Comparative ecology and physiology of four South African mussel species with notes on culture potential

Doctoral Thesis


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University of Cape Town

The objective of this study was to compare the distribution patterns, reproductive cycles, growth rates and physiological processes in four species of South African marine mussels - Aulacomya ater, Choromytilus meridionalis, Mytilus galloprovincialis and Perna perna. The first chapter is a preliminary account of the introduction of the invasive M. galloprovincialis, and its ecological effects. Its rate of invasion is estimated by monitoring changes in the diet of oystercatchers, as well as comparing its resistance to silt and dessication, and its reproductive and its growth strategies with those of other intertidal mussel species. It appears that although presently mainly confined to cool water of the west and south coasts, M. galloprovincialis will soon colonize in the near future the warmer waters of the east coasts. The second chapter consists of a survey revealing the distribution patterns and standing stock of the four species around the South African coasts. The cool upwelled waters of the west coast support the major populations of A. ater, C. meridionalis and M. galloprovincialis, while the warm east coast is mainly colonized by the brown mussel P. perna. The Transkei and Natal coasts are intensively exploited by subsistence gatherers with an annual rate of removal approaching or even exceeding standing stock. The overall wild crop is ca > 1000 metric tons, while aquaculture output is estimated at 800 tons per annum. The third chapter looks at the timing and the intensity of spawning of the four species. Gamete build up and release results in marked variation in flesh yield. The dry flesh weight of M. galloprovincialis can vary three-fold between ripe and spawned conditions at a length of 65mm. Equivalent figures for C. meridionalis, A. ater and P. perna are respectively 2.8, 2.3 and 2.2-fold. It appears that A. ater shows the greatest gamete output with three annual spawnings, followed by M. galloprovincialis and C. meridionalis with two protracted spawnings. The fourth chapter deals with growth performance of the four species under different environmental conditions. A. ater consistently grows much slower than other species, is intolerant to aerial exposure or silt load and grows best in cold waters. C. meridionalis also likes cool water conditions, but is the most tolerant of silt load. M. galloprovincialis and P. perna are the least affected by tidal exposure and both grow faster in warm water conditions. In terms of aquaculture development, M. galloprovincialis is the most appropriate species to grow on the west and south coasts, while P. perna will be the candidate for the east coasts of South Africa. The fifth chapter compares rates of filtration, respiration and ammonia excretion, as well as absorption efficiencies and subsequent scope for growth in the four mussel species. All species maintain a relatively steady scope for growth over the ration levels 1-7 mg 1- . However, rates are the highest in M. galloprovincialis (254 J h-1 at 7 mg 1-1 ration) followed by C. meridionalis and P. perna at 117 and 70 J h-1 and finally A. ater, with only 23 J h-1. Scope for growth in M. galloprovincialis is high at all temperatures (1 o-20°c) while that of A. ater is low and declines rapidly with temperature. The scope for growth of P. perna and C. meridionalis is intermediate, but while C. meridionalis becomes less efficient at higher temperatures P. perna show a rapid increase with increasing temperature. These physiological measures are correlated to the observed distribution pattern and growth rates in the field.

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