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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 ammoco...

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