Enemies with benefits: parasitic endoliths protect mussels against heat stress.

Positive and negative aspects of species interactions can be context dependant and strongly affected by environmental conditions. We tested the hypothesis that, during periods of intense heat stress, parasitic phototrophic endoliths that fatally degrade mollusc shells can benefit their mussel hosts. Endolithic infestation significantly reduced body temperatures of sun-exposed mussels and, during unusually extreme heat stress, parasitised individuals suffered lower mortality rates than non-parasitised hosts. This beneficial effect was related to the white discolouration caused by the excavation activity of endoliths. Under climate warming, species relationships may be drastically realigned and conditional benefits of phototrophic endolithic parasites may become more important than the costs of infestation.

The combination of selection and dispersal helps explain genetic structure in intertidal mussels.

Understanding patterns of gene flow, selection and genetic diversity within and among populations is a critical element of predicting how long-term changes in environmental conditions are likely to affect species distribution. The intertidal mussel Perna perna consists of two distinct genetic lineages in South Africa, but the mechanisms maintaining these lineages remains obscure. We used regional oceanography and lineage-specific responses to environmental conditions as proxies for gene flow and local selection, respectively, to test how these mechanisms could shape population genetic structure. Laboratory experiments supported the field findings that mussels on the east coast (eastern lineage) are physiologically more tolerant of sand inundation and high temperatures than those on the south coast (western lineage). Temperature loggers mimicking mussel body temperatures revealed that mussels experience higher body temperatures during aerial exposure on the subtropical east coast than on the temperate south coast. Translocations showed that, on the east coast, the western lineage suffered higher mortality rates than local individuals, while on the south coast, mortality rates did not differ significantly between the lineages. Nearshore drogues showed remarkably little overlap between the trajectories of drifters released off the south coast and those released off the east coast. Physiological tolerances can thus explain the exclusion of western individuals from the east coast, but they cannot explain the exclusion of the eastern lineage from the south coast. In contrast, however, ocean dynamics may limit larval dispersal between the two lineages, helping to explain the absence of eastern individuals from the south coast. We emphasise the importance of a multidisciplinary approach in a macro-ecological context to understand fully the mechanisms promoting evolutionary divergence between genetic entities. Our results suggest that phylogeographic patterns of Perna perna may be maintained by a combination of local conditions and the isolating effect of the Agulhas Current that reduces gene exchange.