local conditions; development takes longer in cooler climates. The larvae of the Golden-ringed Dragonfly (Cordulegaster boltoni) can take up to 5 years to fully develop! The larvae of dragonflies and damselflies spend their time developing in freshwater ponds, canals, ditches and rivers.

The larvae of dragonflies and damselflies are different. Damselflies have fin-like structures at the end of their abdomen (called caudal lamellae) that act as external gills for breathing while in water, dragonfly nymphs don’t have these structures. Unlike the larvae of butterflies and beetles, there is no pupal stage and when ready the larvae climb out of their freshwater home and transition into an adult with a final moult.

Mayflies (Order Ephemeroptera)

Adult mayflies are very familiar to anglers, especially those that fly fish in rivers using artificial flies made of feathers and fur. Known by many names, most adult Mayflies live for only a day! They spend most of their life as larvae in freshwater feeding and growing, some taking several months or even years before emerging as an adult. Mayfly adults do not feed, their sole purpose is to reproduce and then they die.

Stoneflies (Order Plecoptera)

There are 35 species of stonefly in the UK. Like mayflies the larvae spend most of their life in freshwater feeding and growing – some growing to over 3cm in size clinging to stones in the stream bed. Adults have two pairs of wings that are either folded flat or wrapped around the abdomen – some are short making them poor fliers, and some hardly have any wings at all! The adults of most species don’t feed and live for only a short time. They can often be seen basking on stones, fence posts and bridges.

There are some very rare species of stonefly that are found in only a handful of locations. Adults of the very rare endemic Northern February Red (Brachyptera putata) can be seen resting on fenceposts in the Scottish Highlands from February to March.

Another rare species to look out for is the Critically Endangered Scare Yellow Sally (Isogenus nubecula). A native stonefly, which in the UK is only found along parts of the main channel of the River Dee in Flintshire, Wales.

Caddisflies (Order Trichoptera)

Caddisflies are often known by anglers as ‘sedges’ or ‘caddisworms’. Adult caddisflies are moth-like in appearance, although they have long-thin forward projecting antennae and a characteristic way of holding their wings over their back – like a steep-pitched roof over their bodies. The larvae of caddisflies are found in a wide range of freshwater habitats and several species make elaborate cases out of sand, small stones or vegetation (different species use different materials and it depends on what is available in their vicinity). There are also larvae that don’t make these cases known as caseless caddis, however some species do make a shelter to hide in.

There is a single species of caddisfly in the UK that doesn’t live in water. The Land Caddis (Enoicyla pusilla) lives among dead leaves and moss and can be found in Oak woodland in the county of Worcestershire.

Beetles (Order Coleoptera)

There are almost 300 species of beetle that are fully aquatic in their larval and adult stages, and a further 100 or so that can be found living on the banks and margins of pools, streams, and wetlands. Our largest species of water beetle is the Great Silver Water Beetle (Hydrophilus piceus) which can grow to almost 5 cm in length! While underwater it breathes by trapping air bubbles on its underside, giving it a silvery appearance. Some water beetles have interesting patterns that help them blend into their backgrounds to camouflage them and protect them from predators but also to help when hunting prey.

Leeches (Phylum Annelida)

Despite their gruesome reputation, there are only two leeches in the UK that feed on blood of mammals. The Medicinal Leech (Hirudo medicinalis) was well used in medieval times in the UK to cure a number of ailments. Leeches are still used today in medicine to increase blood circulation. The very first use of the Medicinal Leech was recorded during the rise of civilization inancient Egypt where they have been observed in wall paintings. The Medicinal Leech is the largest of our 17 species of freshwater leeches, and is one of two species capable of penetrating human skin – the other is Placobdella costata which is a rare species in the south of England. The Medicinal Leech is also one of our rarest species, known from only 20 or so sites from across the UK.

Crayfish (Order Decapoda, Phylum Arthropoda)

Related to marine lobsters, crayfish are found in freshwater habitats. There is only one species of crayfish native to the UK – the White-clawed Crayfish (Austropotamobius pallipes). This species has suffered declines since the 1970’s and today is found in patches across England and Wales.

There are at least six other non-native species of crayfish recorded in the UK, including the North American Signal Crayfish (Pacifastacus leniusculus) that competes with the White-clawed Crayfish for food and habitat and can transmit the lethal crayfish plague to our native species.

Discover more about freshwater invertebrates and invasive non-native freshwater invertebrates on our YouTube channel.