On a cool morning off Dinard, in northern Brittany, a trawler’s nets came back heavier than usual. Among the brown and red shapes of the catch, one animal shone an unexpected bright blue, leaving even a veteran sailor momentarily speechless.
A routine trip that turned into a surprise
For 25 years, Breton fisherman Franck Le Doussal has worked the waters off Dinard, on France’s Emerald Coast. He knows these fishing grounds at all seasons, in all weather. So when he says he has never seen something before, locals listen.
As he checked his nets that day, one crab stood out. It was a sea spider crab, a common species in the region, but its shell glowed a striking turquoise instead of the usual spiky red-brown. The crab weighed around 1.5 kg, making it a good-sized specimen, but the colour was what made it extraordinary.
After a quarter of a century at sea, the fisherman came face to face with a creature so rare he doubted his own eyes.
Speaking to regional media, Franck admitted his astonishment, stressing that he had never seen anything like this in his entire career. The catch quickly spread through local fishing circles and marine biology networks, with photos circulating among experts.
Not an isolated case along the Breton coast
This blue sea spider crab is not entirely alone. Just days earlier, another unusually coloured spider crab had been reported near Lorient, further south along the Brittany coastline. That specimen is expected to be transferred to a public aquarium, where it could become a living exhibit for visitors and researchers.
The two finds, so close in both time and place, have sparked curiosity. Marine scientists stress that such animals are still extremely rare, yet the incidents show that the oceans still hold many unexpected variations.
- Location of first find: off Dinard, northern Brittany
- Second reported specimen: near Lorient, southern Brittany
- Species: sea spider crab (Maja brachydactyla)
- Weight of Dinard crab: about 1.5 kg
What makes the crab bright blue?
The unusual colour is not caused by pollution or paint, as some social media users sometimes guess when such photos appear online. Instead, the answer lies in chemistry and pigments inside the shell.
The colour of a sea spider crab’s carapace is mainly driven by two pigments:
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| Pigment | Typical colour effect | Role in the shell |
|---|---|---|
| Astaxanthin | Red to orange | Gives crabs and lobsters their familiar colour after cooking |
| Crustacyanin | Blue | Binds astaxanthin and shifts it toward blue tones in the living animal |
In a normal crab, these two pigments balance each other, producing a range of browns and reds that help the animal blend with rocks and algae. When that balance slips, the result can be dramatic. Excess crustacyanin or a deficiency of astaxanthin can push the shell toward bright blue or turquoise.
Unusual shell colours usually come from a pigment imbalance, not from an external stain or disease.
Heat also plays a role. When crabs or lobsters are cooked, the blue-associated protein crustacyanin is destroyed, leaving only astaxanthin. That is why a blue or even purple lobster still turns red in the pot.
Similar pigment oddities have been reported in lobsters across the North Atlantic. According to figures cited by the BBC for lobsters, the odds of catching a rare colour morph can be roughly one in two million, although precise rates vary with region and species. Spider crabs are less well studied in this regard, but marine biologists agree that bright blue individuals are exceptional.
Life of the sea spider crab
The animal hauled up off Dinard belongs to the species Maja brachydactyla, often called the sea spider crab or simply spider crab. It is one of the largest crustaceans found in European waters and a valued catch for coastal fisheries.
Its lifespan typically ranges from five to eight years. During that time, the crab goes through repeated moults, shedding and renewing its shell as it grows. Each new carapace must harden rapidly, leaving the crab vulnerable for several days.
The spider crab follows a seasonal migration pattern. Adults move toward shallow coastal waters to reproduce, then often head back to deeper areas outside the breeding period. Fishermen usually target them when they are closer to shore and more numerous.
Recognising a spider crab
Spider crabs are immediately recognisable once you know what to look for:
- Rounded, spiky red-brown shell in typical individuals
- Ten legs, including two powerful front claws
- Males with noticeably bulkier claws than females
- A tendency to camouflage themselves by attaching algae and small organisms to their carapace
They are found throughout the Mediterranean and the North-East Atlantic, from Portugal up to the British Isles and the southern North Sea. Brittany sits in a key zone for the species, with rich rocky seabeds and strong tidal currents that bring food.
What happens to rare catches like this?
When a fisherman pulls up an unusually coloured animal, there is no automatic rule for what to do next. Choices depend on local regulations, the condition of the animal and the interest from scientists or aquariums.
In some cases, a rare specimen may be kept alive and passed on to a research lab or public aquarium. There, it can be studied for genetics and health, and shown to visitors as an example of natural variation. Other times, the animal may simply be sold and eaten like any other, especially if it is legal size and no institution has requested it.
For many fishermen, rare catches bring a brief moment of fame, but they still have a working day to finish at sea.
Incidents like the Dinard blue crab highlight how dependent science still is on people who work on the water daily. Crews are often the first to spot unusual species, jellyfish blooms, or shifts in fish populations linked to warming seas.
Why odd colours matter for science
Strange colours in marine animals can sometimes hint at deeper processes. They may come from random genetic mutations, diet differences, disease, or changes in the environment. Each case needs careful examination before any broader conclusion.
For researchers, a rare crab like this offers a chance to study how pigments are controlled in crustaceans. By analysing shell fragments, they can test pigment levels, look for genetic markers, and better understand how often such colour morphs appear in wild populations.
There is also a public side to the story. When an image of a bright blue crab or lobster travels online, it sparks curiosity about species people usually only see cooked on a plate. That curiosity can lead to questions about sustainable fishing, marine protected areas and the health of coastal ecosystems.
From strange crab to broader ocean questions
For readers unfamiliar with the jargon, “pigment imbalance” simply refers to the way an animal’s body produces and arranges colour molecules. In crustaceans, these molecules are packed into cells in the shell and bound to proteins. A small genetic variation or dietary change can alter how many pigment molecules are made or how they are stored, shifting the animal’s appearance without affecting its survival.
Imagining future scenarios, if climate change or pollution were to affect the food sources that provide these pigments, colour variations might become slightly more common. For instance, a change in plankton or algae communities could influence the amount of astaxanthin available to crabs and lobsters. Scientists are watching such trends, while emphasising that isolated oddities like the Dinard crab are not, on their own, proof of a larger pattern.
For coastal communities, rare marine animals also open opportunities. Aquariums can use them to build educational exhibits on biodiversity. Schools can base science lessons on local stories instead of distant tropical reefs. Fishermen can partner with researchers, reporting unusual catches via simple photos and coordinates. Together, these small steps turn one surprising flash of turquoise in a net into a deeper look at how life in European seas keeps changing, one tide at a time.
Originally posted 2026-03-05 02:05:34.