Landlocked populations have small but detectable differences in ionoregulatory physiology compared to anadromous sea lamprey, Petromyzon marinus

Abstract

Life histories of anadromous and landlocked sea lamprey are similar, though landlocked populations lack seawater (SW) exposure, likely experiencing relaxed selection on SW survival traits. This study investigated SW osmoregulation in juvenile sea lamprey from one anadromous and three landlocked populations from the Great Lakes and Lake Champlain. Juveniles in all populations had strongly elevated gill Na+/K+-ATPase activity compared to larvae, indicating all populations underwent osmoregulatory changes associated with metamorphosis. Survival in SW was high in anadromous sea lamprey (90%) and highly variable among landlocked populations (40%–100%). Plasma ions’ levels were higher and hematocrit was lower after SW exposure in landlocked compared to anadromous sea lamprey. Freshwater (FW) gill ion transporter (H+-ATPase; Na+:Cl− cotransporter) mRNA levels were higher in FW and remained high after SW exposure in landlocked relative to anadromous juveniles. Landlocked lamprey had 24%–33% higher gill Na+:K+:2Cl− cotransporter abundance after SW exposure compared to anadromous lamprey. Our results indicate ionoregulatory differences that are consistent with relaxed selection on traits for SW entry and positive selection on FW traits in landlocked populations, suggestive of a recent Great Lakes invasion.