Colour Polymorphism in Relation to Spatial Distribution in Some Intertidal Isopods in Northern Spain

The hypothesis that the level of chromatic polymorphism in intertidal isopods is related to their distribution and seasonal fluctuations in abundance was examined in northern Spain. Four species were used,Dynamene bidentata, Dynamene magnitorata, Cymodoce truncataandCymodoce emarginata. Only forDynamenespecies were results consistent with initial hypothesis. The most common colour morph inDynamene bidentatawasuniformiswhich is the most flexible phenotype, allowing effective concealment in most of the environments where the species occurs.Dynamene bidentatais the most abundant species of isopod in northern Spain, where it is abundant along the shore and throughout the year. Distribution of D.magnitorata, Cymodoce truncataand C.emarginatais restricted to macroalgal assemblages at low tidal levels. Seasonality is also important in D.magnitorataand C.truncata. The number of colour morphs and phenotypic diversity was larger in D.magnitoratathan inD. bidentata. Only one colour morph was found in C.truncataand four in C.emarginata.