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Intertidal invertebrates

Over half of Devon’s wildlife lives in the sea.

Intertidal invertebrates are some of the oldest forms of life on Earth.

A single mussel can filter over 200 litres of seawater per day.

Glaucous pimplet - Keith Hiscock

Glaucous pimplet, Keith Hiscock

St. John's jellyfish, Eleanor Goodall

St. John’s jellyfish, Eleanor Goodall

Snakelocks anemone

Snakelocks anemone

1. About

Intertidal invertebrates are the soft-bodied and shelled animals that thrive along our estuaries and coastlines. This diverse group includes familiar species like crabs, starfish and sea anemones as well as lesser-known creatures that hide in rock pools and that can be found scattered along Devon’s sandy shores.

Bright orange sponges cling to the undersides of rocks, while translucent, globular sea squirts glow from shadowy crevices. Delicate and charismatic sea slugs glide through seaweed, grazing on whatever they can find. Shells appear in every shape and size, from tiny hydrobia that are just a few millimetres long and are vital for wintering birds, to dense beds of mussels and oysters that form rich habitats teeming with marine life.

Important habitats for these species include: 

Rocky shores provide a mosaic of microhabitats for intertidal invertebrates. Crevices, overhangs and shaded rock pools offer shelter from desiccation and predators. Barnacles, limpets, chitons and sea slugs are commonly found clinging to rocks or grazing on algae, while sponges and tube worms anchor themselves in more protected spots.

Though seemingly barren, sandy shores host burrowing invertebrates such as lugworms, razor clams, and sand hoppers. These species play a vital role in sediment turnover and nutrient cycling. The shifting substrate also supports mobile scavengers and filter feeders that emerge when the tide changes.

In sheltered estuarine zones, dense growths of seagrass and seaweed provide excellent cover and feeding grounds. For example, unlike their free-swimming relatives, stalked jellyfish have a stalk they use to attach themselves to seaweed or seagrass.

The LNRS has identified 15 intertidal invertebrates in Devon as Species of Conservation Concern. Five molluscs, two worms (annelids) and eight species of cnidaria (sounds like nih-DARE-ee-uh). Cnidarians are a surprisingly diverse group, with around 12,000 known species; three times more than mammals. They include familiar creatures like jellyfish, corals and sea anemones as well as lesser-known species such as hydroids.

Key pressures and opportunities

Building sea defences, harbours and housing near the coast can reduce habitat, especially in intertidal zones. It disrupts the natural structures, reducing the availability of crevices, pools and sediment types that intertidal invertebrates rely on. 

Runoff from the land can have serious impacts on coastal ecosystems. Sediment is especially harmful, as it can smother species, particularly those with delicate structures such as gills. Runoff often carries excess nutrients, leading to eutrophication and harmful algal blooms, and can degrade water quality. Other pressures include sewage discharge, marine litter, and chemical pollutants from industry, all of which pose risks to intertidal and marine life.

Climate change is having multiple impacts on our coastal and intertidal environments.

Rising sea temperatures and ocean acidification are changing species distributions and disrupting reproductive cycles. For example, cnidarians such as corals and sea anemones may experience stress or bleaching. Coastal erosion is accelerating, removing or smothering important habitats. At the same time, more frequent storms and heavier rainfall are increasing storm overflows and surface runoff. This causes higher levels of nutrients and pollutants to enter our rivers, estuaries and intertidal areas, further reducing water quality.

Rockpooling, trampling and boating can physically damage fragile habitats. Even well-meaning visitors can unintentionally dislodge sea anemones from their footing.

Foraging has become increasingly popular, but it remains unregulated and there is limited understanding of its ecological impact.

Invasive species, such as Sargassum muticum, are often introduced through ballast water or aquaculture. Once established, they can smother the areas where they grow, blocking light, preventing nutrients from reaching native invertebrates and disrupting the delicate balance of intertidal ecosystems.

2. What we need to do and where

Priority

Increase the abundance and distribution of all intertidal invertebrates, particularly those identified as Devon Special Species.  

See Find out more for links to more detailed information on this group including management advice and records. 

Devon Special Species

Glaucous pimplet - Keith Hiscock

Glaucous pimplet

Anthopleura thallia

The glaucous pimplet is a small but distinctive species of sea anemone found on wave-exposed rocky shores on the north coast of Devon, such as Lee Bay, Croyde and Westward Ho! Devon hosts one of the highest populations of this species in the UK, so is a key region for its conservation and study. 

It is typically found in shallow rock pools, crevices and areas among dense mussel beds, often partially buried under gravel or sediment. Its trumpet-shaped body is dotted with sticky bumps, also known as verrucae, that trap sand and bits of debris, helping it blend into its surroundings. 

It can be green, brown or greyish with up to 100 retractable tentacles that are a similar colour or subtly patterned. Its tentacles are arranged in whorls around its mouth and have small outgrowths called acrorhagi that are armed with stinging cells it uses for defence. Although not widely distributed, it can be locally abundant and is thought to reproduce asexually, forming groups of genetically identical individuals. 

