Effects of Cd and Zn on oxygen consumption and ammonia excretion in sipuncula (Phascolosoma esculenta)

Physiological responses (oxygen consumption and ammonia excretion) of the sipuncula (Phascolosoma esculenta) exposed to four concentrations of Cd (0.45, 0.96, 2.04, and 4.46 mg L−1) and four concentrations of Zn (1.09, 2.34, 4.96, and 10.91 mg L−1) were monitored for 21 days, respectively. Oxygen consumption rates of sipuncula at all concentrations of Cd decreased from day 1 to day 6. At low concentrations of Cd (0.45 and 0.96 mg L−1), the oxygen consumption rate was promoted. Time and concentration were significant in affecting oxygen consumption rate, respectively. Oxygen consumption rate decreased significantly with time for Zn-exposed individuals and also decreased significantly with the interaction between the concentration of Cd and time for Cd-exposed individuals. Changes occurred in the ammonia excretion rates and O:N ratios with individual sipuncula experiencing different metal concentrations over time. Although low O:N ratios (<30) were obtained in most of the treatments, no predictable correlation was found between concentrations of metals and values of O:N obtained. The value of using O:N ratio as a stress index is questioned.