The Mediterranean mussel Mytilus galloprovincialis is the most successful marine invasive species in South Africa. Its presence has had significant ecological consequences on the intertidal communities of the west coast. On the south coast, M galloprovincialis co-exists and competes with the indigenous intertidal mussel Perna perna in the lower balanoid zone, where they show partial habitat segregation. The upper and the lower mussel zones are dominated by M. galloprovincialis and P. perna respectively while they co-occur in the mid zone. In this thesis M. galloprovincialis and P. perna are compared in terms of their population genetics and their ecophysiology. The success of an invader depends on its ability to react to new environmental factors, especially when compared to indigenous species. The distribution and diversity of intertidal species throughout the world are strongly influenced by periodic sand inundation and hydrodynamic stress. Occupying the lower intertidal zone, P. perna is more strongly influenced by sand (burial and sand in suspension) than M. galioprovincialis. Despite this, P. perna is more vulnerable to the effects of sand, showing higher mortality rates under experimental conditions in both the laboratory and the field. M. galioprovincialis has longer labial palps than P. perna, indicating a better ability to sort particles. This, and a higher tolerance to anoxia, explains its lower mortality rates when exposed to burial or suspended sand. Habitat segregation is often explained by physiological tolerances, but in this case, such explanations fail. The ability of a mussel to withstand wave-generated hydrodynamic stress depends mainly on its byssal attachment strength. The higher attachment strength of P. perna compared to M. galioprovincialis and of solitary mussels compared to mussels living within a bed (bed mussels) can be explained by more and thicker byssal threads. M galloprovincialis also has a wider shell, is subjected to higher hydrodynamic loads than P. perna and shows a higher theoretical probability of dislodgement, this is borne out under field conditions. The attachment strength of both species increased from higher to lower shore, in parallel to a gradient of a stronger wave action. Monthly measurements showed that P. perna is always more strongly attached than M. galloprovincialis and revealed seasonal fluctuations of attachment strength for both species in response to wave height. The gonad index of both species was negatively cross-correlated with attachment strength. The results are discussed in the context of the evolutionary strategy of the alien mussel, which directs most of its energy to fast growth and high reproductive output, apparently at the cost of reduced attachment strength. This raises the prediction that its invasive impact will be more pronounced at sites subjected to low or moderate wave action at heavily exposed sites. The potential of a species for invasion is also determined by the ability of the invader to disperse. Population genetics provide indirect information about dispersal through a direct measurement of gene flow. The low genetic divergence (measured as mtDNA) of M. galloprovincialis confirms its recent arrival in South Africa. In contrast, the population genetics structure of P. perna revealed strong divergence on the south-east coast, resulting in a western lineage (straddling the distributional gap of the Benguela System), and an eastern lineage, with an overlap region of the two on the south coast between Kenton-on-Sea and Haga Haga. This genetic disjunction may be caused by Agulhas Current acting as an oceanographic barrier to larval dispersal, or by different environmental selective forces acting on regional populations. Over the last ten years, M. galloprovincialis has shown a decrease or cessation of its spread to the east in exactly the region of the genetic disjunction in P. perna, again suggesting either an oceanographic barrier to larval dispersal, or increasing selection driven by sharp gradients in environmental conditions.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:5613 |
| Date | January 2006 |
| Creators | Zardi, G I |
| Publisher | Rhodes University, Faculty of Science, Zoology and Entomology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | 166 leaves, pdf |
| Rights | Zardi, Gerardo Ivan |
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