Crustaceans and myriapods

Fairy shrimps live in temporary ponds and puddles on Devon’s heaths and moors.

Devon is the only place in the world where the flat-backed millipede lives.

Devon’s white-clawed crayfish are likely to be extinct in the wild within the next 20 years.

What we need to do and where

White-clawed crayfish, Wildwood – Lee Harding

1. About

Crustaceans are invertebrates which have a very obvious hard, segmented external skeleton (known as an exoskeleton) providing support and protection. This group includes crabs, lobsters, shrimps, crayfish, barnacles and woodlice (the UK has 37 species of woodlice). They are mainly aquatic, but some (such as woodlice) live on land.

Their diet ranges from dead organic matter to eating other invertebrates or small fish. They are vital to food chains, both as predators and prey, and they play a key role in helping to break down dead organic matter. 

Myriapods are also invertebrates. This group includes millipedes and centipedes which have long segmented bodies with each segment having one or two pairs of legs. They live on land and feed on decaying plant material.

Five species are listed as Devon Species of Conservation Concern. Of these four are Focus Species as they need extra attention. These are discussed below.  The false flat-backed millipede lives in south Devon and isn’t known anywhere else in the world.

Pressures and opportunities relating to Focus Species are discussed below.

2. What we need to do and where

Priority

Increase the abundance and distribution of native crustaceans across Devon particularly those identified as Focus Species.

See Find out more below for any relevant national and local targets.

Focus species

White-clawed Crayfish

Read more

Habitat 

The white-clawed crayfish is globally endangered and legally protected. It is the largest UK native invertebrate and our only native freshwater crayfish. They need clean, well-oxygenated streams and rivers with a neutral to slightly alkaline pH with readily available calcium to form the exoskeleton. White-clawed crayfish are very sensitive to pollutants, especially insecticides and agricultural inputs such as ammonia and to high silt levels. They are occasionally found in lakes and ponds.

They need plenty of hiding places under rocks, vegetation and tree roots and eat plants, algae, invertebrates and small fish. In turn they are prey for a range of species including otters, various waterbirds and fish such as Atlantic salmon. Their lifecycle includes an egg stage, a juvenile stage and an adult stage and they can live for more than seven years.

 In Devon all the rivers are sub-optimal for white-clawed crayfish in terms of both habitat and water quality, which may be why the animals are on average smaller in size than most other populations and live at very low densities.  

Population

White-clawed crayfish were abundant in England and Wales up to the 1950s when the population declined due to pollution (especially chemicals such as organophosphates used in sheep dips) and habitat degradation.  In the 1970s American signal crayfish were introduced as a farmed species. Unfortunately these crayfish escaped and rapidly spread across the country. Signal crayfish dominate native crayfish and will outcompete them for food and habitat. They also carry a fungal disease called crayfish plague which is fatal to native crayfish.  As a result of all these pressures there was a huge decline in the white-clawed crayfish population during the 1980s and 90s with up to 95% of populations lost in southern England. The majority are now found in northern England but populations continue to decline.

White-clawed crayfish have never been widespread in Devon as our rivers are generally too acidic. Historically they were present in East Devon catchment rivers, the Clyst, Culm, Axe, Otter and Creedy Yeo. They are now only found on the Culm and Creedy Yeo but due to the presence of signal crayfish, pollution the populations continue to decline. Low rivers levels in summer and damaging floods in winter contribute to their decline and they are likely to become extinct in the next 20 years. See Find out more for the detailed paper on white-clawed crayfish written for the LNRS by the EA. This includes information on the hatchery at Wildwood Devon and the Saving Devon’s Native Crayfish Project.  

Actions

Improve habitat in the Creedy Yeo and Culm for white-clawed crayfish and prevent extinctions through continuing to fund the hatchery and develop Ark sites.

