The spiny dogfish Squalus acanthias L. maintains osmolyte balance during long-term starvation

Over 56 days of starvation, spiny dogfish Squalus acanthias lost c. 0.3% body mass per day and condition factor and hepato-somatic index declined, but plasma osmolality levels remained higher than seawater levels for the entire period; major osmolytes in the plasma, such as urea, trimethylamine oxide and inorganic ions did not change appreciably. Urea was always the dominant nitrogen waste and was excreted at a constant rate over the 56 day starvation period, suggesting that a minimum rate of urea loss to the environment is unavoidable. Significant amounts of unknown-nitrogen compounds were also excreted at rates higher than that of ammonia. The dogfish can maintain its osmolytes constant despite losing large amounts of nitrogen-rich urea, and therefore maintains plasma hyperosmotic regulation over long-term starvation.