As the glaucous pimplet can be found in mussel beds it is vulnerable to trampling and accidental removal. It is also highly sensitive to fine sediments, which can clog its feeding structures and reduce the availability of oxygen. Any decline in mussel populations, whether from harvesting or environmental stress, can further threaten the pimplet by reducing its habitat.

Stalked jellyfish

Stalked jellyfish are a rare and unusual type of jellyfish that live in shallow coastal waters. Unlike their free-floating relatives, they use a stalk to attach themselves to seaweed, seagrass, or rocks. They are small (usually under 5 cm), often beautifully patterned, and can be hard to spot due to their camouflage and preference for sheltered habitats like rockpools and estuarine lagoons.

There are ten species recorded in the UK, with four more commonly found in Devon. Each species has a central bell-shaped body with eight arms, each ending in clusters of tentacles. These tentacles are often tipped with stinging cells used to catch tiny prey such as plankton. Their presence is a strong indicator of clean, undisturbed intertidal habitats.

Spotted kaleidoscope jellyfish

Haliclystus octoradiatus

This species is named for its vivid appearance: a bell-shaped body patterned with greens, browns, or oranges, and distinctive white spots on its tentacles. It is typically found in rockpools, estuarine lagoons, and seagrass beds in Devon.

Kaleidoscope jellyfish

Haliclystus auricula

This species closely resembles the spotted kaleidoscope jellyfish, though it features a sleeker, spot-free appearance. What sets it apart are the small, rounded lobes nestled between its arms. Its coloration is equally vivid, often displaying shades of green, brown, or red.

St. John's jellyfish, Eleanor Goodall

St John’s jellyfish

Calvadosia cruxmelitensis

This jellyfish has a short stalk and is often found attached to red seaweeds on sheltered rocky shores. Its tentacles are arranged in a cross-like pattern and it is often seen attached to red seaweeds.

Calvadosia campanulata - Keith Hiscock

A stalked jellyfish

Calvadosia campanulata

This species has a smooth, bell-shaped body and lacks the cross-shaped tentacle arrangement of Calvadosia cruxmelitensis. It is less vividly coloured than other species but still plays a role in indicating healthy intertidal ecosystems.

Celtic sea-slug traversing empty dog whelk egg cases (August 2002) - Peter Barfield

Intertidal molluscs

Spiny cockle, celtic sea slug and native oyster are listed as Devon Special Species. For more information on intertidal molluscs see the Molluscs page.

Actions for all intertidal invertebrates

Habitat management and creation

Intertidal invertebrates are found across Devon’s coastlines and will benefit from the actions set out on the Coast page. Anyone who is managing or restoring habitat with rare species should be aware of their needs.

Reduce impacts to sensitive habitats 

  • Identify and map habitats where sensitive intertidal invertebrates, such as the glaucous pimplet, are known to occur, particularly wave-exposed rocky shores with gravelly rock pools. Include these habitats in local marine conservation designations and shoreline management plans.
  • Use field surveys and modelling to understand how sediment affects intertidal invertebrates.
  • Promote land management practices that limit erosion, such as buffer strips, catchment-sensitive farming, and sustainable drainage systems (SuDS).
  • Work with local authorities, developers, and landowners to ensure coastal infrastructure and planning decisions support sediment control and habitat protection.
  • Use signage, codes of conduct and public education to promote responsible rockpooling and coastal recreation in areas with vulnerable species. 
  • Collaborate with local tourism operators to reduce trampling and disturbance in sensitive intertidal zones.

Research and monitoring 

  • Fund research into reef resilience, the impact of climate change and potential restoration techniques. 
  • Explore pilot projects to restore reefs in degraded areas using natural or artificial substrates. 

3. Inspiration

This section will be finalised following public consultation. Please share any case studies and places to visit.

Case studies

Wembury Marine Centre (Devon Wildlife Trust)

Wembury Marine Centre runs guided rock pool safaris and educational activities that help people identify and understand the importance of intertidal species like sea anemones and stalked jellyfish. The centre builds community support for marine conservation and encourages responsible shoreline behaviour.

Where to visit

Always follow the Countryside Code and keep to footpaths and sites that are managed for public access.

Wembury 

This rocky shoreline along Devon’s stunning south coast is one of the best places in the UK to discover the hidden wonders of marine life. At low tide the rock pools come alive with colourful sea anemones, darting shrimp, feathery worms and even the occasional starfish or sea urchin. 

Lee Bay, Ilfracombe

Lee Bay’s peaceful setting and rich biodiversity make it an ideal spot for rock pooling, nature walks and discovering the fascinating invertebrates that live here.

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

4. Find out more

 This section will be tidied up and finalised following public consultation. Please share any links.

Marine Invertebrates – Field Studies Council 

Beneath the waves | Devon Wildlife Trust 

Home | Wembury Marine Centre 

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