Objective: Integrating population genetic and morphological variation over the entire geographical range of Littorina striata; separating genetic and phenotypic elements in shell polymorphisms and determining their biological significance in order to investigate what factors/mechanisms are responsible for the macro- and microgeographic maintenance of shell polymorphisms in het presence of extensive gene flow (i.e. selection vs phenotypic plasticity); phylogenetic relationships of different littorinids; study of littorinid myoglobins. Design: The whole geographic range of L. striata (Macronesian archipelagos: Azores, Madeira, Canary Islands, Cape Verde) has been intensively samples; at several places, sampling involved detailed microgeographic patterns; as such weveral thousands of individuals have been (and still are) analyzed for morphometric and genetic variation. Field transplantation experiments. Methodology: Electrophoresis (isoelectric focusing and PAGE) of allozymes and radular myoglobins; random amplified polymorphic DNA; development of microsatellite DNA markers and Single Strand Conformation Polymorphisms(SSCP); morphometrics of shell features; ribosomal DNA sequencing (18S rDNA genes and internal transcribed spacers). The population genetic structure of other periwinkle species at various geographical scales and under different environmental conditions, including contaminant stress, are investigated with the same techniques as outlined above.