Details of this species in South Africa Status: Alien
Invasiveness: Invasive
Occurrence: Established and expanding
Source: Branch and Stephanni 2004 Arrival Date: Late 1970s
Introduction: Unintentional (accidentally) Species Notes for this Location:
M. galloprovincialis was first recorded in South Africa in the late 1970s. Its initial detection was, incidentally, the result of a culinary hunt by one of the authors, which revealed an unexpected orange colour to the gonads of the mussels sizzling in the pan, instead of the normal cream or chocolate brown of the local species; but confirmation of the identity of the species required the greater sophistication of electrophoresis (Grant and Cherry 1985, in Branch and Stephanni 2004).
M. galloprovincialis now covers some 2000 km of the west coast of South Africa and Namibia, and has also become established on the south coast as a result of a mariculture venture. It is now rated as the most abundant alien marine species to have invaded South Africa (De Moor and Bruton, 1988, in Branch and Stephanni 2004). As to the method of introduction, shipping is held to be the most probable original mode (Grant et al. 1984, in Branch and Stephanni 2004).
M. galloprovincialis has colonised the rocky intertidal zone in parts of the open coast shore in South Africa (Carlton 2003), and is now a dominant species on the west coast (Griffiths and Day 2004). It was present for many years before it was recognised as invasive by researchers (Calvo-Ugarteburu and McQuaid 1998, in McEnnulty et al. 2001). It can attain standing stocks of over 200 metric tons per kilometre of rocky coast (van Erkom Schurink and Griffiths 1990, in Hammond and Griffiths 2004). Management Notes for this Location:
No control or eradication attempts have been carried out but further introductions have been prevented. Impacts:
Competition: Relative to the three native species, Mytilus galloprovincialis grows faster in absolute terms under optimal conditions, and its growth is proportionally less diminished by exposure to air (Van Erkom Schurink and Griffiths 1993, in Branch and Stephanni 2004). Its annual reproductive output expressed as a percentage of its body mass, exceeds 120% as it reproduces more than once each year; its total annual output is between 20% and 200% greater than that of any of the indigenous species (Van Erkom Schurink and Griffiths 1991, in Branch and Stephanni 2004). This high reproductive output translates into an exceptionally high rate of recruitment, with densities of up to 20,000 recruits per 100cm2 being recorded on the west coast (Harris et al., 1998, in Branch and Stephanni 2004). M. galloprovincialis is also more tolerant of exposure to air than any of the indigenous species. Survivorship of M. galloprovincialis (measured over 24 months at midtide levels where the mussels experienced about 50% exposure to air) is more than double that of any of the other species. Confirmation of the relative tolerance of the four species to aerial exposure was provided by experiments in which mussels were held for 42 weeks at the high-tide level where they experienced up to 7 days of continuous exposure to air. Under these conditions, survivorship of M. galloprovincialis was 92%, but 78% for Perna perna, 37–46% for Choromytilus meridionalis and 0–10% for Aulacomya ater (Hockey and van Erkom Schurink 1992, in Branch and Stephanni 2004). M. galloprovincialis has a lower parasite load than the native P. perna, giving it a competitive advantage (Calvo-Ugarteburu and McQuaid 1998, in McEnnulty et al. 2001).
Ecosystem change: From the perspective of predators, Mytilus galloprovincialis constitutes an additional source of food because it has substantially increased the biomass and vertical extent of mussels on the west coast (Van Erkom Schurink and Griffiths 1990, in Branch and Stephanni 2004). Two predators are known to have been affected: the whelk Nucella cingulata has increased in overall density on shores invaded by the mussel, increasing from 4 to 220•m-2 of shore (G.M. Branch, Unpub. Data, in Branch and Stephanni 2004). Secondly, the African Black Oystercatcher, Haematopus moquini, has switched its diet to feed on M. galloprovincialis, with a resulting increase in reproductive potential (Hockey and van Erkom Schurink 1992, in Branch and Stephanni 2004).
Habitat alteration: The presence of Mytilus galloprovincialis results in the loss of habitat for the native limpet Patella granularis (Hockey and van Erkom-Schurink 1992, in Morton 1996).
Reduction in native biodiversity: The indigenous ribbed mussel A. ater was progressively displaced from semiexposed and exposed shores as the cover of M. galloprovincialis rose there. At sites where M. galloprovincialis was experimentally removed, there were no declines of A. ater (G.M. Branch Unpub. Data, in Branch and Stephanni 2004). Last Modified: 9/05/2006 9:13:37 a.m.
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