Ecological impacts of invasive mussels (Mytilus galloprovincialis) colonising the canopy of kelp forests in False Bay, South Africa

Master Thesis


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University of Cape Town

Kelp forests are amongst the most productive, diverse and dynamic ecosystems on earth and kelp are ecosystem engineers, which provide a structurally-complex habitat for many marine plants and animals. The Mediterranean mussel (Mytilus galloprovincialis) is an aggressive invader that has been in South Africa since the late 1970s, and does not normally occur in the subtidal zone, however in 2016 this mussel was first observed growing on the heads and stipes of kelp plants (Ecklonia maxima) in False Bay, South Africa. The overall aim of this project was to gain a better understanding of the ecological impacts of the invasive mussel colonising the canopy of kelp forests in False Bay. It was found that M. galloprovincialis were more likely to occur on the outer edge of kelp beds, where there is more water movement, and mussel masses are much more frequently found on kelp heads than on stipes. In addition, mussels infecting the kelp beds were likely younger than three years, indicating that colonisation has been a recent event. While the buoyancy of kelp plants was reduced by the invasion of mussels, it was not enough to sink most kelp plants. The mussel masses created habitat that increased species richness on both infected heads and stipes, and six species that are alien to the Western Cape were identified on the infected kelp plants. As detached kelps can raft vast distances, there is thus potential to spread not only alien species, but also native South African species to locations where they could become invasive. Lastly, a new species of amphipod that burrows into the primary blades of kelp heads was discovered during the course of the study. Further investigation is required to determine if the infection rate of kelp forests in False Bay is increasing and whether this phenomenon occurs in other locations.