AbstractReduced growth and reproductive potential are common sublethal stress responses. Bioenergetic alterations as evidenced by scope for growth or O:N ratio measurements are also sensitive sublethal stress responses for certain animals. The O:N ratio, which is the ratio of oxygen consumed to nitrogen excreted in atomic equivalents, reflects the relative utilization of protein in energy metabolism. The purpose of the present study was to determine whether concentration‐dependent alterations in the O:N ratio for mysids(Mysidopsis bahia)could be detected at various times during a life‐cycle toxicity test and to assess the utility of this index for diagnosing bioenergetic responses that result in reduced growth and reproductive inhibition.JuvenileM. bahiawere exposed to cadmium (CdCl2) in a continuous‐flow proportional diluter system. The exposure concentrations were monitored routinely throughout the test and varied less than ±10 from the nominal concentrations. The O:N ratio measurements were made at 4, 10 and 18 d of exposure. Dry weights and cadmium body burdens of individual mysids were made at day 18 and day 33, the termination of the test.Significant alterations in the O:N ratio were evident by 4 d and indicated a shift toward lipid utilization with increasing cadmium concentration. Growth was significantly reduced in the lowest concentration (4 ppb) at 18 d and a linear concentration‐dependent increase in the cadmium body burden was observed at the termination of the test. These results suggest that alterations in the O:N ratio are a sensitive and early diagnostic index for detecting sublethal stress responses. We feel that this approach could be used in place of the standard life‐cycle toxicity test for determining the concentration of a toxicant at which no effect on growth and reproductive processes
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