Around 65 species of crab occur in the waters of the British Isles, yet many beachgoers consistently misidentify the most common ones they encounter.
You’re not alone if you’ve confused a velvet swimming crab for a shore crab or missed spotting hermit crabs hiding in plain sight.
Understanding the types of crabs in the UK and avoiding these identification pitfalls will transform your next coastal adventure into a successful marine discovery expedition.
This guide reveals the most frequent mistakes people make when identifying British crab species and provides clear identification tips to help you spot these fascinating creatures correctly during your next beach exploration.
1. Brown Crab
Also called edible crab, the brown crab (Cancer pagurus) sports a large, oval, reddish-brown shell that many beachgoers mistake for other species.
You’ll often confuse juvenile brown crabs with shore crabs due to their similar coloring, but the brown crab’s distinctive “pie-crust” scalloped shell edge sets it apart.
These substantial crabs can reach up to 25cm across and prefer rocky substrates where they hunt for mollusks and worms.
The biggest identification mistake involves assuming all large, brown-colored crabs are the same species. Brown crabs have notably robust claws – the right claw is typically larger for crushing, while the left is more pointed for cutting.
Marine life identification guides emphasize checking for the characteristic oval carapace shape rather than relying solely on color.
Pro Tip: Look for the distinctive scalloped edge around the shell – this “pie-crust” pattern is unique to brown crabs and won’t be found on similar-sized species.
2. Velvet Swimming Crab
The velvet swimming crab (Necora puber) is aggressive and features striking red eyes with a distinctly velvety shell texture.
Many rock poolers make the mistake of avoiding these crabs entirely due to their intimidating appearance and defensive posture.
Their flattened rear legs act like paddles, making them excellent swimmers – a feature that distinguishes them from most other British crab species.
You’ll typically find these crabs in deeper rock pools and under large stones during low tide. Their aggressive nature isn’t necessarily dangerous to humans, but they will readily defend themselves when cornered.
The velvety texture of their dark carapace, combined with those unmistakable bright red eyes, makes proper identification straightforward once you know what to look for.
Rock pooling safety guides recommend observing these fascinating creatures from a respectful distance rather than attempting to handle them directly.
Key Takeaway: The red eyes and velvety shell texture are diagnostic features – no other common UK crab species shares both characteristics.
3. Spider Crab
The spider crab (Maja brachydactyla) possesses long, spindly legs and a distinctly spiny carapace that often confuses observers.
The most common mistake involves missing these crabs entirely due to their remarkable camouflaging abilities.
Spider crabs actively decorate their shells with algae, sponges, and other marine growth, making them nearly invisible against rocky backgrounds.
These fascinating creatures can have leg spans reaching up to 50cm, yet their bodies remain relatively small compared to their impressive limb length.
Their triangular carapace and pronounced rostrum (pointed projection) between the eyes help distinguish them from other large crab species when the decoration is removed or minimal.
Young spider crabs are particularly challenging to spot as they’re more heavily decorated than adults. Marine conservation websites describe their molting process, during which they temporarily lose their camouflage and become more visible to observers.
| Feature | Spider Crab | Brown Crab | Shore Crab |
|---|---|---|---|
| Leg length | Very long | Medium | Short |
| Carapace shape | Triangular | Oval | Square-ish |
| Decoration behavior | Yes | No | No |
| Size range | Large | Large | Small-medium |
4. Shore Crab
The shore crab (Carcinus maenas) represents the most common intertidal species, typically greenish in color but highly variable in appearance.
This variability causes the biggest identification confusion – shore crabs can range from bright green to dark brown, red, or even purple depending on their environment and molt cycle. You’ll encounter these adaptable crabs in virtually every suitable habitat along the British coast.
Their square-shaped carapace with five pointed projections on each side provides the most reliable identification feature.
Shore crabs demonstrate remarkable tolerance for varying salinity levels, allowing them to colonize estuarine environments where other species cannot survive.
This adaptability has made them one of the world’s most successful invasive species in regions beyond their native range.
Coastal ecology studies highlight their ecological importance as both predator and prey species in British marine ecosystems.
Common Mistake: Don’t rely on color alone for identification – the carapace shape and side projections are far more reliable than the highly variable coloration.
5. Norwegian King Crab
The Norwegian king crab (Paralithodes camtschaticus) remains rare in UK waters, being more common in northern waters around Scotland and beyond.
Many rock poolers make the mistake of assuming any large, spiny crab must be a king crab, when they’re more likely observing spider crabs or large brown crabs.
True king crabs have an asymmetrical abdomen tucked under their body and only three visible pairs of walking legs – the fourth pair is reduced and hidden.
These impressive crustaceans can reach enormous sizes, with leg spans exceeding one meter in mature specimens.
However, encounters with actual king crabs in British waters remain extremely rare, with most sightings occurring in deeper offshore waters rather than accessible tide pools.
Fisheries research data indicates that while populations exist in northern European waters, they rarely venture into the shallow coastal areas where most recreational rock pooling occurs.
6. Masked Crab
The masked crab (Corystes cassivelaunus) features distinctively long antennae and a mask-like pattern on its elongated shell.
Most beachgoers completely overlook this species because it spends daylight hours buried in sand with only its antennae visible.
The antennae form a breathing tube that allows the crab to remain hidden while still accessing oxygenated water above the sediment surface.
You’re most likely to encounter masked crabs during night-time beach walks or when they’re disturbed from their sandy hideouts during extreme low tides.
Their elongated carapace and disproportionately long antennae make identification straightforward when they are visible, but their secretive nature means many coastal visitors never observe them.
