Ionoregulatory changes during metamorphosis and salinity exposure of juvenile sea lamprey (Petromyzon marinus L.)

Ammocoetes of the anadromous sea lamprey Petromyzon marinus L. spend many years in freshwater before metamorphosing and migrating to sea. Metamorphosis involves the radical transformation from a substrate-dwelling, filter feeder into a frees-swimming, parasitic feeder. In the present work we examined osmoregulatory differences between ammocoetes and transformers ( metamorphic juveniles), and the effects of salinity acclimation. We measured the expression of key ion-transporting proteins [Na+/ K+-ATPase, vacuolar (V)-type H+-ATPase and carbonic anhydrase (CA)] as well as a number of relevant blood parameters (hematocrit, [Na+] and [Cl-]). In addition, immunofluorescence microscopy was used to identify and characterize the distributions of Na+/ K+-ATPase, V-type H+-ATPase and CA immunoreactive cells in the gill. Ammocoetes did not survive in the experiments with salinities greater than 10%, whereas survival in high salinity ( >= 25-35%) increased with increased degree of metamorphosis in transformers. Plasma [ Na+] and [ Cl-] of ammocoetes in freshwater was lower than transformers and increased markedly at 10%. In transformers, plasma ions increased only at high salinity (> 25%). Branchial Na+/ K+- ATPase levels were similar to tenfold higher in transformers compared to ammocoetes and salinity did not affect expression in either group. However, branchial H+- ATPase expression showed a negative correlation with salinity in both groups. Na+/ K+- ATPase immunoreactivity was strongest in transformers and associated with clusters of cells in the interlamellar spaces. H+-ATPase (B subunit) immunoreactivity was localized to epithelial cells not expressing high Na+/K+-ATPase immunoreactivity and having a similar tissue distribution as carbonic anhydrase. The results indicate that branchial Na+/ K+-ATPase and salinity tolerance increase in metamorphosing lampreys, and that branchial H+- ATPase is downregulated by salinity.