The construction of shipping canals such as the Suez (across Egypt joining the Mediterranean and Red Seas), Panama (across the isthmus of central America joining the Caribbean Sea and the Pacific Ocean), and Kiel (linking the North Sea with the Baltic through Germany), provided unrivalled opportunities for marine organisms greatly to extend their geographical ranges. Physical factors such as freshwater lochs and temperature, and physical barriers, have limited or prevented the spread of species through the Panama and Kiel Canals but around 500 species of Red Sea organisms have spread into the Mediterranean through the Suez Canal. Only about 10 species have moved in the other direction, probably because of prevailing current flows, but the full reasons for this are as yet unknown. Most of the invading Red Sea species have not caused major problems although a few native Mediterranean species have been ousted by their Red Sea counterparts, e.g. the cushion star Asterina gibbosa has been replaced in some areas of the Mediterranean by the Red Sea Asterina wega. Plankton communities too have been altered by the influx of Red Sea species.
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