Spider crabs Cornwall are one of the most fascinating marine species you can witness along the Cornish coast. Each year, these impressive crustaceans appear in large numbers, offering wildlife enthusiasts, photographers, and families a unique chance to see them in their natural habitat. From their enormous size to their extraordinary moulting behaviour, spotting spider crabs in Cornwall is an unforgettable experience.
What Are Spider Crabs?
Spider crabs (Maja brachydactyla) are marine animals known by many names, including the European spider crab, Cornish king crab, and the common or spiny spider crab. This species is the largest spider crab found in the British Isles and is a migratory animal. Spider crabs are invertebrate crustaceans that belong to the phylum Arthropoda and are part of a larger crustacean group that includes animals such as crabs, lobsters, shrimps, crayfish, barnacles, and even woodlice, all of which share similar body structures and characteristics. Spider crabs live up to their name with their long arachnid-looking legs and distinctive spiny shells, often found washed up on beaches.
Physical Characteristics, Defining Features and Identification
Spider crabs are large marine crustaceans that can grow up to 20–25 cm in body length, with a leg span reaching around 80 cm. They are typically orange to brownish-red in colour and are easily recognised by their long, spindly, spider-like legs and oval, raised shell, known as the carapace.
The carapace is covered in many short, sharp spines for protection and is often coated in algae, giving the crab a green, hairy appearance that helps it blend into its surroundings. At the front of the shell, between the eyes, is a short, strong, forked rostrum with two diverging points that may spread outward or lie almost parallel.
Male spider crabs are usually larger than females and have noticeably bigger claws. Their claws are equal in size and, together with the strong walking legs, feature small spines along each segment.
Spider Crab Habitat & Where They Live
Deep-Sea Environments
Spider crabs are mainly found along the south and west coasts of England, Wales, and Ireland, with their range thought to extend further north toward Scotland. Spider crabs in Cornwall are especially common around the entire coastline, particularly along the north and west coasts of Cornwall.
These crabs inhabit both rocky and sandy marine environments, living from the low-water mark down to depths of around 50–80 metres, often favouring areas rich in seaweed or kelp that provide food and natural camouflage. As juveniles, spider crabs typically remain in shallow waters, but once they moult and reach maturity, adults begin seasonal migrations.
In autumn, they move from shallow coastal areas into deeper waters, sometimes travelling more than 100 miles, and may reach depths of 120–150 metres during the winter months, primarily for mating.
Although they are well known for forming large gatherings in shallow water, this lifecycle migration to deeper habitats is a key part of their annual behaviour.
Spider Crabs in Cornish Waters
Spider crabs are a non-threatened and relatively common species, with the British Isles representing the northern edge of their natural distribution.
Spider crabs in Cornwall are widespread along the entire coastline, with especially strong populations on the north and west coasts, and are frequently recorded around locations such as Falmouth, St Ives, Newquay, and St Austell Bay.
Spider crabs can be found throughout the year, but they undergo large seasonal migrations, moving into shallow coastal waters during the summer and returning to deeper offshore areas in winter. In Cornwall, spider crabs are most commonly observed between July and September, when these migrations bring them closer to shore.
Catching Spider Crabs in Cornwall
Spider crabs in Cornwall are best caught around rocky reefs, kelp beds, harbour walls, and sandy seabeds where there’s structure and slightly deeper water close to shore. The most effective method is using baited crab pots set on the seabed and left for a few hours, but they can also be caught from piers or rocks using crab lines with oily fish bait like mackerel.
They’re most commonly targeted from spring through early autumn, when they move closer inshore, and calm conditions with a steady tide tend to give the best results. If you’re fishing recreationally, rules set by Cornwall Inshore Fisheries and Conservation Authority limit you to five shellfish per day (from the combined group of lobster, crawfish, edible crab, and spider crab), and spider crabs must measure at least 130mm across the carapace before you can keep them – so always check the latest local regulations before heading out.
