Bull. Environ. Contam. Toxicol. (1989) 43:850-857 9 1989 Springer-Verlag New York Inc. Environmental ~Contamination land Toxicology Effects of Chromium and Cadmium upon Respiration and Survival of Callinectes similis Patricia Ramfrez, 1 Guadalupe Barrera, 1 and Carlos Rosas 2 Laboratorio de Contaminaci6n, Bioensayos e Impacto Ambiental, Departamento de Hidrobiologia, Universidad Aut6noma Metropolitana Iztapalapa, Apartado Postal 55-535, 09340 Mexico, D.F. and Laboratorio de Ecofisiologfa, Departamento de Biologia, Facultad de Ciencias, Universidad Nacional Aut6noma de Mexico, 04510 M~xico, D.F. Studies using toxicity tests for determining the effects of heavy metals on aquatic organisms have traditionally been carried out under controlled laboratory conditions. In these tests, the organisms are studied in artificial environments, where the hardness, alkalinity and pH or salinity of the water are controlled. However, the results thus obtained are of limited value in establishing criteria for limiting the discharge of these contaminants. One method which produces more realistic results is the performance of tests under similar conditions to those existing in the organism's original habitat. Different forms in which such studies have been carried out include the analysis of the accumulation of metals over a period exceeding two years (Amiard et al. 1986), the evaluation of the growth and reproductive processes of the population (Peternac and Legovi~ 1986), and the study of changes occurring in the structure of the community being studied (Weber 1981). However, due to the fact that studies based on the calculation of the indices of alteration may be excessively time consuming and expensive, the in situ measurement of the organisms metabolic rate has also been proposed as a possible way of determining the effects of heavy metal contamination (Weber 1981). Although in Tamiahua laggon (in the state of Veracruz, Mexico) never have been registered mortality of organisms which can be associated with heavy metal pollution, the actual total concentrations in water are 246.6 ug Pb/L, 50.6 ug Cr/L and 35.0 ug Cd/L, concentrations which are above the established limits by Mexican Legislation (Rosas et al. 1988). In the southern zone of this coastal lagoon (the zone of marine influence), one may find large populations of crabs Callinectes similis, in which levels of certain heavy metal accumulation have been detected. These crabs are currently fished commercially for human consumption under conditions lacking adequate sanitary control. For the above reasons, and for our conviction that field laboratory tests are more adequate for establishing safety criteria related to heavy metal contamination, we chose to orient this study on determination of the effects of lethal concentrations of Cd and Cr +6 850