Molluscs

Devon’s freshwater pearl mussels are some of the last remaining in the world. Once harvested for pearls they are now restricted to the Taw and Torridge and haven’t bred since the 1960s. 

What we need to do and where
A native oyster is photographed on a grey rock with yellow lichen like plant matter growing sporadically around its edges. The oyster itself is a rounded semi-circle shape and light grey in colour with a hard matte shell.

Native oyster, Keith Hiscock

A sandbowl snail moves across a long thing leaf against a blurred background od dead vegetation lying on the floor. The snails dark brown- black head is visible poking outside of the shell with antennae raised. The conical shell appears three or four times the length of the head of the snail and gets darker brown-black towards the tip and is a light brown- amber towards the head of the snail.

Sandbowl snail, Niccola Bacciu

1. About

To add (explain molluscs – including other Devon Species of Conservation Concern and more common species – filter feeding – water purification benefits – lack of awareness and appreciation etc). 

Species in this group vary hugely and so pressures and opportunities are discussed for each Focus Species and in the relevant habitat pages. 

Key pressures and opportunities

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Climate change

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Water quality

2. What we need to do and where

Priority

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

Focus species

A sandbowl snail moves across a long thing leaf against a blurred background od dead vegetation lying on the floor. The snails dark brown- black head is visible poking outside of the shell with antennae raised. The conical shell appears three or four times the length of the head of the snail and gets darker brown-black towards the tip and is a light brown- amber towards the head of the snail.

Sandbowl snail

Quickella arenaria
Read more

Read about the sandbowl snail on the sand dune invertebrates page.

Freshwater pearl mussel

Margaritifera margaritifera
Read more

Habitat 

Freshwater pearl mussels live in fast flowing, nutrient poor stream and rivers with clean cobbles and gravels. They are one of the longest living invertebrates, capable of living to over 130 years. They can form mussel beds which provides habitat for other wildlife and, as they are filter feeders, cleans the water.  

To reproduce, females release larvae into the water which attach to the gills of trout or salmon for approximately 9 months. They then drop off and settle in clean gravel beds where they will grow into fully sized mussels, reaching maturity at around 12 years old. As filter feeders they use their gills to catch tiny bits of plants and animals floating in the water.   

Pearl mussels need rivers with high water quality (sediments will clog their gills), stable gravel beds and high oxygen level.  Significant changes in river flow can also be harmful—low flows can dry out gravels and leave them stranded, while floods can wash them into unsuitable areas.  

Population 

Freshwater pearl mussels were once widespread in our rivers. Their pearls have been traded since Roman times, and in the 16th century river bailiffs were employed to ensure all valuable pearls were kept for the king.  However, this historic exploitation for pearls along with a decline in the quality of river habitat has led to huge declines-  90% across their UK? range by the end of the? 20th century. They are now globally threatened.   

In southwest England the only remaining populations are on the lower sections of the Taw and Torridge rivers but in very low and decreasing numbers. These populations have not bred successfully in the wild since the 1960s and so the remaining individual mussels are now over 80 years old.  The Torridge population may be the oldest population in UK, and likely date back 20- to 30,000 years.  

Habitat in the Taw and Torridge is now very degraded. Most cobbles and gravels have been eroded away or are smothered by fine silts. Juvenile mussels were reintroduced in the lower Torridge in 2017-18 through a Devon Wildlife Trust project. However the success of this cannot be confirmed until at least 2027 when the mussels will be big enough to see.  The project developed a ten year strategy but no funding has been available since 2018.  The population will be extinct if no action is taken. 

For a detailed overview please read the Freshwater pearl mussel paper written for the LNRS, see Find out more. 

Actions

Prevent the extinction of freshwater pearl mussels in Devon’s rivers through habitat restoration and a captive breeding programme. 

  • Establish a captive breeding hatchery facility and long term ‘ark’ habitat for adults in order to develop a population which can be re-introduced into Devon’s rivers when habitat is restored.  
  • Build on existing work to identify small catchments for habitat restoration (water quality and gravel/cobble beds) – see Watercourse corridors for related actions. 
  • Monitor existing populations and the reintroduced juvenile mussels. 
  • Find funding for the above and build on the work and learning from the DWT freshwater pearly mussel project. 

Consultation question:  can our old mussels still breed or do we bring in mussels from elsewhere for breeding? 

Detailed actions are set out in the Freshwater pearl mussel profile written for the LNRS, see Find out more.

Where to focus action

Further work is needed to identify catchments where habitat restoration should take place prior to re-introductions. The scale of restoration needed on the Torridge is not achievable. The priority is to focus on the selection of smaller, manageable catchments for prioritisation.  Possible areas include the Upper Torridge, Mole, Bovey and Upper Dart but further research is needed and sites could include rivers in Somerset or Cornwall including the Barle, Upper Tamar (likely the Ottery) or Camel (likely the DeLank). 