Spider Crab Behaviour & Diet
Feeding Habits & Scavenging
Spider crabs are slow-moving, bottom-dwelling crustaceans that spend most of their time crawling across rocky or coarse sandy seabeds, where they rely on camouflage and limited movement to avoid predators.
They are considered opportunistic feeders, showing flexible behaviour depending on food availability and habitat conditions. Like many majid crabs, they are primarily omnivorous predators and scavengers, feeding on a wide range of benthic organisms as well as dead organic material. Their diet commonly includes molluscs, small crustaceans, worms, echinoderms, algae, and carrion found on the seabed.
Rather than actively hunting fast prey, spider crabs often use their claws to pick, tear, or crush slow or sessile organisms and to scavenge dead animals, making them important recyclers of organic matter in coastal ecosystems.
Feeding typically occurs while they slowly search along the seabed, using sensory organs to detect food. This opportunistic scavenging behaviour allows them to adapt to seasonal changes in prey abundance around the UK coastline, particularly during their summer migrations into shallow Cornish waters, where food availability is higher.
Movement & Lifestyle
Spider crabs around Cornwall have a multi-stage life cycle that ties closely to seasonal changes in their environment. After hatching, larvae spend a short planktonic phase in the open water (about 2–3 weeks), passing through several larval stages before settling to the seabed and becoming juvenile crabs.
These juveniles grow in shallow, sheltered habitats where they feed and develop over the first couple of years before reaching sexual maturity, typically around 1–2 years old. Like many other majid crabs, spider crabs undergo a terminal moult at maturity, after which they no longer grow significantly and focus their energy on reproduction.
Adults then participate in seasonal migrations between deeper offshore waters (often in winter) and shallower coastal areas in summer. During these summer months, they mate and frequently gather in large groups to moult their old shells, a vulnerable time that drives conspicuous aggregations in Cornish shallows that are now observed annually.
This lifecycle – from planktonic larvae to benthic juvenile, growth to maturity, seasonal migration, and reproduction – typically spans several years and reflects adaptations to UK coastal conditions
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8 Fascinating Facts About Spider Crabs
- Spider Crabs in Cornwall have a typical lifespan of 5-8 years; however, some species of spider crab, such as the giant or Japanese spider crab, have been known to live to 100 years old.
- During the summer, thousands of spider crabs swarm Cornish waters
- Covered in green algae as ‘hairy camouflage.’
- After molting, spider crab bodies are soft, making them vulnerable, hiding until their new shells harden.
- Not picky-eaters, spider crabs will eat even jellyfish
- Because they have poor eyesight and move slowly, they rely on taste and sensing organs located on their legs to identify food
- Once ignored by fishermen as a nuisance, they are now a significant, sustainable, and economically important catch for the UK and French seafood markets
- Spider crabs have a very low vulnerability to fishing – 12%” meaning this species is considered to have a relatively low risk of population decline due to fishing pressure
Types of Spider Crabs
Spiny Spider Crab (Maja brachydactyla)
Often called the common spider crab, this is the largest species, with a carapace up to 20cm in length. They are common in the South West and frequently found inshore during summer to moult and mate.
Great Spider Crab (Hyas araneus)
Also known as the sea toad, this species is smaller with a pear-shaped, tubercle-covered shell. It is found around all UK coasts, particularly in cooler northern waters.
Long-legged Spider Crab (Macropodia rostrata)
Small, delicate, and highly camouflaged creatures found in rockpools and shallow waters.
Slender Spider Crab (Macropodia tenuirostris)
A specific, smaller, and very long-legged species within the Macropodia genus.
Giant Spider Crabs
The Giant or Japanese spider crabs are the largest arthropods in the world, famous for their enormous leg span that can reach up to 12 feet (3.7 meters) from claw to claw. Native to the waters around Japan, these deep-sea creatures live at depths of 150 to 1,000 feet, where they slowly crawl along the ocean floor.
Despite their intimidating size and long, spindly legs, they are generally gentle scavengers, feeding on dead animals, shellfish, and plant material. Their tough, spiny shells help protect them from predators, and they can live for decades in the cold, dark depths of the Pacific Ocean.
