Maja squinado

The European spider crab, known scientifically as Maja squinado, is a migratory crustacean.

It is commonly called the spiny spider crab or the spinous spider crab. These crabs live in the Mediterranean Sea. While they look like the much larger Japanese spider crab, they are not close relatives. The European species belongs to the family Majidae. The Japanese species belongs to a different family called Macrocheiridae. Understanding these crabs helps scientists study marine life and ocean ecosystems.

The life cycle of Maja squinado involves many complex stages. After hatching, the larvae are tiny, measuring just over 1mm in length. They weigh approximately 0.12 mg at this stage. Within 4 to 8 days, the larva undergoes several molts. A molt is when an animal sheds its hard outer shell to grow. During this time, the cephalothorax, or the head and chest region, develops further. In a second phase, the carapace, which is the hard upper shell, reaches about 2mm in length. The larva then undergoes metamorphosis to become a juvenile. This change shifts their lifestyle from planktonic, meaning they float in the water, to benthic, meaning they live on the sea floor.

The growth of a juvenile crab follows a specific sequence of molts. After hatching, the first juvenile instar performs its first molt about 21 days later. This leads into the second juvenile stage. During this stage, the carapace grows significantly to about 4.51 mm. The second molt leads to the third juvenile stage. At this point, the crab looks like an adult, with a carapace length of about 5.63 mm. However, the crab is still not sexually mature. Juvenile crabs spend about two years molting and growing before they can reproduce. They typically live in shallow waters, hiding between rocks in coastal kelp forests during the winter. In the summer, they move to small rocky reefs at depths of only 4 meters.

Scientists have worked to clarify the identity of this species through taxonomy. Taxonomy is the science of naming and classifying living things. A review of the species complex helped researchers distinguish between different crabs. They found that specimens from the Atlantic Ocean are actually a separate species. This Atlantic species is named Maja brachydactyla and was identified by Balss in 1922. The name "squinado" has a long history. It comes from a Provençal name recorded by Rondelet as early as 1554. This shows that humans have been observing these crabs for centuries.

The physical development of the crab changes as it reaches maturity. After the two-year juvenile period, crabs go through two critical molting periods. The first is the prepubertal molt, which occurs in April. The second is the pubertal molt, which happens between July and October. The average time between these two important molts is 104 days. As crabs get larger, their shells do not increase in size as much during each molt. For example, the prepubertal molt increases carapace length by about 36%. In contrast, the pubescent molt only increases length by about 27%. In some very large captive crabs, scientists have even seen three molts or instances where a molt was lost or late.

Maja squinado has several interesting behaviors related to survival and diet. These crabs are migratory, meaning they move long distances. In the autumn, some crabs travel over 160 km during an eight-month period. When they molt, they become vulnerable to predators because their new shells are soft. To stay safe, they become gregarious, which means they gather in groups for defense. Their diet includes macroalgae and various benthic invertebrates. A 1992 study in Galicia showed they eat seaweeds from the Laminariaceae family. They also consume mollusks, such as gastropods and bivalves like Mytilus spp., as well as echinoderms.

Humans interact with this species through large-scale commercial fisheries. Every year, over 5,000 tonnes of these crabs are caught. Most of the catch comes from the coast of France, which provides over 70% of the total. Other significant sources include the United Kingdom, Spain, and Ireland. Because the species is currently endangered, it is becoming harder for fishermen to find them. To help manage this, the European Union requires a minimum landing size of 120 mm. Some countries have even stricter rules. For instance, Spain bans the landing of egg-bearing females, and France has closed seasons to protect the population. These regulations are vital for preventing environmental degradation and managing fishery health.