Invertebrates on the River Annan
Stonefly move from one stage to the next by shedding their exoskeleton (called 'moulting') over a period of 1-3 years. During this phase they are soft with white colouration and extremely vulnerable.
The stonefly is another good pollution indicator as they are very sensitive to pollution and drops in oxygen levels. There are 34 species of stonefly in the UK and the large stonefly (Perlodes microcephala) is one of the largest with the larvae reaching almost two inches in length. As their name suggests, stonefly larvae live amongst and under the stones of the river bed where they feed on algae and predate on other invertebrates. They hatch into winged adults by crawling onto exposed rocks in the shallows, once hatched they leave behind the larvae case on the rocks and can be seen either flying or drifting downstream rarely moving far from the river.
Stoneflies (Order: Plecoptera)
Flat Bodied Stone Clingers (Family: Heptageniidae)
Agile Darters (Family: Baetidae)
Mayfly (Family: Ephemeriidae)
Freshwater invertebrates are an important aspect of the ecology of the river for a number of reasons. They are an important part of the food chain for many fish, birds and mammals, they help to break down organic matter in the river and they are useful biological indicators for water quality. If you are interested in helping to monitor the water quality of the river while learning how to identify some of the freshwater invertebrates found in the river please click here.
To find out more about the freshwater invetebrates click on the name below or scroll down to browse the list.
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The family heptageniidae is commonly known as stone clingers or flat heads. This includes a number of species commonly found on the Annan, the march brown (Rithrogena germanica) is the most iconic of these and the olive upright (Rithrogena semicolorata) is the most prolific in terms of numbers. The yellow may (Heptagena sulphurea) is a very striking bright yellow fly that hatches during late spring and summer in small numbers as does the late march brown (Ecdyonurus insignis) and the large brook dun (Ecdyonurus torrentis). The larvae act as very good pollution indicators as they are very pollution sensitive, the march brown is absent from a good number of UK rivers and in decline in many of the rivers where it is found, on the river Annan we are fortunate to have a fairly healthy and stable population.
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Known as agile darters due to their swimming action, the family Baetiidae covers a number of species which are found in the river Annan and the Stillwater’s within the Annan catchment. The main species found in the Annan belong to the genus Baetis and include common names that will be familiar to most anglers such as large and small dark olives (Baetis rhodani), pale watery (Baetis fuscatus)and iron blue dun(Baetis niger, B. muticus). Like many freshwater invertebrates they provide a valuable food source for fish, birds and other invertebrates and they have an important place within the ecosystem.
The mayfly larvae are burrowers that live in tunnels in the beds of rivers and lakes, they use adapted breathing filaments to create a current over their backs allowing them to extract oxygen from static water. Mayflies (Ephemera danica) are not common on the river Annan with only the odd specimen being seen or reported each year although in the last two years mayfly larvae have been found more often in kick samples and there have been more confirmed sightings of adult flies. The larvae usually live in their burrow for two years before maturity and emerges from the river bed to migrate towards shallower water before ascending to the surface to hatch. The adult is the largest up-winged fly in the UK and can reach 30mm in length, the male is typically only two thirds the size of the female but apart from a pair of claspers appended from the abdomen and larger eyes there appearance is very much the same
Freshwater Shrimp (Gammarus pulex )
Caddisflies (Order: Trichoptera)
Found in fast-moving streams, the caddis fly plays an important ecological role and not just by providing a food source for fish. There are in total 198 species of caddis fly found in the UK and Ireland and they can be split into three distinct groups. Those that carry their case around with them (Integripalpia) and they are known as true cased caddis, those that have a case fixed in an area that they can retreat to (Annulipalpia) known as true caseless caddis and a very primitive form of caddis including Rhyacophila, Glossosomatidae and Hydroptilidae which only builds a case prior to pupating. The case (depending on which group) is used for security, as a method of trapping food and also as a balance weight to help anchor them to the bottom. Once the pupae emerges the caddis fly itself looks very different (like a small moth) the difference between the caddis and its distant relative is that moths have a scaly wing whereas the caddis wings have hairs on them.
Although commonly known as freshwater shrimp, gammarus are actually freshwater amphipods. Gammurus is a common species in the River Annan and act as good indicators of water quality due to their requirement for oxygen rich water and intolerance of pollution. They are generally found in flowing water with a preference for rocky substrate but are infrequently found ponds. Adults are typically around 14mm but males can grow up to 22mm. Their antennae contain sensory organs which are used to find food which is larger made up of decomposing material (both plant and animal). Although principally detritivors they will eat small invertebrates including other gammurus. Gammurus are an important and abundant food source for trout and salmon parr as well as stone loach, minnows and sticklebacks. Unlike many of the invertebrates found in the River Annan they do not have a larval stage and instead hatch fully developed from eggs, maturing and reproducing very quickly.
Dragonflies & Damselflies (Order: Odonata)
Dragonflies & Damselflies are found around the still waters, ponds and Lochs within the catchment. They are a distinct group of insects with large eyes and 2 pairs of wings. Dragonflies are usually much larger than the smaller, more delicate damselfly. They can be differentiated at rest as the dragonfly holds its wings horizontally from the body while the damselflies wings are held in a closed position over the abdomen.
Most of the life span is spent underwater as larvae or nymphs and only a few weeks are spent in the adult stage. Dragonflies & Damselflies are voracious carnivores and will take any flying insects they can catch. Their prey usually includes flies, midges and mosquitoes but the larger species can sometimes be seen to take moths and butterflies. They show the same ferocity as nymphs and they are skilled underwater predators at this stage eating anything smaller than them.