The invasion of the east coast of North America by Carcinus maenas, especially after the migration above Cape Cod, gives a prime example of the effects of this crab when introduced to new communities. The significant losses in the soft shell clam industry in the 1950s are reason enough for people to be concerned when the crab is introduced to new areas. Washington and Oregon, on the west coast of North America, have multimillion dollar shellfish industries that are endangered by the presence of this crab. The further expansion of its range should be halted for economic reasons alone.

Another significant factor to consider in C. maenas invasions is the effect on the already existing animal communities. In Europe, the crab is known to be a significant factor in the evolution of the armor of mollusk shells (Le Roux et al. 1990). Both gastropods and bivalves have evolved heavier armor where C. maenas is present in their ranges. Many prey species are also able to react to the presence of the crab, or to conspecific juices when other prey have been attacked by the crab. In its introduced ranges, this evolution had not occurred. Some animals use reproductive methods that are highly vulnerable to C. maenas attack, but were well suited for life before the crab came (Brenchley 1982).

Other predators are also effected. Decapod crustaceans that inhabit similar ecological niches to C. maenas, such as the rock crab Cancer irroratus, have been negatively impacted by the presence of the crab (Elner 1981).

With the example of eastern North America, we can predict lower numbers for bivalves and similar crustaceans in areas where C. maenas is introduced. However, areas such as San Francisco Bay are very unpredictable, due to the high rate of species introductions (Cohen and Carlton 1995). Eventually, evolution will select for more armored shells in prey species, as has happened in Europe. Similar crustacean species will hopefully be able to survive, perhaps by altering their niches. Ultimately, the majority of species in the introduced ecosystem will probably be impacted.

We can see that C. maenas is adaptable. It is able to tolerate a fairly wide range of salinity, temperature and habitat. The lesson to be learned from this is that quite a few marine habitats around the world are in potential danger of being invaded by C. maenas. A few larvae in ballast water are all it takes to spread this animal halfway around the world.

So what can we do to halt the spread of C. maenas and other nuisance invaders? Ballast water control is probably the simplest and most effective method to do this. Ballast water carries an astonishing number of species on regular treks around the world (Carlton and Geller 1993). By cutting down on the animals transported through ballast water, the number of introduced species would probably decrease dramatically.

Another good method of control is to limit disturbance to the marine environment. It is suspected that less disturbed systems are better able to ward off invaders (Vermeij 1996). Keeping the marine environment at a natural state will potentially help fisheries, as well as keep invaders away.

Invasions have been occurring for millions of years, and they will continue to occur. However, we must not allow human mitigated invasions to upset the delicate evolutionary and ecological balance that these invasions have followed for most of history. Too many invasions to a habitat may irreparably damage the existing communities, and there is no way of predicting what may eventually happen if this occurs.