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

Abstract

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.