Abstract
Climate change is causing the oceans around South Africa to warm resulting in changes to the habitats of intertidal organisms. This study was conducted to determine the thermal tolerance variability of two mussel species found within the intertidal zone in South Africa. The study consisted of field-based thermal ramping exposures and laboratory-based exposures to thermal treatments.
Three sites were chosen for in-field analysis. Garden Route National Park (GRNP) was chosen as it is bordered by the warm Indian Ocean’s Agulhas Current. The other two sites were the Namaqua National Park (NNP) and West Coast National Park (WCNP), chosen as they are bordered by the cold Atlantic Ocean’s Benguela Current. The in-field data showed that mussels from the GRNP had a higher Arrhenius Breakpoint Temperature (ABT) and Flatline Temperature (FLT) compared to organisms from the NNP and WCNP. These results indicate that the mussels from GRNP were more resilient to increases in temperature compared to the mussels from NNP and WCNP. Perna perna had lower ABT but higher FLT than Mytilus galloprovincialis. This was thought to be due to the gaping behaviour of P. perna that M. galloprovincialis does not exhibit. The complete closure of the shell allows for a higher temperature to be tolerated due to less water loss, but gaping allows for a higher FLT temperature. Submerged conditions elicited lower ABT and FLT values as the mussel’s body cavity is in direct contact with warming water with no ability to regulate internal temperature.
Only two mussel species from the GRNP and NNP were used for laboratory studies. Laboratory exposures consisted of six thermal treatments (11, 15, 19, 23, 27, and 31°C) followed by cardiac performance determination, Heat Shock Protein 70 (Hsp70) analysis and biomarker analysis. Results from the laboratory exposures showed that M. galloprovincialis exhibited higher beats per minute (bpm) and higher Hsp70 expression compared to P. perna from both GRNP and NNP from emerged and submerged exposure conditions. This indicates the vulnerability of M. galloprovincialis to increasing temperatures. The cardiac performance, Hsp70 analysis and biomarker results showed localized acclimation of the mussels preferred coastline temperature.
Perna perna were more vulnerable to increases in temperature when cold-water acclimated (NNP) compared to when they were warm-water acclimated (GRNP). Whereas M. galloprovincialis were more vulnerable when warm-water acclimated compared to cold-water acclimated. Since P. perna is a warm-water species and M. galloprovincialis are a cold-water species, these results were expected. Emerged exposure conditions resulted in higher Hsp70 expression compared to submerged conditions. However, emerged conditions resulted in less oxidative stress and damage compared to submerged conditions. As mussels close their shells during emerged conditions, they were able to reduce their metabolic rate and avoid increases in ROS activity. Increases in temperature showed an increase in bpm experienced by both species. There was also an increase in Hsp70 expression; oxidative stress; protein structure damage and aerobic respiration. The findings of this study suggest that intertidal mussels in South Africa are vulnerable to the effects of climate change and further increases in temperature could cause a range shift or mass die-offs of mussel beds.