Lateral Plate Polymorphism in Marine and Estuarine Populations of the Threespine Stickleback (Gasterosteus aculeatus) along the Coast of Norway

Marine populations of the threespine stickleback (Gasterosteus aculeatus) have mostly been described as monomorphic for the complete lateral plate morph, especially in northern waters. An examination of stickleback populations from 27 marine and estuarine sites along the coast of Norway showed that polymorphism is more common than expected; in fact, only 15% of the samples were monomorphic. Populations from estuarine habitats (low salinity) were more polymorphic than populations from marine habitats. A study of morphometric characters shows that the plate morph variation reflects true polymorphism, not just immigration of freshwater sticklebacks or hybridization. Plate number variation within each plate morph was also recorded, but no pattern of variation could be detected, either geographically, or between estuarine and marine habitats.