Upland rainforest snails

Read more

This group of rare snails live in woodlands in areas with heavy rainfall and cool mild climates. A large percentage of the world population is therefore found in old oak woodlands on the western Atlantic fringes of Europe.  

The snails are largely found in cool, damp areas within woodlands on bark, dead wood or in leaf litter.  In Devon they are largely found in the upland rainforest oak woods on and around Dartmoor and Exmoor.  

Pressures include impacts to this damp habitat due to woodland management (especially loss of dead wood) and climate change.  

Western tree snail

Balea heydeni

The name is disputed, and so the species is also known as Balea sarsii. This species was previously misidentified as the widespread Balea perversa, so has often been overlooked. The snail feeds mostly on lichen and its presence is based primarily on food availability. Most commonly found in trees covered in mosses and lichens, the snail can also be found under bark or in other suitable crevices within the tree.  

English chrysalis snail

Leiostyla anglica

The snail can usually be found attached to twigs or larger branches on the ground and is not known to climb trees or walls. It has a rich, glossy red-brown shell when fresh but this may be oxidised to a dull whitish colour. It has been commonly recorded in hazel woods with undisturbed leaf litter, particularly where native holly is present.

Brown snail

Zenobiella subrufescens

The brown, or dusky, snail can be found along tree trunks (often hazel, beach and oak) during winter and within dry weather they often retreat into the leaf litter and dead leaf bases. It is a species noted for a particularly thin shell.

Plaited snail

Spermodea lamellata

This snail prefers deep leaf litter made up of durable slow to decompose leaves, such as that of beech and oak. It is threatened by warmer, drier summers, loss of broadleaf habitats, and less consistent rainfall. In Devon, this species is now restricted to North Devon. 

Hollowed glass snail

Zonitoides excavatus

The hollowed glass snail is often described as ‘near endemic’ to Britain and Ireland as so much of the world’s population is found here. Devon is a hotspot for the species in southern England. The snail lives predominantly in acid woodlands or in areas of base-poor rocks such as acidic scrub or the margins of peat bogs. It is one of the few calcifuge (lime-hating) molluscs in Britain. 

Actions

Maintain damp areas with lots of deadwood, leaf litter and lichens in Dartmoor and Exmoor oak woodlands  

Where to focus action

All Dartmoor and Exmoor woodlands and Temperate rainforest restoration areas on Dartmoor and Exmoor.  See Mapping 

Spiny cockle

Acanthocardia aculeata
Read more

The spiny cockle is found in the sandy substrates of Devon’s shallow coastal waters where it can burrow into the sand to avoid predation and harsh environmental conditions.  Spiny cockles are filter feeders. They filter plankton and small particles from the water, whilst improving water quality and recycling nutrients.   

They are easily recognisable by its robust, heart-shaped shell adorned with prominent radial ribs and spines. These spines, which give the species its common name, serve as a protective mechanism against predators. The shell’s exterior ranges in colour from pale yellow to brown, often exhibiting darker bands or streaks. The interior of the shell is smooth and typically white or light-coloured.  

Devon boasts a notably higher population of spiny cockles compared to other regions in the UK, with this abundance particularly evident in areas like the Salcombe estuary. Despite their relatively high numbers in Devon, the spiny cockle is considered a nationally rare species.  

Actions

Implement and enforce regulations on cockle harvesting to prevent overfishing and ensure sustainable uses of the resource.

Establishing seasonal closures during breeding periods to allow populations to reproduce and recover. 

Where to focus action

All estuarine environments, in particular, intertidal mudflats and sandflats. See Mapping.

The tiny slug sits on a someones palm in a zoomed in photo emphasising its small size. The slug is facing away from the camera, meaning the all black bodies ribbed texture is visble as well as two small antennae on its head.

Celtic sea slug

Onchidella celtica
Read more

The Celtic sea slug is a small, air-breathing mollusc found on north Devon’s rocky shores from April to November, especially in areas like Woolacombe, Saunton, and Croyde. It shelters in crevices and among barnacles and mussels, emerging at low tide to graze on microscopic algae, diatoms, and detritus. It’s often seen near whelk eggs, feeding on algae growing around the cases. 

Dark green-black and well-camouflaged, the slug’s body is oval and fleshy, up to 13 mm long, and covered in coarse, evenly spaced rounded bumps that help it blend into its surroundings. 

