Patterns of nitrogenous waste excretion and gill urea transporter mRNA expression in various species of marine fish

Many prior studies of nitrogenous waste excretion in marine fish have examined excretion patterns for short time periods, and with relatively coarse sampling schemes (e.g., an initial and a final sample point). Recent studies of a ureotelic marine fish (the gulf toadfish, Opsanus beta) have demonstrated that urea excretion in this species occurs in brief but massive bursts, lasting from 0.5 to 3 h, and often only once per day. The present study sought to determine if prior sampling protocols may have underestimated the amount of urea being excreted by marine fish. A survey of 16 marine species (the teleosts: Myoxocephalus octodecemspinosus, Scophthalamus aquosa, Cyclopterus lumpus, Lophius americanus, Aprodon cortezianus, Cymatogaster aggregatus, Parophrys vetulis, Microstomus pacificus, Hippoglossoides elassodon, Bathyagonus nigripinnus, Ophiodon elongatus, Hemilepidatus spinosus, Icelinus terrius; the elasmobranch: Raja rhina; and the hagfish: Eptatretus stoutii) was undertaken for ammonia-N and urea-N excretion using a long sampling period (48 h) and hourly sample collection. Apart from the obvious exception of an elasmobranch, ammonia excretion was confirmed to be predominant in marine fish, with urea excretion constituting between 1.4 and 23.8% of the total of ammonia plus urea excreted. Notably, no pulses of urea excretion were detected. Despite the relatively low level of urea excretion, expression of urea transporter-like mRNA (detected using the toadfish urea transporter, tUT, cDNA as a probe) was discovered in gills of many of the species surveyed for nitrogen excretion patterns, although no signal was detected in the hagfish. These results suggest that urea excretion takes place through a specific transport pathway. Finally, more detailed analysis of nitrogen excretion in one of the surveyed species, the plainfin midshipman (Porichthys notatus) demonstrates that "total" nitrogen excretion estimated by summing ammonia and urea excretion underestimates true total nitrogen excretion by 37-51%