Living mainly in the North Atlantic and in the Mediterranean, the spider crab is distinguished by a thorny shell yellow brown or red with a heart shape. Close-up on the species frequenting the European coasts.
Identity card of the spider crab
The term spider crab refers to crabs of the family of Majidae the best known of which are Maja squinado (Mediterranean) and Maja brachydactyla (Northeast Atlantic). Since 1922, the name of Maja squinado was applied to two species which were differentiated on morphological bases in 1998 then genetic in 2008. Like all shellfish, the spider crab has a exoskeleton, an external anatomical attribute that supports and protects the animal.
Description of the spider crab
Highly prized for their flesh, these decapods (ten legs) can be up to 25cm long and 18cm wide and weigh from 250g to 3kg. Their color varies from reddish brown to yellow brown. Portrait of the species found along the European coasts:
- The Maja squinado. Its triangular shell is rounded and rounded. Its lateral edges have broad spines, followed by several small teeth. The dorsal surface sports a row of numerous pointed tubers. A first pair of smooth legs is provided with clamps while the other four, thinner, are covered with bristles;
- The Maja brachydactyla. In large individuals, the carapace is larger and more oval than that of the squinado. Strongly domed and rounded, it is characterized by side bands equipped with five large thorny teeth. Its back has smaller tubercles while its rostrum bears two strong divergent points. Its long legs have developed hairiness and end in a strong claw;
- The Maja crispata (red-brown or yellow-brown color) and the Maja goltziana (purplish red). Small in size (between 8 and 11 cm maximum), these two species share a similar geographical area but to a lesser extent than the first two.
Distribution of spider crabs
Four species of spider crabs live along European coasts. Zoom :
- The Maja squinado is only found at sea Mediterranean. This crustacean frequents rocky and sandy bottoms where it evolves among algae and posidonia (underwater flowering plants), between the surface and 50 meters deep;
- The Maja brachydactyla is confined exclusively to the temperate waters of the east Atlantic. Its area of distribution encompasses the east of the English Channel, the west of Ireland to the coasts of Western Sahara. Like the squinado, it is observed between the surface and 50 m deep, above rocky and sandy bottoms, among algae;
- Maja crispata evolves into Mediterranean and Atlantic, from Brest via the Bay of Biscay, Portugal, Morocco and Senegal. The littoral benthic species is found in shallow water (generally 0 to 30 m) and up to 100 m on rocky or sedimentary coasts covered with algae. She often frequents the ports;
- The Maja goltziana occurs in the tropical and temperate eastern Atlantic, from Portugal to the Congo. She is present but rare in Mediterranean.
Diet of the spider crab
The larval phase spider crab feeds on plankton while juveniles and adults vary their menu by consuming organisms fixed or not very mobile: bivalves (mussels, modioles), gastropods (whelks), echinoderms (sea urchins, starfish, brittle stars), small crustaceans, algae (corallines, kelp) and dead fish.
Behavior of the spider crab
Spider crabs sometimes congregate in heap of several dozen individuals. This approval occurs mainly at the end of the terminal moult (in summer or autumn) when their consistency is soft. The accumulation could constitute a strategy of defense against certain predators with a dominant situation of males who position themselves above the heaps. In September-October, the adults migrate beyond 50 meters of depth then move towards the large to winter. In March, they rise between the surface and 50 meters, approaching the coast to stay there all spring and summer.
Reproduction of the spider crab
Around the age of 2, spider crabs reach sexual maturity after a terminal moult. From then on, the adult stops his growth, can no longer regenerate its lost legs and its longevity is shortened. Females ovigerous carry the eggs under their feet for at least three months. Between 50,000 and 500,000 eggs (depending on the size of the spawners) hatch six to seven weeks after laying. The larvae are pelagic (they swim) and retain their larval state for about fifteen days. During its first two years, the spider crab performs between twelve and thirteen moults before beginning its life to the rhythm of migrations seasonal to coastal areas for breeding.
Protection of spider crabs
From predators spider crabs include octopus, lobster and carnivorous fish (bass, conger), which attack juveniles in particular. These decapods enjoy protection under the Convention of Bern (relating to the conservation of wild life and the natural environment in Europe). Their catch is regulated (in size and quantity) for professional and recreational fishing. If not captured, the spider crab displays a longevity from 5 to 8 years old.