Mussel Invasives

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INVASIVE MARINE MUSSELS

IntroductionPaleontological evidence of shell remains along the South African coast suggests that modern humans were eating shellfish 164 000 years ago, and that shellfish may well have played a crucial role in ensuring our survival, following a volcanic eruption 75 000 years ago (Anderson & De Wit, 2011). Shellfish remain very important for our survival. In the 1980’s, shellfish made up 8% of the protein in the diets of local coastal populations in the central region of the Wild Coast, and harvesting rituals have become an integral part of local culture (Hockey et al, 1988).

Mussels form an important component of the shellfish we consume, but the type of mussels we eat today may well be different to those we ate in the past – thanks to an “alien invasion”. Marine species have been introduced continuously into South Africa for more than 400 years, since the arrival of the first European explorers. Some 86 marine species are currently regarded as introduced to the region, with a further 39 considered of unknown origin (Griffiths et al, 2009). This article focusses on the impact and implications of invasion of South Africa’s rocky shores by a marine mollusc, Mytilus galloprovincialis.

Alien InvasionWe accidently introduced the Mediterranean mussel, Mytilus galloprovincialis, to the West Coast in the 1970’s and not so accidentally to East Coast in the 1980’s. It is thought to have arrived in Saldanha Bay in the ballast water of ships but in the late seventies we began farming M galloprovincialis in the Bay (Nangammbi, 2007). After its introduction to the West Coast it spread rapidly along the coast, eventually rounding Cape Point and advancing up the south coast. In the late eighties we began farming M galloprovincialis in a mussel farm in Port Elizabeth harbour and true to form it began spreading from this point of origin as well. Since it had never before been recorded in this region, it represented a unique opportunity to gain some insight into the dispersal of mussel larvae. Contrary to what one might expect for larvae that spend around 3 weeks in the ocean before settling out onto rocky shores, most of the surviving recruits were found only a few kilometres

from the Harbour – at least initially. This has important implications for shellfish conservation and suggests that localized currents have some influence on larval dispersal (Phillips, 1994). Current reports indicate that the blue mussel (as it is popularly called) has halted its advance up the East Coast at East London, possibly due to ocean currents that limit their dispersal (Zardi et al, 2007)

ImpactSince its arrival, M galloprovincialis has had a profound effect on the ecology of South Africa’s rocky shores. The cold, nutrient rich conditions on the West Coast suited the invasive. It grew well on submerged ropes at a mus-sel farm in Saldanha Bay, but in the natural environment the indigenous black mussel Choromytilus meridionalis still seems to hold sway sub-tidally and at the interface of rocky and sandy shores. The situation on rocky intertidal shores is somewhat different. Here single layered beds of the indigenous ribbed mussel Aulacomya ater have been replaced with thick multi-layered beds of M gallo-provincialis, substantially altering the invertebrate com-munity living in mussel beds (Robinson et al, 2007a). The invasive even managed to gain a foothold on a sand bank in the middle of Langebaan Lagoon (tempo-rarily as it turned out) and converted the community to one resembling that of a rocky shore (Robinson et al,

The invasive has taken over from indigenous mussels on West Coast rocky intertidal shores.

Mussel Invasives

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2007b)! M galloprovincialis also successfully competes with limpets for space on rocks. A rapid assessment in the Northern Cape Alexander Bay mining area indicated that there may be sufficient limpet resources to support small scale harvesting. However, a few years later when the opportunity for experimental harvesting arose, it was found that M galloprovincialis had taken over to such an extent that harvesting limpets was no longer viable (Fishing and Mariculture Development Association, pers com). On the West Coast M galloprovincialis is highly prolific, 20 – 200% more so than indigenous species (Branch & Steffani, 2004), and while it should be consid-ered “out of control and uncontrollable” (Hockey & van Erkom Schurink, 1992), under certain conditions it could support small-scale fisheries (Robinson et al, 2007 c&d).

P perna does better low on the shore, M galloprovincialis does better higher on the shore, with mixed beds in be-tween Hannekom (2008). This is because P perna has stronger byssus threads and can withstand stronger wave action low on the shore. Higher on the shore mussels are exposed to air for longer and are prone to water loss and desiccation. In these conditions P perna suffers as it shells gape when exposed to air, while M galloprovincialis does well because it holds shells tightly closed. P perna has fewer endolithic parasites to weaken it than M galloprovin-cialis (Zardi et al, 2009), but P perna beds provide shelter for invasive mussel larvae settling out the water column (Rius, 2005). These types of counterbalancing factors are thought to facilitate the coexistence of the two species on the South Coast (Rius & Mcquaid, 2009).

With a myriad of biological and physical interactions de-termining community structure (Bell, 2011) and causing site to site variations (Zardi et al, 2009) it is difficult to predict what exactly would happen if the invasive began to spread north of East London along the Wild Coast. Mussel stocks have been severely depleted on the East Coast, so for indigenous local communities on the Wild Coast and in Northern Kwazulu-Natal, the arrival of M galloprovincialis could well be a blessing in disguise. However, from an ecological perspective this scenario would likely have far reaching, unpredictable and not necessarily desirable impacts. On the East Coast, where we still have a choice, conservation measures and inno-vative programmes engaging communities in managing indigenous mussel stocks in the face of heavy demand, would appear to be more favourable than introduction (accidental or otherwise) of an invasive species.

ImplicationsSome 75 000 years ago shellfish resources helped humanity survive a period of rapid and severe climate change. This article illustrates the profound, unpredict-able impact an invasive marine species can have on our shellfish and how this can influence harvesting of these resources. We clearly need to be extremely care-ful about introducing invasive species, and should focus on conserving and managing our shellfish resources, at a time when we are once again faced with rapid, extreme changes in our climate, albeit for different reasons.

On the South and East coasts M galloprovincialis met a much stronger competitor in the indigenous brown mussel, Perna perna. Contrary to expectations (Erland-sonn et al, 2011), the two mussels now coexist on rocky shores (Rius & MacQuaid, 2009). People on the South Coast seem to find both species equally tasty, harvesting both without discrimination and both mussel stocks are depleted in accessible areas (Rius, 2005).

Indigenous and invasive mussels coexist on the South and East Coast. Photo Claudio Velasquez Rojas/Homebrew films

The invasive blue mussel Mytilus galloprovincialis is clearly visible among the indigenous brown mussel Perna perna. Photo: Claudio Velasquez Rojas/Homebrew films

ReferencesPhillips TE (1994). Dispersal, settlement and recruitment: Their influ-ence on the population dynamics of intertidal mussels. PhD Thesis, Rhodes University.Branch GM, Steffani CN (2004). Can we predict the effects of alien species? A case history of the invasion of South Africa by Mytilus galloprovincialis (Lamarck), J. Exp. Mar. Biol. Ecol. 300 (2004), pp. 189–215.Robinson TB, Branch GM, Griffiths CL, Gov-ender A, Hockey PAR (2007a). Effects of the invasive mussel Mytilus galloprovincialis on rocky intertidal community structure in South Africa. Marine Ecology Progress Series 340: 163-171.

ContributorDr Tracey Phillips; Gondwana Alive, Kommetjie, Cape Town

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