Cheating the locals: invasive mussels 'steal' the cooling effect of indigenous mussels

Rocky intertidal communities on the south coast of South Africa are dominated by two intertidal mussels with different thermoregulatory behaviours, the indigenous species Perna perna and the invasive mussel Mytilus... [ view full abstract ]

Rocky intertidal communities on the south coast of South Africa are dominated by two intertidal mussels with different thermoregulatory behaviours, the indigenous species Perna perna and the invasive mussel Mytilus galloprovincialis. Whilst both species form dense aggregations, P. perna also gapes during periods of aerial heat-stress. This behaviour is linked to maintaining aerobic respiration and has no effect on the body temperatures of isolated individuals, but when surrounded by conspecifics, beneficial cooling effects of gaping emerge. It is uncertain, however, whether the increasing presence of M. galloprovincialis limits the ability of P. perna for collective thermoregulation. In this context, we investigated how naturally occurring and manipulated varying relative densities of P. perna and M. galloprovincialis may influences the thermal properties of mussel beds during emersion under various conditions of heat-stress. A combination of infrared (IR) thermography, temperature data loggers and thermocouples was used to measure temporal and spatial variability in heat-stress of both natural and artificial mussel beds. Body and shell surface temperatures of P. perna within mixed artificial beds increased faster and reached higher temperatures than for individuals placed within conspecific beds, indicating that the presence of M. galloprovincialis limits the thermoregulatory effects of gaping. In contrast, body and surface temperatures of M. galloprovincialis within mixed artificial mussel beds increased slower and exhibited lower temperatures than for individuals in beds comprised entirely of M. galloprovincialis. Irrespective of species composition, habitat temperatures and heating rates were greater in artificial beds comprising small individuals than those comprising large individuals. Noticeably, the small-scale patterns of thermal stress detected within manipulated beds were not observed within naturally occurring mixed mussel beds during any of the eight sampling events. We propose that small-scale differences in topography, size-structure and mussel bed size may mask the effects of gaping behaviour within natural mussel beds. The results from our manipulative experiment nevertheless represent the first report of the behaviour of one ectothermic species influencing the body temperatures in another. This may have significant implications for predicting how the co-existence of these two species may change as global temperatures continue to rise.