Human-Mediated Spread of Alien Crabs

The introduction and spread of alien species is now recognized as one of the most significant modifiers of biodiversity. In the absence of their normal predators and parasites, alien crabs often establish high population densities and tend to compete fiercely with local fauna for food and shelter. A total of 73 species of brachyuran and crab-like anomuran decapods are known as alien species, of which 48 (65.8\%) have become established. Three groups stand out with their high number of alien species: namely the Portunoidea (swimming crabs, such as Carcinus maenas), Grapsoidea (shore crabs, such as Hemigrapsus takanoi) and Majioidea (spider crabs, such as Pyromaia tuberculata). Canals, ballast water and hull fouling are the primary vectors/routes by which crabs are spread. Transfer of crabs with shellfish, combined with the live seafood trade, are also important. The Mediterranean Sea has the highest number of alien brachyuran species as many have invaded through the Suez Canal, making the Mediterranean the meeting place of Atlantic and Indo-West Pacific faunas. We used egg size as an indicator of life history strategies and a comparison of established alien species with a matched control group of crabs shows that mean egg size of alien crabs is smaller, but it shows wide variation. The Erythrean invaders from the Red Sea are a representative sample of aliens that shows the same pattern even though their transfer agent was a canal rather than shipping. Deliberate transfers to establish new fisheries has been successful in some cases (e.g., Paralithodes camtschaticus to the Barents Sea), but some species are still expanding their range and so their ultimate effects are unknown. The impact of most aliens remains uncertain, hut recent work on Hemigrapsus sanguineus provides an excellent model of the kind of experimental field work that needs to be done. The current focus of attention on coastal aliens has resulted in the unfortunate agreement, at the international level, that ballast water can be dumped with impunity on the high seas, without any knowledge of its impact.