Unlike typical marine sea slugs that breathe through gills, the Celtic sea slug breathes air through a lung-like cavity. That’s because it evolved not from a gilled marine ancestor, but from a land-dwelling or amphibious slug, making it uniquely suited to life in the intertidal zone. This adaptation allows it to survive long periods out of water during low tide, a key to thriving in north Devon’s rugged coastal habitats. 

Though small and often overlooked, Onchidella celtica plays an important ecological role and is a sign of a healthy, balanced shoreline ecosystem. 

Actions

Maintain healthy unpolluted rocky shores along Devon’s north coast – see Coast. 

Where to focus actions

All estuarine and rocky shore environments on the north Devon coast. 

A native oyster is photographed on a grey rock with yellow lichen like plant matter growing sporadically around its edges. The oyster itself is a rounded semi-circle shape and light grey in colour with a hard matte shell.

Native oyster

Ostrea edulis)
Read more

Native oysters start life as males and can change sex based on their environment. They reach maturity between eight and ten months old and can live up to six years, with some living as long as 15 years. They feed by filtering water to extract nutrients; a single adult native oyster can filter over 140 litres of water each day. 

They live in estuaries and shallow coastal waters with mud, rocks, sandy mud, and gravel and can be found from the low tide line down to about 80 meters deep. Native oyster reefs (to explain in a bit more detail or link to Coast) provide a valuable refuge for juvenile fish.  

The invasive non-native pacific oyster (Magallana gigas)  compete with native oysters for space and food and have resulted in a decline in native oyster populations, which are already under threat. Controlling the spread of Pacific oysters is challenging and resource intensive. Efforts to manage their populations include culling and monitoring, but these measures require significant effort and coordination. They are also threatened by diseases such as Bonamia ostreae (needs explaining)   

Consultation question: are they found in all suitable intertidal Devon habitat? 

Actions

Maintain healthy unpolluted intertidal habitats. See Coast for more detailed actions

Monitoring and managing diseases that impact on native oysters through regular health checks and implementing biosecurity measures to prevent the spread of diseases. 

See Invasives for actions relating to Pacific oysters 

Where to focus action

All estuarine and rocky shore environments, in particular, intertidal mudflats and sandflats.  The Environment Agency has identified areas where oyster reefs could potentially be restored.  See Mapping.     

Actions for all molluscs

Habitat management 

Molluscs are found in most habitats across Devon and will benefit from actions outlined in the habitats pages. See Habitats for details. 

Anyone who manages habitat with rare molluscs should be aware of their needs. For more information see the list of Devon Species of Conservation Concern and see links in Find out More below. 

Awareness and surveys 

  • Increase awareness and appreciation of molluscs. 
  • Increase research , survey and monitoring to better understand Devon’s mollusc populations. 

3. Inspiration

Sections below to be finalised through Consultation 

Case studies

Devon Wildlife Trust’s freshwater pearl mussel project (2014-18)

Lessons were learnt from this relatively low budget project and recommendations can be found in the final report (Unpublished: Report on Short-term Captive Breeding of the Torridge Freshwater Pearl Mussel 2016 to 2018 V2) for any future endeavours of this approach.   

Freshwater pearl mussel project | Devon Wildlife Trust 

Devon and Severn IFCA Stock Assessments 

The Devon and Severn Inshore Fisheries and Conservation Authority (IFCA) conducts regular stock assessments for various marine species, including intertidal mussel and cockle beds. Stock assessments in areas like the Exe Estuary and Taw-Torridge Estuary help monitor the health and size of mussel populations. This information is crucial for maintaining sustainable harvesting practices and ensuring the long-term viability of mussel beds. 

Molluscan Research in D&S IFCA’s District | Devon & Severn IFCA | Inshore Fisheries and Conservation Authority 

Where to visit

Always follow the Countryside Code and stick to footpaths and sites which are managed for public access.  

Sites which are accessible to the public and are good places to see lots of molluscs include:

Salcombe Estuary 

Nestled in South Devon’s National Landscape, the Salcombe Estuary is a rich and diverse habitat teeming with marine life. Its sheltered waters, mudflats, rocky shores, and sandy beaches create ideal conditions for a wide variety of molluscs. From colourful sea slugs to grazing limpets, the estuary offers a unique opportunity to observe these fascinating creatures up close. 

For information on these and other sites which are open to the public please see the Explore Devon website.    

4. Find out more

To be finished through Consultation. 

Welcome | The Conchological Society of Great Britain and Ireland  

Freshwater pearl mussel project | Devon Wildlife Trust 

Molluscan Research in D&S IFCA’s District | Devon & Severn IFCA | Inshore Fisheries and Conservation Authority 

Molluscs and their shells are very varied and beautiful….. – Coastwise North Devon 

Freshwater pearl mussel full expert profile.docx

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