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It is important to keep lists of invasive and alien species up to date, but it can be difficult to determine when species should be removed from such lists. One such example is the urchin, Tetrapygus niger (Molina, 1782), which is believed to have been unintentionally introduced to Alexander Bay on the west coast of South Africa with oyster spat imported for aquaculture (the only recorded alien population of T. niger globally). This species is a kelp grazer in its native range, capable of converting healthy kelp populations to barren landscapes. This study involved the re-survey of two aquaculture dams in September 2014, which previously contained the urchin, as well as intertidal and subtidal transects of the surrounding coast. Tetrapygus niger were absent from all sites, despite the presence of a healthy population of native urchins Parechinus angulosus (Leske, 1778). There was also little indication of urchin grazing on kelp stipes. As a result of this absence record, it is recommended that T. niger be removed from the South African list of introduced marine species, and from global lists of invasive species.

The Chilean black urchin, Tetrapygus niger (Molina, 1782) in South Africa: gone but not forgotten