| Summary: | Until recently, marine bioinvasions have received little attention in South Africa, and the status of intertidal marine aliens was last assessed in 1990. I thus investigated the present distribution and status of three intertidal alien species (Mytilus galloprovineialis, Care in us maenas and Careinus aestuarii) and documented the presence and status of a previously unknown invasive, the Japanese oyster, Crassostrea gigas. M galloprovincialis was recorded along the entire west coast of South Africa, with populations extending up to central Namibia, and along the south coast of South Africa to 40 km west of East London. Along South African shores a total stock of 35 403.7 tons (± 7 241.4 SE) was recorded, with the vast majority of the stock occurring on the west coast (31 054.5 tons ± 6 274.1 SE). C. maenas supports a substantial population of 133 568 individuals (95 % confidence range == 97 694 - 166 862) in Table Bay Harbour which appears to be acting as in invasion incubator for surrounding areas. A new population of 9 180 individuals (95 % confidence range 5 870 - 12 003) was recorded in Hout Bay Harbour. This represents a range extension along the Cape Peninsula. The low intertidal abundance of this crab between these localities is thought to reflect the inability of C. maenas to inhabit exposed habitats. C. aestuarii was not recorded during this study despite previous documentation of its presence. In 2003, naturalised populations of C. gigas were documented in the low shore zone of the Breede, Goukou and Knysna Estuaries, but not on the open coast. This may again be indicative of the limiting effect of wave action on species alien to South African shores. As such, the presently undetermined impact of this species may well be focused on estuarine habitats. To quantify the threat posed to intertidal communities by M galloprovincialis, changes in community structure on the rocky shore of Marcus Island and the sandy shore of Langebaan Lagoon were measured. In both habitats, the invasion significantly altered ...
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