Abstract
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.
Highlights
There are many sources of uncertainty in invasive and alien species lists (McGeoch et al 2012)
We present the case of Tetrapygus niger (Molina, 1782), Family Arabaciidae, in South Africa
To estimate urchin densities and the extent of urchin feeding on kelp, counts of Tetrapygus niger, the native urchin Parechinus angulosus (Leske, 1778) and stipes of the native kelp Ecklonia maxima (Osbeck) Papenfuss were made
Summary
There are many sources of uncertainty in invasive and alien species lists (McGeoch et al 2012). Tetrapygus niger is native to the Pacific coast of South America from northern Peru to southern Chile (Clark 1910) where it is found on rocky substrata or artificial structures in rocky habitat (Dumont et al 2011), in areas with extensive kelp forests and strong wave surge. The urchin can prevent the colonisation of the mussel Semimytilus algosus (Gould, 1850), indirectly reducing the diversity of associated species (Hidalgo et al 2013). In Peru and Chile, T. niger is the most abundant urchin, reaching densities of up to 85 individuals per m2 (Vasquez and Buschmann 1997; Rodriguez 2003). At these high densities, urchin barrens can develop, whereby the growth of kelp and
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