  • Develop at least 10 Ark sites for the Creedy Yeo and Culm catchments by 2026 and a further 10 buy 2033. These should be newly dug ponds away from flood plains and need a clean water source without signal crayfish and crayfish plague. Populate the Ark sites with captive bred white-clawed crayfish and wild white-clawed crayfish from the Creedy Yeo and Culm.
  • Continue to fund the hatchery at Wildwood Devon, Escott.
  • Deliver river restoration and habitat enhancement work on both the Creedy Yeo and the Culm to improve water quality, water chemistry, river-bed condition and in-stream habitat complexity. See Watercourse Corridors for related actions.
  • Target habitat improvements to areas which may benefit native crayfish in the short term by improving their resilience to predation and their reproductive success and in areas considered suitable future release sites.
  • Explore opportunities for controlling signal crayfish such as introducing genes which cause infertility (known as gene drive technology).

Where to focus action

High Opportunity Areas: River Creedy Yeo and River Culm which are on the Watercourse Corridor layer. See Mapping.

Fairy shrimp

Chirocephalus diaphanus
Read more

Habitat

Though their transparent body can make them hard to spot, fairy shrimps are unmistakeable once seen. They swim upside down and can grow up to 3 cm. They have a long, forked tail and 11 pairs of swimming legs.

Fairy shrimps live in small ponds and puddles which dry out in the summer. These are generally found on heaths, moors and grasslands where livestock trampling maintains open, sparsely vegetated areas or where quarry or miliary vehicles create deep, wet rutted areas along tracks. They are thought to prefer temporary ponds as they have fewer predators such as fish and dragonfly larvae.

Eggs lie dormant in dry mud and hatch when rain refills the pool. Fairy shrimp feed mainly on algae (which can turn them green) but also eat organic material and small invertebrates like water fleas.

Fairy shrimps are a flagship species for temporary pools, and important invertebrate habitat supporting many rare species.

Population

The Fairy Shrimp has always been a very scarce and threatened species in the UK and its population is vulnerable to extinction. It is a protected species which makes it illegal to knowingly disturb or harm it.

In Devon there are fairy shrimp scattered across the county. The commons of southwest Dartmoor have the greatest concentration of colonies and there are also recent sites on clifftop grassland in the Branscombe area and on the north Devon culm grasslands. Most sites have no formal protection.

Fairy shrimp habitat has been lost on some sites where a reduction in livestock has led to open water becoming overgrown or densely shaded by tall vegetation such as rushes and/or scrub.

Note that the National Biodiversity Network map doesn’t include most of the recent records of this species in Devon and that there have been no recent targeted surveys in Devon.

Actions

Maintain and create temporary pools on heaths, moors, culm and coastal grasslands for fairy shrimps (and other invertebrates)

  • Survey all sites with existing records and develop a targeted management strategy across Devon. Ensure that records are shared with DBRC.   
  • Graze sites with traditional breeds of cattle of ponies (during the winter where possible) to create microhabitats of shallow, open temporary pools and hollows. Lightly graze during the summer to control scrub. Clear scrub mechanically where needed.  Target management to sites with existing populations.
  • Create new pools close to existing fairy shrimp colonies.

Where to focus actions

High Opportunity Areas:

  • South West Dartmoor Downs  – including Lee Moor quarries (mapped under Heath, mire and rush pasture mosaics)
  • North Devon Culm mosaic (ditto)
  • Sidmouth to Dorset Coastal Wildbelt (mapped under Coast)

See Mapping.

British cave shrimp

Niphargus glenniei
Read more

The British cave shrimp is only known from Devon and Cornwall. Along with its close Irish relative (N.irlandicus) it’s thought to be the oldest animal species in the British Isles and to have been here for at least 19.5 million years!

Little is known about this species, but it’s widely distributed in Devon’s underground groundwater aquifers and is found in caves, mines, springs, wells and boreholes. As they live in total darkness that are blind and have extra-long legs and antennae to detect food (microorganisms and organic matter). The main pressures are thought to be pollution of groundwater from sewage and fertiliser run-off.  

