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

Responses to moderate and extreme salinity change were investigated in Whit e Sea Littorina saxatilis of different genetically determined shell colour morphs in order to test a hypothesis about physiological selection as a dri ving force of the change of phenotypic structure of this species along a sa linity gradient in White Sea estuaries. Some of the studied physiological r esponses did not differ in the snails with different shell coloration inclu ding oxygen consumption rate, rate of salt loss in extremely low salinity a nd rates of behavioural isolating and opening responses. However, snails wi th the brown tessellated unbanded shell (which are predominantly found in t he estuaries) demonstrated better survivability under conditions of extreme ly low salinity and combination of low salinity and freezing temperatures a s compared to the conspecifics of the purple tessellated unbanded morph whi ch is more frequently found in the marine sites. Periwinkles with brown tes sellated unbanded shells also tended to be more responsive to an unfavourab le 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 advant age of the brown tessellated unbanded morph under extremely fluctuating sal inity and temperature regime of the White Sea estuaries, and thus a conside rable 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. (C) 2000 Elsevier Sc ience B.V. All rights reserved.