Physiological variation related to shell colour polymorphism in White Sea Littorina saxatilis

Responses to moderate and extreme salinity change were investigated in White Sea Littorina saxatilis of different genetically determined shell colour morphs in order to test a hypothesis about physiological selection as a driving force of the change of phenotypic structure of this species along a salinity gradient in White Sea estuaries. Some of the studied physiological responses did not differ in the snails with different shell coloration including oxygen consumption rate, rate of salt loss in extremely low salinity and rates of behavioural isolating and opening responses. However, snails with the brown tessellated unbanded shell (which are predominantly found in the estuaries) demonstrated better survivability under conditions of extremely low salinity and combination of low salinity and freezing temperatures as compared to the conspecifics of the purple tessellated unbanded morph which is more frequently found in the marine sites. Periwinkles with brown tessellated unbanded shells also tended to be more responsive to an unfavourable salinity change, so that relatively more animals of this morph isolated themselves inside the shell shortly after placement in low salinity. It is suggested that these physiological differences may provide selective advantage of the brown tessellated unbanded morph under extremely fluctuating salinity and temperature regime of the White Sea estuaries, and thus a considerable increase of the relative abundance of this morph towards the head of the White Sea estuaries may be a result of physiological selection on pheno(geno-) typic structure of L. saxatilis populations.  2000 Elsevier Science B.V. All rights reserved.