The mask-like facial pattern that gives this species its common name becomes clearly visible when viewed from the front, resembling a theatrical mask or tribal face painting.
Pro Tip: Look for pairs of long antennae protruding from sandy areas in shallow water – this often indicates a buried masked crab beneath.
7. Pea Crab
The pea crab (Pinnotheres pisum) represents one of the tiniest crab species you’ll encounter, living inside mussels and other bivalves.
The biggest mistake involves assuming these minuscule crabs are juvenile stages of larger species rather than recognizing them as fully mature adults.
Female pea crabs remain inside their host shellfish throughout their adult lives, while males venture out to find mates.
These remarkable crabs measure only 6-12mm across and have soft, translucent shells that allow them to squeeze into their bivalve homes.
When you’re collecting mussels or examining opened shells, you might discover these tiny inhabitants nestled within the mantle cavity of their hosts.
Marine symbiosis research describes their fascinating lifecycle and relationship with host species, noting that they feed on food particles filtered by their bivalve hosts without significantly harming them.
Key Features of Tiny UK Crabs:
- Pea crabs: Live inside shells, soft carapace
- Young shore crabs: Hard shell, found under rocks
- Young brown crabs: Scalloped shell edge, harder carapace
- Juvenile spider crabs: Spiny shell, longer legs
8. Edwards’ Hermit Crab
Edwards’ hermit crab (Pagurus prideaux) commonly inhabits rocky shores throughout Britain, often living in partnership with sea anemones.
The most frequent identification mistake involves confusing this species with the common hermit crab due to their similar habitats and shell-dwelling lifestyle.
Edwards’ hermit crabs typically carry Calliactis sea anemones on their adopted shells, creating a distinctive symbiotic partnership that helps with identification.
These fascinating creatures demonstrate complex behaviors when selecting new shells as they grow, often forming “chains” where multiple crabs line up to exchange shells in size order.
The sea anemone partnership provides protection for the crab while giving the anemone mobility and access to food scraps.
You’ll recognize Edwards’ hermit crabs by their reddish coloration and the presence of sea anemones on their shells, though not all individuals carry anemone partners.
9. Common Hermit Crab
The common hermit crab (Pagurus bernhardus) represents the most widespread hermit species, typically living in whelk shells and other large gastropod homes.
Many observers make the mistake of trying to identify hermit crabs by their shell choice rather than examining the crab itself.
The common hermit crab’s left claw is notably larger than the right, and their eyestalks are longer than their antennae – key features that distinguish them from Edwards’ hermit crabs.
These adaptable crabs inhabit various shell types depending on availability, but show strong preferences for whelk shells when possible.
Their scavenging behavior makes them important cleanup crew members in marine ecosystems, consuming dead and decaying organic matter.
Marine behavioral studies reveal complex decision-making processes involved in shell selection and exchange behaviors.
Identification Table: Hermit Crab Species
| Species | Left Claw | Eyestalk Length | Common Partners | Shell Preference |
|---|---|---|---|---|
| Common Hermit | Larger | Longer than antennae | None typical | Whelk shells |
| Edwards’ Hermit | Similar size | Shorter than antennae | Sea anemones | Various shells |
10. Long-clawed Porcelain Crab
The long-clawed porcelain crab (Pisidia longicornis) appears small and flat with distinctively long antennae that many mistake for legs.
These delicate-looking crabs aren’t true crabs at all but belong to a group called anomurans, more closely related to hermit crabs than to typical crabs.
Their extremely long antennae and flattened body shape help distinguish them from other small crab species found in similar habitats.
You’ll find these fascinating creatures under stones in the mid to low tide zones, where they filter-feed using specialized feeding appendages.
Their ability to autotomize (deliberately shed) limbs when threatened often surprises observers who witness this defensive behavior.
Despite their fragile appearance, porcelain crabs are remarkably well-adapted to their rocky shore environment, using their flattened profile to squeeze into narrow crevices where predators cannot follow.
Common Mistake: Don’t confuse the long antennae for additional legs – porcelain crabs have the same number of walking legs as other crab species, but their antennae are proportionally much longer.
11. Broad-clawed Porcelain Crab
The broad-clawed porcelain crab (Porcellana platycheles) lives flat under stones on the lower shore, distinguished from its long-clawed relative by broader claws and shorter antennae.
Many rock poolers overlook these cryptic creatures entirely because they remain motionless when their stone shelters are lifted, relying on camouflage rather than escape behaviors.
These specialized filter-feeders use their broad, flattened claws to create feeding currents that bring suspended particles within reach of their feeding appendages.
Their extremely flattened body profile allows them to inhabit spaces that would be inaccessible to other crab species of similar size.
The broad-clawed species typically inhabits lower shore positions than its long-clawed relative, requiring more consistent submersion for successful feeding. Rocky shore ecology guides describe their role in lower intertidal communities and feeding strategies.
Pro Tip: Look for the broader, more robust claws and proportionally shorter antennae to distinguish this species from the long-clawed porcelain crab – both may be found under the same rocks.
Understanding these common identification mistakes will dramatically improve your success rate when exploring British coastlines.
Each species has evolved specific adaptations for their particular niche, from the brown crab’s powerful crushing claws to the masked crab’s ingenious breathing tube system.
Next time you’re exploring rock pools or sandy shores, take time to observe these remarkable adaptations rather than rushing through your coastal discoveries.
The key to successful crab identification lies in observing multiple characteristics rather than relying on single features like color or size, which can vary significantly within species.
With practice and patience, you’ll soon develop the skills needed to accurately identify the fascinating types of crabs in the UK during your marine adventures.
