The introduction of species into new environments is a process that has been occurring for millions of years. The factors associated with these introductions are many and varied, but human mitigated introductions are becoming increasingly common, especially in the past several decades (Vermeij 1996). There are a minimum of 4,500 nonnative species established in the United States, and they represent up to 8% of the taxonomic groups examined. Approximately 15% of these species are termed nuisance species, those with significant ecological or economic impact to the areas in which they have settled (Ruiz et al. 1997).

The transport of marine organisms in ballast water has led to an increase of introduced marine species to systems world wide, especially coastal systems (Carlton 1996a). One ship's ballast water was shown to contain 367 separate species. These species included all major animal phyla, most minor phyla, and representatives of all trophic groups (Carlton and Geller 1993). The five most common groups represented, which were 80% of the taxa, were crustaceans, polychaetes, turbellarian flatworms, cnidarians, and mollusks (Carlton and Geller 1993). It is estimated that 3000 species move around the world in a single day (Carlton 1996a).

One such species is the green, or shore, crab Carcinus maenas. A native of the shores of Europe, the crab has been introduced to western North America, Australia, and South Africa all in the past two decades (Zeidler 1988, Le Roux et al. 1990, Cohen et al. 1995). It has been a source of particular concern due to its past history. The green crab did not begin its trek around the globe recently. The crab was introduced to the east coast of North America approximately 150 years ago, with disastrous effects to the commercial shellfish industries in the area (Lafferty and Kuris 1996). It is this past history of voracious predatory behavior that has people in the newly affected areas worried. The states of Washington and Oregon have large aquaculture and shellfish industries that may be badly affected by the predatory behavior this crab (Raloff 1998).

Invasion biology is a relatively new field with many questions to answer (Vermeij 1996). Why are certain environments more susceptible to invasions? Why are certain species better invaders? The green crab may offer the answer to these questions. Invading species have long-lasting impacts on community structure, impacts that we may never fully understand. If invasions have been occurring for millions of years, we have no way of tracking every invading species. There are many species that are cryptogenic; it is not known if the species is native or introduced. Shallow water marine systems do not leave good fossil records, making a cryptogenic label inevitable for many species (Carlton 1996b). We may never know what the total effects of invasions are, because we may never know the true scope of all invasions over time. What we do know is that invasions bring change, and change brings evolution. Invasions may be the greatest source of evolution, and our world would be vastly different without them.

The earlier invasion of eastern North America by C. maenas gives us a good opportunity to study the effects of an invasion on evolution in a particular community. While 200 years will not allow large scale speciation, it will allow noticeable changes in morphology and behavior, and in some cases, extinction. This older invasion may also allow us to predict the mechanisms of future invasions to some degree. Invasions by the same species are idiosyncratic, and aren't totally reliable in predicting further invasions (Grosholz 1996), but they are still valuable for seeing certain patterns.

The newer invasions, especially of Western North America, give an interesting opportunity to contrast success in different habitats. These are very new invasions, and success is not assured. We do not yet know how the crabs will react to the established animal communities, or how the communities will react to the crabs. Over the next several years, we will begin to see patterns, and these patterns are valuable in understanding the mechanisms of invasions.

What enables species to spread? They must be tolerant of a wide range of conditions. For example, tropical species don't often migrate into colder water. Animals must also be able to adapt physiologically and morphologically to new conditions. Studies have shown that only species sufficiently different from those already there will be able to establish themselves successfully (Vermeij 1996). One particular problem with invading species is that they often arrive with no natural predators, whether animal or pathogen (Lafferty and Kuris 1996). This allows extremely high population densities.

Vermeij (1996) outlines three arbitrary stages in the process of an invasion: arrival, establishment, and integration. Arrival is the dispersal of individuals into a recipient region. Establishment occurs when the new arrivals can now sustain themselves through local reproduction and recruitment. At this stage, the donor region is no longer the reason for the population's persistence at the new site. Integration occurs when the new species begins to forge ecological links with the 'native' species, and evolution occurs, reflecting a new regime in the recipient community.

This paper will examine the natural range of C. maenas in Europe, and compare it to its introduced ranges, with a concentration on the East and West Coasts of North America. Evolutionary relationships will be examined and compared to the new ranges, and possible methods of control will be discussed.