Rivers and estuaries in the US have been invaded by innumerable exotic marine creatures, including clams and crabs, that threaten native species. The invading species can multiply to the extent of damaging harbours and waterways if there are no natural enemies in the new environment. Trochammina hadai , a single-celled member of the foram family (forams are amoeba-like creatures with shells of calcium carbonate), apparently entered the San Francisco Bay in the ballast of a ship arriving from Japan. Ecologists fear that the new forams will affect native forams, causing irreparable damage to the coastal ecosystem ( Popular Science , Vol 250, No 4).
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