Invasion
Invasion Description
1st record: South Africa/Saldanha Bay (1979, Robinson and Griffiths 2005). Molecular analyses indicate that South African populations of Mytilus galloprovincialis are derived from European-north African Atlantic stocks (Daguin and Borsa 2000).
Geographic Extent
South Africa/Saldanha Bay (1979, Robinson and Griffiths 2005; Haupt et al. 2010, oyster farms); Loderlitz-Cape Agulhas/Namibia-South Africa/Atlantic Ocean (Robinson and Griffiths 2005); South Africa/Alexander Bay (2007, Haupt et al. 2010, oyster farms); South Africa/Knysna Estuary (2008, Haupt et al. 2010, oyster farms); South Africa/Langebaan Lagoon (mid-1990s-2001, Robinson et al. 2007, heavy colonization in 1990s, on sand bottom, but complete die-off in 2001).
Vectors
Level | Vector |
---|---|
Probable | Fisheries Accidental (not Oyster) |
Alternate | Hull Fouling |
Alternate | Ballast Water |
Regional Impacts
Ecological Impact | Competition | |
Mytilus galloprovincialis grows faster and has greater reproductive output than the three mussel species native to South Africa, Aulacomya ater, Choromytilus meridionalis, and Perna perna (Branch and Stefanni 2004). Part of this advantage may come from the near-absence of parasites in M. galloprovincialis (Calvo-Ugarteburu and McQuaid 1998). On the west coast of the Cape region of South Africa, M. galloprovincialis largely replaced the slower-growing native mussel Aulacomya ater and the native limpet Scutellastra granularis in the mid-intertidal zone. The abundance of A. ater decreased by 80% between 1979 and 1988. Mussels also excluded larger adult S. granularis by competing for substrate. A second native limpet, Scutellastra argenvillei, is excluded by M. galloprovincialis on exposed shores, but dominates on semi-exposed shores where M. galloprovincialis is less abundant due to lower recruitment (Branch and Steffani 2004; Griffiths et al. 2005; Branch et al. 2008). Another native mussel, Choromytilus meridionalis, was less affected because it extended further into subtidal areas and is more tolerant of sand than M. galloprovincialis (Griffiths et al. 1992). Overall, the invasion of M. galloprovincialis increased the structural complexity of the upper zones of the rocky intertidal, crating microhabitats. By 2012, the density of M. galloprovincialis declined, as it was partially replaced in the upper intertidal by the invading barnacle Balanus glandula. By this time, structual complexity had decreased to pre-1980 levels. A new mussel invader, Semimytilus algosus, native to the Pacific coast of South America was predicted to become established, but not dominant, because of lower efficiency of fof utilization, based on single=species feeding experiments (Alexander et al. 2015). | ||
Ecological Impact | Habitat Change | |
On the west coast of South Africa, Mytilus galloprovincialis colonized a wider tide range than the native mussels, since it is more desiccation resistant (Hockey and Van Erkom Schurink 1992). Its beds are composed of multiple layers of mussels, in contrast to the single layers of other species. Consequently, they provide shelter from wave action and desiccation for other biota dwelling among them. The spreading beds increased the complexity of the habitat and also decreased the patchiness of the intertidal habitat (Robinson et al. 2007). The dense beds of M. galloprovincialis, while largely replacing the native Aulacomya ater, permitted small A. ater to extend their range upward on the shore, by providing shelter within the matrix of the invading mussels' shells. Similarly, the shells of M. galloprovincialis, while excluding the larger adult limpets Scutellastra granularis, provided substrate for young recruiting limpets (Griffiths et al. 1992; Branch and Steffani 2004; Branch et al. 2008). Mussel beds of M. galloprovincialis actually support a very high density of S. granularis, although the size and fecundity of the limpets are reduced. Another species of limpet, S. argenvillei, is more severely affected because it can just barely reach reproductive size when growing on mussels, and so is quite rare in M. galloprovincialis beds (Branch and Stefanni 2004; Branch et al. 2008). By 2002, the invasion of M. galloprovincialis greatly increased the structural complexity of the upper intertidal, enabling many lower-intertidal species to colonize new microhabitats higher on the shore. By 2012, M. galloprovincialis had declined in the upper intertidal, and was largely replaced by the barnacle Balanus glandula. As a result, habitat complexity returned to pre-invasion (1980) levels (Sadchatheeswaram and Branch 2015). | ||
Ecological Impact | Food/Prey | |
The increased biomass provided by M. galloprovincialis provided increased food stocks for predators, such as the African Black Oystercatcher, Haematopus moquini, which shifted its diet from A. ater to M. galloprovincialis, and increased its brood size (Griffiths et al. 1992; Hockey and Van Erkom Schurink 1992). A native whelk, Trochia cingulata shifted its food preference from the native Aulacomya atra to the introduced M. galloprovincialis and Semimytilus algosus, and also shifted its drilling behavior (Alexander et al. 2015). | ||
Economic Impact | Fisheries | |
Mussels in South Africa have been traditionally important for subsistence harvesting. Mytilus galloprovincialis has increased the available stock, and also created opportunities for aquaculture (Hockey and Van Erkom Schurink 1992). The mussel culture industry in South Africa is based on M. galloprovincialis (Robinson et al. 2005). | ||