Over 100 species of freshwater mussels (Unionidae) are endangered or threatened in the United States, and another dozen species support a declining commercial harvest of shells for the cultured pearl industry in Asia. Because of these Significant declines in abundance, a study was undertaken to develop a diet for rearing juvenile mussels, with the goal of long-term propagation of rare species. Three trials were conducted to test various tri-algal and commercial diets and to determine the influence of silt in survival and growth of the rainbow mussel (Villosa iris) and giant floater (Pyganodon grandis). After 45 days post-metamorphosis, juvenile V. iris fed algae with silt exhibited a two-fold increase in shell length (532 μm), and 63.5% survival. Juvenile P. grandis exhibited similar results at 45 days post-metamorphosis. In comparison, all juvenile mussels fed algae without the presence of silt exhibited no increase in shell length after 45 days post-metamorphosis. However, survival varied between species. Survival after 45 days was 5.0% for V. iris and 43.3% for P. grandis. P. grandis is probably more tolerant of a variety of environmental conditions. Analysis of covariance showed that growth rate over time (120 days) of P. grandis was significantly greater than that of V. iris. Shell lengths of P. grandis juveniles fed algae in the substrates kaolin, sterilized silt, sterilized silt plus Aqua Bacta-Aid, and bacteria-colonized silt were Similar, indicating that bacteria were not essential to juvenile digestion or nutrition. Shell lengths of V. iris juveniles fed algae in kaolin or algae in bacteria-colonized silt also were similar after 60 days. Juvenile mussels appear to be pedal-feeding for approximately 120 ± 30 days, depending on the species; hence, silt probably serves as a physical substratum for pedal-feeding mussels to collect food particles. Subsequent tests indicated that growth was significantly correlated with algae high in oils which contain polyunsaturated fatty acids (PUFA). V. iris juveniles fed a tri-algal diet, consisting of Neoehloris oleoabundans~ Phaeodactylum tricornutum, and Bracteacoccus grandis (NPB), with silt substratum showed the best growth over time. Individuals achieved a mean shell length of 1747 μm and had 30.0% survival after 140 days postmetamorphosis. All other tri-algal diets tested enhanced growth over the commonly used green tri-algal mix of Chlorella, Ankistrodesmus, and Chlamydomonas (CAC), and all algae diets enhanced growth over a silt-only diet. Commercial yeast diets did not support growth. After 272 days post-metamorphosis, V. iris fed CAC in silt achieved a maximum shell length of 4520 μm (17-fold increase in length), with a mean length of 2968 μm and approximately 5% survival. After 195 days postmetamorphosis, P. grandis achieved a maximum shell length of 7846 μm (22- fold increase in length), with a mean of 4877 μm and approximately 12% survival. Results of all feeding trials indicate that algae are a suitable food source for rearing early juvenile freshwater mussels. A tri-a1gal diet high in oils resulted in greater growth than all other diets tested. Resident bacteria in riverine sediments were not essential to growth and survival of juvenile mussels. Silt provided some nutritional value, but primarily served as a physical substratum for pedal-feeding juveniles. / Master of Science

Mytilus galloprovincialis is invasive on rocky shores on the west coast of South Africa where it has become the dominant intertidal mussel. The success of this species on the west coast and its superior competitive abilities, have led to concern that it may become invasive on the south coast at the expense of the indigenous mussel Perna perna. On shores where these species co-occur, there appears to be habitat segregation among zones occupied by mussels. M.galloprovincialis dominates the high-shore and P.perna the low-shore, with a mixed zone at mid-shore level. This study examined the factors responsible for these differences in distribution and abundance. The study was conducted in Plettenberg Bay and Tsitsikamma (70km apart) on the south coast of South Africa. Each site included two randomly selected locations (300-400m apart). A third mussel species, Choromytilus meridionalis, is found in large numbers at the sand/rock interface at one location in Plettenberg Bay. Aspects of settlement, recruitment, growth and mortality of juvenile and adult mussels were examined at different tidal heights at each site. Quantitative analysis of mussel population structure at these sites supported the initial observation of vertical habitat segregation. Post-larvae were identified to species and this was confirmed using hinge morphology and mitochondrial DNA analysis. Size at settlement was determined for each species to differentiate between primary and secondary settlement. Adult distribution of C.meridionalis was primarily determined by settlement, which was highly selective in this species. Settlement, recruitment and growth of