Effects of zinc and mercury on the white shrimp, Penaeus setiferus

Methodologies were developed and utilized to assess effects of zinc and mercury (singly and in combination) on the white shrimp, Penaeus setiferus. The importance of modifying variables (such as temperature, weight, molting, sex, and season) were assessed in bioassay techniques which utilized measurements of locomotor activity, mortality, and oxygen consumption. Four-day mortality results (the number or percentage dead) allowed reasonable conclusions concerning zinc and mercury toxicity, but other methods of expressing data yielded additional information. Expressing mortality (time to death for individuals) as the independent variable in a regression equation was the preferred technique. Several positive interaction effects (synergism) were statistically significant: zinc x mercury, temperature x zinc, temperature x season of the year, temperature x zinc x season of the year. Mortality rates of large animals exceeded those of small animals in bioassays utilizing low zinc and mercury concentrations (2.03 ppm Zn and 37 ppb Hg). No significant weight effect was found in acute bioassays employing up to 40.6 ppm Zn and 148 ppb Hg. Mortality rates were greater at 30°C than at 22°C, and the process of molting significantly affected death rates of shrimp exposed to a range of zinc and mercury concentrations. Shrimp detected and responded (by increased locomotor activity) to zinc and mercury concentrations that were well below acutely toxic levels. The effects of zinc and mercury were less than additive when assessed by the locomotor response. Isolation of the effect of locomotor activity on oxygen consumption permitted the respiratory effects of zinc and mercury to be investigated. Average oxygen consumption rates decreased (10%) in mercury-treated shrimp and increased (25%) in zinc-treated shrimp after a three-week exposure to 37 ppb Hg and 2.03 ppm Zn. However, the individual responses were quite variable and the changes in oxygen consumption rates were not statistically significant. The relationship between oxygen consumption and weight in P. setiferus was found to be similar to that reported in the literature for other shrimps