Nitrogenous waste excretion by the larvae of a phylogenetically ancient vertebrate: the sea lamprey (Petromyzon marinus)

Abstract

Larval sea lampreys (Petromyzon marinus) (ammocoetes) excreted significant quantities of urea, which composed 15-20% of the total nitrogenous waste excreted. Compared with teleosts of similar size, ammonia and urea excretion rates (JAmm and JUrea, respectively) in ammocoetes were relatively low, reflecting the low metabolic rate of these burrow-dwelling suspension feeders. Analyses of liver enzymes indicated that ammocoetes had all the enzymes necessary to produce urea via uricolysis, but not those of the ornithine-urea cycle (OUC). Further, exposure to 2 mmol·L-1 total ammonia for 5 d was accompanied by a 3-fold elevation of JUrea, but did not lead to greater OUC activity. Internal ammonia levels increased markedly, however, exceeding 2000 μmol·L-1 in plasma and 5000 μmol·L-1 in muscle after the 5-d exposure period. This high resistance to internal ammonia accumulation was related to the very high glutamine synthetase activities measured in ammocoete brains. The excretion and production of urea by ammocoetes demonstrates for the first time that agnathans are capable of producing physiologically relevant amounts of urea. Given the ancient origins and conserved evolution of lampreys, these observations also suggest that at least some of the early jawless vertebrates were able to produce and excrete urea.