Low temperature trumps high food availability to determine the distribution of intertidal mussels Perna Perna in South Africa

Explanations of species distributions often assume that the absence of a species is due to its inability to tolerate an environmental variable. Recent modelling techniques based on the dynamic energy budget (DEB) theory offer an effective way of identifying how interacting environmental parameters influence distributions through non-lethal effects on growth and development. The mussel Perna perna is an abundant ecosystem engineer around the coasts of Africa, South America and the Arabian peninsula, with an unexplained 1500 km lacuna in its distribution on the west coast of South Africa. We used a DEB approach to explain its distribution in southern Africa and test the hypothesis that this lacuna is caused by sublethal effects of low temperatures on its metabolism. We parameterized a standard DEB model for P. perna using eco-physiological parameters measured in the laboratory, validated by comparison with the body size and reproductive effort of animals from the field throughout South Africa. The model highlighted the importance of reproductive failure under the cold water conditions of the west coast, despite particularly high food availability there, and the surprisingly good performance of P. perna under the warm, highly oligotrophic conditions of the east coast. The results suggest that P. perna is well adapted to low food conditions, but is reproductively vulnerable to low temperatures. DEB models accurately described and explained the anomalous biogeography of this intertidal mussel, allowing us to disentangle the interaction of antagonistic stressors and reveal the critical importance of sublethal temperature effects on reproduction to the species' distribution.