Actions

Improve groundwater quality for the British cave shrimp.

See Watercourse corridors for more detailed actions relating to water quality. 

Where to focus action

Across Devon.

False flat-backed millipede

Anthogona brittanica
Read more

This threatened millipede is found around Kingsbridge and Dartmouth in south Devon and isn’t known anywhere else in the world.  

It was first found in 1992 at Slapton Ley NNR on coastal vegetated shingle. A 1995 survey recorded it in at least ten other sites in a restricted area of south Devon stretching from Kingsbridge in the west to Brixham. The millipede has been found in leaf litter under broadleaved trees in woodlands (often where there is a layer of Ivy in the understorey) on sea cliffs and scrubby coastal grasslands.

The vegetated shingle at Slapton is under considerable threat from rising sea levels associated with climate change and areas have been lost through recent storm surge events.  At its southern end the shingle is constrained from moving inland by the ley, A379 road and coastal defences.  None of the other sites have any formal protection making them vulnerable to loss, particularly sites close to Brixham and Dartmouth where development pressure is high.

No surveys have been undertaken since 1995 and there are no current conservation projects focusing on this threatened species. 

Actions

Maintain, enhance and link a range of habitats for the false flat-backed millipede

  • Re-survey sites to understand presence and habitat requirements and then carry out targeted habitat management.
  • Maintain and enhance stands of stable shingle grassland along the shingle ridge at Slapton Ley. 
  • Manage and link other sites by creating blocks and corridors of native woodland, hedgerows and scrubby grassland.   

Where to focus action

High Opportunity Areas:

  • Brixham to Start Point Coastal Wildbelt: Initial re-survey to be targeted to Slapton Ley NNR (vegetated shingle, Slapton Wood and Mill Hill Copse)) and Burlestone Wood, Dartmouth.
  • Start Point to Bolt Tail Coastal Wildbelt

See Mapping.

Actions for all crustaceans and myriapods

Habitat management and creation

Woodlice, millipedes and centipedes will benefit from actions to manage, restore and connect most habitats across Devon.  Crustaceans will particularly benefit from actions relating to coastal and aquatic habitats.  See Habitats page.

Survey, monitoring and engagement    

  • Support efforts to conserve and raise awareness of Devon’s crustaceans and myriapods.

3. Inspiration

Case studies

Saving Devon’s native crayfish project

Text to be added – do we include this or too early days?    

Fairy shrimps at Lee Moor quarries?

Where to visit

Text to be added. 

Woodlice, millipedes and centipedes can be seen in gardens and parks…..go rock pooling to see crabs! 

4. Find out more

To be finalised + overview papers on Focus Species written for the LNRS by experts to be added

Crayfish

American Signal Crayfish: England Case Study – Biological Recording 

Crayfish Conservation Manual, Ian Marshall and Jen Nightingale 20219Rostra publications).  ISBN 978-1-7398868-0-6 

White-clawed crayfish: advice for making planning decisions – GOV.UK (www.gov.uk) 

Ecology of the White-clawed crayfish, Conserving Natura 2000 Rivers Ecology Series No. 1  David Holdich 

Why are white-clawed crayfish endangered?

Saving Devon’s Native Crayfish Project 

White-clawed crayfish – Buglife

Fairy shrimp

A good online description to aid identification of fairy shrimps can be found on the Freshwater Habitats Trust (FHT) website

BRATTON, J. H. & FRYER, G. 1990. The distribution and ecology of Chirocephalus diaphanus Prévost (Branchiopoda: Anostraca) in Britain. Journal of Natural History 24 (4): 955-964.

BRATTON, J. H. (ed.) 1991. British Red Data Books: 3. Invertebrates other than insects. Joint Nature Conservation Committee, Peterborough.

NBN ATLAS – Fairy Shrimp Chirocephalus diaphanus. Accessed 04/11/2024.

British cave shrimp

British Cave shrimp – Bug Directory – Buglife

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