Divergent Pathways of Ammonia and Urea Production and Excretion during the Life Cycle of the Sea Lamprey.

Little is known about nitrogenous waste (N waste) handling and excretion (J_{N waste}) during the complex life cycle of the sea lamprey (Petromyzon marinus), an extant jawless fish that undergoes a complete metamorphosis from a filter-feeding larva (ammocoete) into a parasitic juvenile that feeds on the blood of larger, jawed fishes. Here, we investigate the ammonia- and urea-handling profiles of sea lampreys before, during, and after metamorphosis. The rates of ammonia excretion (J_amm) and urea excretion (J_urea) significantly decreased after the onset of metamorphosis, with the lowest rates observed during midmetamorphosis. Near the completion of metamorphosis, rates of J_{N waste} ($J_{\mathrm{N waste}}=J_{a\mathrm{mm}}+J_{u\mathrm{rea}}$) significantly increased as sea lampreys entered the juvenile period. Feeding juvenile lampreys had greater than 10- to 15-fold higher J_amm and fivefold higher J_urea compared to nonfed juveniles, which corresponded to higher postprandial (postfeeding) concentrations of plasma ammonia and urea. The routes of J_amm and J_urea completely diverged following metamorphosis. In larvae, J_amm was equally split between branchial (gills) and extrabranchial (skin plus renal) pathways, but following metamorphosis, >80% of ammonia was excreted via the gills in nonfeeding juvenile lampreys, and >95% of ammonia was excreted via the gills in adult sea lampreys. Urea, on the other hand, was predominantly excreted via extrabranchial routes and, to a lesser extent, the gills in larvae and in nonfeeding juveniles. In adults, however, virtually all urea was excreted via urine. Reverse transcription polymerase chain reaction and in silico analyses also indicated that a urea transporter encoded by a slc4a2-like gene is present in lampreys. The branchial expression of this transporter is modulated throughout sea lamprey life history, as it is higher in the larvae and steadily decreases until the adult stage. We conclude that the divergent pathways of J_amm and J_urea during the sea lamprey life cycle reflect changes in their habitat, lifestyle, and diet. Further, the near-complete reliance on renal routes for J_urea in adult sea lampreys is unique among fishes and may reflect the ancestral condition of how this N waste product was handled and excreted by the earliest vertebrates.