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The upper Mytilus galloprovincialis band (larger paler mussels) meeting with the new jet black Semimytilus algosus coating the lower shore.

The Mediterranean mussel Mytilus galloprovincialis is the most significant marine invasive species along the South African coast but C·I·B core team member Professor Charles Griffiths and researchers from the University of Cape Town have recently found another introduced mussel, the Pacific South American mussel (Semimytilus algosus). It has been found to occur at huge densities of thousands per square metre along most of the West Coast of South Africa.

Mussels are often keystone consumers and bioturbators and their ability to filter selectively and to process large quantities of suspended material influence the dynamics of rocky shore systems, with consequent implications for local patterns of biodiversity. Mussels are also defined as ecosystem engineers, because they create beds that structure communities and because they are also likely to invade and alter rocky shore communities. Thus, the decline or displacement of these species may have cascading effects on ecosystem structure and functioning of a rocky shore.

Student Kimon de Greef on a completely Semimytilus algosus invaded shore at Elands Bay.

In Namibia mats of Semilmytilus algosus is well known on the rocky intertidal zone north of Lüderitz. This species has been known to have been present in Namibia since the 1930’s, but has not previously been recorded in South Africa, despite detailed recent surveys of mussel stocks in the region by previous C·I·B funded researcher Dr Tammy Robinson.

An investigation in the spatial distribution of Semimytilus algosus species along the West Coast found that they appear to inhabit the lower parts of the shore opposed to the mussel Mytilus galloprovincialis, which invade the upper shore. Much of the west coast is currently dominated by three bands of invasive invertebrate species – the barnacle Balanus glandula on the upper shore, Mytilus galloprovincialis, in the mid-shore and now Semimytilus algosus on the lower shore. A similar pattern has been observed on the present day Peruvian Coast where Semimytilus occupies the mid-lower half of the intertidal zone, while Perumytilus occupies the mid-upper intertidal, thus forming two distinct bands.

“It is not yet clear whether this is a new invasion or (more likely) a dramatic new expansion of the Namibian stock”, says Prof. Griffiths.