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

Life histories of anadromous and landlocked sea lamprey are similar, though landlocked populations lack seawater exposure, likely experiencing relaxed selection on SW survival traits. This study investigated seawater osmoregulation in juvenile 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 seawater was high in anadromous lamprey (90%) and highly variable among landlocked populations (40-100%). Plasma ions levels were higher and hematocrit was lower after seawater exposure in landlocked compared to anadromous lamprey. Freshwater gill ion transporter (H+-ATPase; Na+:Cl- cotransporter) mRNA levels were higher in freshwater and remained high after seawater exposure in landlocked relative to anadromous juveniles. Landlocked lamprey had 24-33% higher gill Na+:K+:2Cl- cotransporter abundance after seawater exposure compared to anadromous lamprey. Our results indicate ionoregulatory differences that are consistent with relaxed selection on traits for seawater entry and positive selection on freshwater traits in landlocked populations, suggestive of a recent Great Lakes invasion.