The Atlantic spider crab, a valuable invader

For several years now, the spider crab has been invading the Normandy and Brittany coasts, predating on bouchot mussels. To such an extent that several counter-measures are being tested. At the same time, the recovery of numerous interesting compounds from this decapod enables the creation of high value-added products for different markets, and could be a solution for strengthening resilience in the face of the consequences of global warming.

 

The spider crab Maja brachydactyla

 

The spider crab is a member of the Majidae family, the best-known of which are Maja squinado (Mediterranean) and Maja brachydactyla (Northeast Atlantic). Highly prized for its meat, it can measure up to 25 cm long and 18 cm wide, and weigh from 250 g to 3 kg. It grows in successive molts. Its carapace is strongly rounded and domed, with lateral bands equipped with five large spiny teeth. Its back has smaller spines, while its rostrum bears two strong divergent spikes. Its long legs end in a strong claw.

 

Maja brachydactyla is found in the waters of the Northeast Atlantic, English Channel and North Sea. In September-October, adults migrate to depths of over 50 meters, then head offshore to overwinter. In March, the spider crab rises between the surface and 50 meters and moves closer to the coast, where it remains throughout spring and summer.

 

The spider crab first feeds on plankton, then on fixed or immobile organisms such as mussels, gastropods, sea urchins, starfish, small crustaceans, algae and dead fish.

 

The spider crab is protected under the Bern Convention. Its catch is regulated (in size and quantity) for both professional and recreational fishing.

 

The proliferation of spider crabs on the coasts of Brittany and Normandy

 

For several years now, the Maja brachydactyla has been invading the coasts of Brittany and Normandy, predating on bouchot mussels. Production losses for mussel farmers are increasing every year, and are now reaching record levels (up to 70% of production).

To counter this phenomenon, mussel farmers have obtained prefectoral authorization to trap the invader with nets or traps and release it offshore. But this technique had no effect on the phenomenon, as the invader kept returning to the mussel beds.

 

Given the persistence of the phenomenon, a scientific protocol drawn up by IFREMER was launched in February 2023 to gain a better understanding of the proliferation phenomenon and find ways to drive it out. The spider crab is highly sensitive to vibrations and noise. Last summer, a sound scare test was carried out on the Channel coast. Vibrations and noise from chains mounted on small trawlers were used to scare away spider crabs during high tides. The test proved positive: although it didn’t totally eliminate predation, it greatly reduced the presence of spiders. Scientific work on this protocol is still in progress.

 

The valorization of spider crab

Straight to the plate

Spider crab is very popular in Spain, where it sells for around two euros a kilo. The queen of seafood platters, or a main course, it is a product sold mainly in fishmonger’s shops and auctions in France. Spider crab can be found both live and cooked.

 

Other uses

 

Spider crab meat

Rich in protein, its fine flesh contains very little lipid, mainly omega-3 fatty acids. It can be recovered and packaged as frozen pulp for use in many ready-to-eat recipes (pies, gratins, salads, spring rolls, etc.). The spider is also an excellent source of minerals and trace elements, notably copper, zinc, selenium and phosphorus.

 

 

The spider’s shell

The shell of the spider crab is also a source of valuable compounds: proteins, marine calcium, chitin and astaxanthin.

 

Upcyclink is developing processes to transform these compounds (concentrates, powders, flavors, etc.) into high value-added products for the wellness (e.g. chitosan), beauty (e.g. astaxanthin), human food (e.g. calcium powder, flavors, protein concentrate), animal food (feed, petfood) and fishing (palatability enhancers) markets.

 

 

So there are some interesting solutions and prospects for adding value to the invasive spider. At a time when global warming is causing Maja brachydactyla to proliferate in the same areas and other marine species (whelk, for example) to become rarer, this type of development makes perfect sense for strengthening resilience in the face of climate change.

 

 

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