These days, with globalization driving long-distance dispersal of species ranging from fungi to insects to plants, the term “invasive species” gets used a lot. To me, there is a difference between “non-native” and “invasive”. Although both terms refer to an organism that was not found here prior to colonialism, the term “invasive” is typically reserved for organisms that have not only established outside their native range, but have thrived and spread to the point of being a nuisance and can often have large-scale ecological or economic consequences. 

Asian shore crabs (Hemigrapsus sanguineus) are firmly in the “invasive” category. Asian shore crabs have a squarish green to purple carapace with three points on either side of their eyes and banding on their legs that alternates in light and dark bands. These crabs are opportunistic omnivores, which essentially means that they will eat anything they can get their mouths around – a great strategy if you’re trying to colonize a new area that may not contain the types of food you were eating in you native range. Although they will eat almost anything that is an appropriate size, they do tend to favor mollusks over algae. Because of this, they can significantly affect blue mussel populations by preying on the juvenile mussels in rocky intertidal habitats. This generalist crab is so successful that it general outcompetes native crabs, dramatically reducing species diversity, either through direct predation of other crustaceans or through competition. On a recent trip to a rocky intertidal area on one of the Boston Harbor Islands, I found Asian shore crabs in such high numbers that there was at least one under almost every rock I checked, and in some cases more than half a dozen crabs under a single stone.

Another characteristic that has allowed Asian shore crabs to become invasive is that they can tolerate a wide range of salinities and temperatures, facilitating their spread into a variety of habitats and regions. For example, although they are most commonly found in rocky intertidal shores, where they hide under rocks and in crevices at low tide, they can also survive in salt marshes as well.

The first record of Asian shore crabs in the United States was in New Jersey in 1988; it is believed that the larvae were transported by ballast water of cargo ships coming from Asia. Through the 1990s Asian shore crabs spread both north and south and can now be found in great numbers from Maine to North Carolina. Due to the Asian shore crab’s tolerance of a wide range of environmental conditions, it is likely that the invasion will continue along the US coastline. Two additional factors contribute to the Asian shore crab’s success along the United States shoreline. First, parasites, which help control Asian shore crab populations in their native range, are not present along the US Atlantic coast. Second, the Asian shore crab has a higher rate of reproduction than native crabs; its breeding season extends from May to September, which is about twice the length of native crabs’ breeding season, giving it a competitive advantage.