Other Notable Crabb Species Found in UK Waters
- Edible crab (Cancer pagurus) – Also called the brown crab, this is one of the largest and most commercially important crabs in the UK. It has a broad, reddish-brown shell and powerful black-tipped claws.
- European green crab (Carcinus maenas) – A small but highly adaptable shore crab, usually green or brown. It’s common in rock pools and estuaries and is known globally as an invasive species outside its native range.
- Velvet swimming crab (Necora puber) – Recognizable by its bright red eyes and blue-tinted legs, this aggressive and fast-moving crab is often found under rocks along rocky coasts.
- Hermit crab (Pagurus bernhardus) – While not a “true crab,” this species is very common in UK waters and is known for living inside discarded snail shells for protection.
Spider Crabs in Cornish Marine Life
Cornwall and UK populations of the spider crab play several important roles in marine ecosystems because they act as both predators and scavengers on the seabed.
They are mainly benthic (bottom-living) animals that feed on a wide range of organisms such as molluscs, worms, small crustaceans, algae, and dead animals, which makes them key recyclers of organic material. By consuming carrion and decaying matter, spider crabs play a crucial role in breaking down waste and returning nutrients to the ecosystem, thereby supporting overall seabed health and nutrient cycling.
Spider crabs also help regulate populations of smaller benthic invertebrates. Because they are opportunistic feeders, they influence the balance between species living on rocky and sandy seabeds, preventing any one prey group from becoming overly dominant.
At the same time, they are an important food source for larger predators such as fish and other marine animals, meaning they form a key link in the coastal food web. Their seasonal migrations between deeper offshore waters and shallower summer habitats further help transfer energy and nutrients between different parts of the marine ecosystem.
Why Spider Crabs Matter to the Region
Spider crabs in Cornwall play a vital role in the area’s coastal ecosystem. As natural cleaners of the seabed, they scavenge dead material and help reduce disease risk, while also linking the food web by transferring energy from small invertebrates and detritus to larger predators. By feeding on a variety of species, they help maintain biodiversity and ecological balance, and their presence also supports local coastal fisheries, making healthy spider crab populations important for both marine life and the region’s livelihoods.
Conclusion
Understanding spider crabs in Cornwall is an opportunity to connect with the region’s unique coastal ecosystem and understand the vital role these crabs play in maintaining biodiversity. Whether you’re a wildlife enthusiast, a photographer, or a family looking for a memorable outdoor experience, witnessing spider crabs in their natural habitat is an unforgettable adventure. With Padstow Sealife Safaris, you can enjoy this incredible spectacle safely and responsibly, while supporting local conservation efforts that keep Cornwall’s marine life thriving.
Frequently Asked Questions
Are spider crabs dangerous?
No. Spider crabs are not aggressive toward humans. They may raise their claws if disturbed, but they are generally slow-moving and defensive rather than dangerous.
Why do spider crabs gather in huge groups?
During late spring and summer, they migrate into shallow waters to moult and mate. They often form large aggregations for protection during moulting, when their new shells are soft, and they are vulnerable to predators.
Where are spider crabs found in the UK?
They are mainly found along the south and west coasts of England, parts of Wales, and around Cornwall. The British Isles mark the northern edge of their distribution. They are mostly absent from the North Sea and northern Scotland.
When is the best time to see spider crabs in Cornwall?
Between July and September, they migrate into shallower coastal waters.
How long do spider crabs live?
They typically live for several years. They reach maturity in about 1–2 years and undergo a terminal moult, after which they stop growing.
Can you eat spider crabs?
Yes. They are commercially fished in the UK and are considered good to eat, although they are less commonly sold than brown crab.
Why do spider crabs look “hairy”?
Their shells often collect algae, seaweed, and small organisms. Some species of spider crab even deliberately decorate themselves for camouflage.
What eats spider crabs?
Large fish, octopus, and sometimes humans. They are an important part of coastal food webs.
Are spider crabs endangered?
No. They are considered a common, non-threatened species in UK waters and have very low vulnerability to fishing compared to many marine species.