P.perna decreased with increasing tidal height, while post-settlement mortality and adult mortality increased higher upshore. Thus all aspects of P.perna’s life history contribute to the adult distribution of this species. Presumably, the abundance of P.perna on the high-shore is initially limited by recruitment while those that survive remain prone to elimination throughout adulthood. M.galloprovincialis displayed the same patterns of settlement and recruitment as P.perna. However, post-settlement mortality in this species was consistently low in the low and high zones. Juvenile growth also decreased upshore, suggesting that M.galloprovincialis may be able to maintain high densities on the high-shore through the persistence of successive settlements of slow-growing individuals. The low cover of M.galloprovincialis on the lowshore appeared to be determined by adult interactions. M.galloprovincialis experienced significantly higher adult mortality rates than P.perna in this zone. There were seasonal variations in the competitive advantages enjoyed by each species through growth, recruitment or mortality on the low-shore. In summer, P.perna had higher recruitment rates, faster growth and lower mortality rates, while M.galloprovincialis had slightly higher recruitment rates and faster growth rates in winter. P.perna is a warm water species while M.galloprovincialis thrives on the cold-temperate west coast of South Africa. Therefore both species appear to be at the edge of their optimal temperature regimes on the south coast, which may explain the seasonal advantages of each. Nevertheless, P.perna has maintained spatial dominance on the low-shore suggesting that it may ultimately be the winner in competition between these species. M.galloprovincialis appears to have a refuge from competition with P.perna on the high-shore due to its greater tolerance of desiccation stress, while being competitively excluded from the low-shore. Warm water temperatures coupled with poor recruitment rates at most sites may limit the success of M.galloprovincialis on this coast.

Mussels play an important supplementary role in the diet of local communities on the Transkei coast in the Eastern Cape province of South Africa. The exploitation of mussels date back to about 1350 years ago, but in the last 3 decades, exploitation of the brown mussel Perna perna has become unsustainable with mussels collected as small as 30-40mm when they are only just sexually mature. Dye and Dyantyi (2002) developed a technique to rehabilitate areas denuded of adult mussels. The government sponsored Mussel Rehabilitation Project (MRP) to use this technique but only some sites have been successfully rehabilitated, reaching c. 80 % cover within a year whilst others only reach about 5%. At an unexploited site (Riet River), I tested the effects of mussel size and wave strength on the effectiveness of the rehabilitation technique, hypothesizing that different size classes may respond differently due to differences in their energy allocation (growth vs reproduction), while wave action determines food supply. Small (1-2cm) and large mussels (3-4cm) were deployed for rehabilitation at 2 exposed and 2 sheltered sites, separated by 100s m. A similar study was repeated in Coffee Bay where shores are exploited. Six sites were selected, 3 sites that had been successfully rehabilitated and 3 that were unsuccessful according to the MRP. Again, two size classes were used but these differed from the first experiment. Mussels of 3-4cm size were now rated as small and 5-6cm as large. Two methods were used to re-attach mussels, the original and the same method with the addition of mesh bags during mussel deployment. Treatments were examined on three occasions at approximately one month intervals. At Riet River, the sites chosen did not show differences in wave strength (measured using dynamometers) or water flux (measured using erosion of cement balls) so that water motion was excluded from the analyses. Small mussels grew faster and had weaker attachment than large mussels. There was no difference in condition index between small and large mussels, or in the numbers of recruits settling among the byssus threads of deployed mussels of the two size classes. In Coffee Bay, there was no relationship between rehabilitation success and maximum wave force, and no difference in bulk water flux among sites. Small mussels deployed using mesh bags survived better than non-meshed or large mussels of either treatment. There was no difference in condition index (CI) between mesh and no-mesh, or between small and large mussels. As in the case of Riet River, small mussels grew faster than large mussels, but large mussels attached stronger than small mussels, with no effect of mesh. Although the factors that improve reseeding of mussels can be identified (use of mesh, use of small mussels, choice of sites with high recruitment rates), successful long-term rehabilitation requires appropriate subsequent management of re-seeded sites.