Inter-specific differences in responses to copper by natural populations ofEctocarpus

Two species of Ectocarpus, E. siliculosus (Dillw.) Lyngb. and E. fasciculatus Harv., occupying the same habitat at Port St Mary, Isle of Man, and apparently subject to the same environmental conditions, exhibit markedly different responses to copper. The greater tolerance shown by E. siliculosus is unlikely to be attributable to direct local selection, but may result from immigration of copper-tolerant individuals from highly selective habitats nearby. E. siliculosus would appear to be a more variable species than E. fasciculatus, thereby being more readily affected by the forces of natural selection.