Carcinus maenas

The European green crab, known scientifically as *Carcinus maenas*, is a highly successful marine crustacean. It is native to the North-east Atlantic Ocean and the Baltic Sea, as well as the coasts of North Africa.

While it is a common part of its home ecosystem, it is also recognized as one of the world's 100 worst invasive alien species. This means that when it moves into new environments, it can cause significant damage to local wildlife and industries. Because it can survive in so many different places, it has become a global concern for marine biologists and fisheries managers alike.

To understand why this crab is such a successful invader, we must look at its biology. The crab possesses a hard shell called a carapace, which can reach a width of 80 millimeters.

A key feature of its anatomy is the presence of five short teeth along the rim of the carapace behind each eye. Additionally, there are three undulations, or bumps, located between the eyes. These bumps are a vital way for scientists to distinguish *C. maenas* from a similar species, *Carcinus aestuarii*. In *C. aestuarii*, the carapace lacks these bumps and extends further forward past the eyes.

The color of the crab is not always green, despite its common name. It can appear in shades of brown, grey, or red. This color variation is partly due to genetics, but local environmental factors play a massive role. For instance, crabs that delay the process of moulting, or shedding their old shells, often become red. These red individuals tend to be stronger and more aggressive than their green counterparts. However, they are also less able to tolerate environmental stresses like hypoxia, which is a lack of oxygen, or low salinity.

*Carcinus maenas* is also an incredibly hardy animal due to its euryhaline nature. This term means it can tolerate a wide range of salinity, from 4 to 52 parts per thousand. This allows the crab to thrive in estuaries, where fresh water and salt water mix. It can also survive a wide range of temperatures, allowing it to live in very cold climates under winter ice.

The crabs spread to new locations through several mechanisms. They can hitch rides on ship hulls or hide in seaweed used as packing material. They also move via ocean currents as larvae or by being transported in bivalves used for aquaculture.

The history of its invasion shows how rapidly it can move. It was first observed on the east coast of North America in Massachusetts in 1817. On the Pacific Coast, the species was found in San Francisco Bay in 1989. It began a rapid northward expansion in 1993, reaching British Columbia by 1999. This allowed the species to extend its total range by 1,000 kilometers in just ten years.

In Australia, it was likely introduced as early as the 1850s and has since spread across many coastal regions.

As a predator, the green crab has a massive impact on marine food webs. It is a diurnal hunter, meaning it is mostly active during the day, though its activity also follows the tides. The crab eats a variety of organisms, including worms, small crustaceans, and many types of mollusks. A single crab can eat up to 40 clams in one day.

This voracious appetite can devastate local fisheries. For example, it preys heavily on soft-shell clams, scallops, and northern quahogs. It can even compete with native Dungeness crabs for food and habitat.

Efforts to control these populations vary by location. In some areas, native species provide natural checks. In eastern North America, the native blue crab can help control green crab numbers. On the West Coast, native rock crabs, such as *Cancer productus*, also prey on them.

Humans also use active methods, such as trapping or declaring disasters to request emergency funding. Scientists have even studied the parasitic barnacle *Sacculina carcini* as a potential biological control. However, using such a parasite is risky because it might also infect native species like the Dungeness crab.