Africa Invaded: Mediterranean mussel

Mytilus galloprovincialis is the mainstay of South Africa’s mussel-farming industry, predominantly located in Saldanha Bay on the west coast. However, the species is an invasive alien, and has caused dramatic changes to intertidal communities.

Rapid spread

The indigenous mussel communities on South Africa’s rocky shores are dominated by the ribbed mussel Aulacomya ater and the black mussel Choromytilus meriodionalis on the cool west coast, and the brown mussel Perna perna on the warmer south and east coasts. The Mediterranean mussel closely resembles the black mussel, so it was only identified in 1984, when genetic analysis confirmed that it was a separate species. By that time it had already spread along the entire west coast, from Cape Point to Lüderitz in southern Namibia. By the early 1990s it had reached Port Elizabeth on the south-east coast, and had become the dominant intertidal organism along the west coast.

The Mediterranean mussel – native to Europe – is thought to have been introduced to South Africa in the late 1970s, probably on the hull of a ship. Its success as an invader can be attributed to its rapid growth rate, high fecundity, and increased tolerance to desiccation. The Mediterranean mussel grows faster and extends higher into the intertidal than the indigenous ribbed mussel, which it has largely displaced.

Out-competing limpets

The Mediterranean mussel has also been found to outcompete the limpet Scutellastra argenvillei, which grows to almost 100 millimetres. Before the alien mussel arrived, the limpet and the ribbed mussel were the dominant invertebrates in the west coast‘s mid to low intertidal, but there were large patches of open space, kept clear by the limpet’s grazing. Now the alien invader forms a solid band of mussels on the shore, leaving little room for the large limpets.

Recent studies have indicated that the competitive interaction between the Meditteranean mussel and the limpet is influenced by the degree of wave action at different sites. On semi-exposed shores, the limpet dominates much of the rock space, probably because wave action there is unfavourably low for the filter-feeding mussel. However, at more exposed sites the mussel dominates – sometimes accounting for more than 90% of cover – and competitively displaces the limpet. Wave action periodically clears gaps in the mussel beds, allowing limpet patches to expand temporarily, but before long the mussel recolonises the cleared rock space and excludes the limpet again.

The mussel beds do provide a good settlement and recruitment ground for juvenile limpets, but as they grow they are unable to find a foothold on the rock substrate, and are eliminated. In fact, at the invaded sites studied, less than 3% of limpets reached the size of sexual maturity.

Beneficial to some...but at a price

Mediterranean mussel beds also support a denser invertebrate community than the indigenous ribbed mussel beds, and tend to provide refuge for larger infaunal organisms such as marine worms, because the mussel shells are thicker and the beds more structurally complex. This, together with the overall increase in mussel standing stock that has resulted from the invasion, is probably advantageous to rocky shore predators, such as fish, rock lobsters, starfish, predatory whelks, octopuses and shorebirds. Indeed, the invasion seems to be beneficial to the endangered African black oystercatcher, which feeds predominantly on mussels and limpets.

Since the alien mussel occurs higher on the shore than the indigenous species, it is providing an easily accessible and abundant food source, and the oystercatchers have evidently responded by raising more young. Nevertheless, the ongoing invasion by the Mediterranean mussel is cause for concern from a biodiversity perspective, and there are fears that it will spread further up the south and east coast, where the brown mussel Perna perna is dominant. In 1992, the alien species was estimated to make up only 1% of overall mussel standing stock on the southern Cape coast, but a survey in 2000 revealed that while it still occurred in relatively low densities in most places, it had become abundant in the Plettenberg Bay area and was well established in the adjacent Tsitsikamma National Park. The Mediterranean mussel may have a competitive advantage as it appears to be free of trematode parasites that are normally present in the brown mussel, affecting growth, reproduction, adductor muscle strength and water loss.

There are currently no initiatives to control the Mediterranean mussel in South Africa, apart from encouraging poverty-stricken communities on the west coast to harvest the species for subsistence use and small-scale commercial ventures.

Reference: Matthews S. & Brandt K.   Africa Invaded: The growing danger of invasive alien species  Global Invasive Species Programme 2004