Using 15N to determine the metabolic fate of dietary nitrogen in North Pacific spiny dogfish (Squalus acanthias suckleyi).

Marine elasmobranchs are ureosmotic, retaining large concentrations of urea to balance their internal osmotic pressure with that of the external marine environment. The synthesis of urea requires the intake of exogenous nitrogen to maintain whole-body nitrogen balance and satisfy obligatory osmoregulatory and somatic processes. We hypothesized that dietary nitrogen may be directed toward the synthesis of specific nitrogenous molecules in post-fed animals; specifically, we predicted preferential accumulation and retention of labelled-nitrogen be directed towards the synthesis of urea necessary for osmoregulatory purposes. North Pacific spiny dogfish (Squalus acanthias suckleyi) were fed a single meal of 7 mM 15NH4Cl in a 2% ration by-body-mass of herring-slurry via gavage. Dietary labelled-nitrogen was tracked from ingestion to tissue-incorporation and synthesis of nitrogenous compounds (urea, glutamine, bulk amino acids, protein) in the intestinal spiral valve, plasma, liver, and muscle. Within 20 h post-feeding, we found labelled-nitrogen was incorporated into all tissues examined. The highest 𝛿15N values were seen in the anterior region of the spiral valve at 20 h post-feeding, suggesting this region was particularly important in assimilating the dietary labelled-nitrogen. In all tissues examined, enrichment of the nitrogenous compounds was sustained throughout the 168 h experimental period, highlighting the ability of these animals to retain and use dietary nitrogen for both osmoregulatory and somatic processes.