Distribution and ecosystem effects of Semimytilus patagonicus on the West Coast of South Africa, 10 years after its detection

Shongwe, Thembeka

Distribution and ecosystem effects of Semimytilus patagonicus on the West Coast of South Africa, 10 years after its detection

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

Invasive mussels are important components of many rocky shore communities and 95 marine invasive species have been recorded on the South African coast, but only three of these have been documented to have significant ecosystem effects across a large geographic region. Of these two are mussels, the Mediterranean mussel, Mytilus galloprovincialis, and the Chilean mussel, Semimytilus patagonicus. The introduction of S. patagonicus along the West Coast is relatively recent, being detected only in 2009, compared to M. galloprovincialis, which was detected in the 1980s. As an ecosystem engineer, S. patagonicus is likely to have significant effects on the rocky shore community. This dissertation aimed to (1) review the temporal change in distributional patterns of South African mussels along the West and South-West Coast of South Africa, (2) investigate the changes in abundance and vertical zonation of S. patagonicus on the rocky shores along the West Coast of South Africa and (3) evaluate changes of intertidal biodiversity 10 years after the S. patagonicus invasion. Surveys were conducted at approximately 100km intervals along the West Coast of South Africa during November 2021 to assess the abundance and zonation patterns of S. patagonicus and compared to surveys conducted in 2011. Univariate and multivariate analyses were used to test for differences in community structure and diversity of intertidal assemblages and the abundance of mussel species, ten years after the S. patagonicus invasion. Comparing historical and present data, the abundance of West Coast native mussel species, Aulacomya atra and Choromytilus meridionalis was reduced, with these species absent on some shores. By contrast, the range of the more tropical Perna perna, has retracted from the South-West Coast towards the South Coast of South Africa. In False Bay on the South-West Coast, where this species was once abundant, it was now rare and/or absent. Present-day survey data were also compared with those from 10 years ago revealing that S. patagonicus abundance had increased significantly in Bloubergstrand, on the West Coast. S. patagonicus was found in high abundance in low and mid intertidal zones, with the low intertidal zone having the highest abundance, at all three sites. M. galloprovincialis abundance increased on both the mid and the high intertidal zone in 2021 compared to 2011 at all three sites, though not always significantly. This is indicative of facilitation between the two species on the mid shore, where they co-occur. Community diversity increased from 2011 to 2021, suggesting that the formation of complex mussel habitats creates habitat for additional species among the communities on the West Coast of South Africa.