1994 Environmental Toxicology and Chemistry, Vol. 24, No. 8, pp. 1994–2001, 2005  2005 SETAC Printed in the USA 0730-7268/05 $12.00 + .00 METAMORPHOSIS OF TWO AMPHIBIAN SPECIES AFTER CHRONIC CADMIUM EXPOSURE IN OUTDOOR AQUATIC MESOCOSMS STACY M. JAMES,*² E DWARD E. LITTLE,‡ and RAYMOND D. SEMLITSCH² ²Division of Biological Sciences, 105 Tucker Hall, University of Missouri-Columbia, Columbia, Missouri 65211-7400, USA ‡U.S. Geological Survey Columbia Environmental Research Center, 4200 New Haven Road, Columbia, Missouri 65201-8709 ( Received 9 November 2004; Accepted 31 January 2005) Abstract—Amphibian larvae at contaminated sites may experience an alteration of metamorphic traits and survival compared to amphibians in uncontaminated conditions. Effects of chronic cadmium (Cd) exposure on the metamorphosis of American toads (Bufo americanus) and southern leopard frogs (Rana sphenocephala) were determined. The two species were reared separately from shortly after hatching through metamorphosis in outdoor mesocosms (1,325-L polyethylene cattle tanks) that simulated natural ponds and enhanced environmental realism relative to the laboratory. Both species exhibited a decrease in survival with increasing initial nominal aqueous Cd concentration. Cadmium treatment did not influence mass at metamorphosis for either species when survival was included as a covariate, but increased the age at metamorphosis for the American toads. The whole body Cd content of metamorphs increased with aqueous Cd treatment level for both species, and the American toads tended to possess more elevated residues. Cadmium quickly partitioned out of the water column and accumulated in and altered the abundance of the tadpoles’ diet. Cadmium-contaminated sites may produce fewer metamorphs, and those that survive will metamorphose later and contain Cd. Interspecific differences in the response variables illustrate the importance of testing multiple species when assessing risk. Keywords—Bufo americanus Cadmium Mesocosm Metamorphosis Rana sphenocephala INTRODUCTION Chemicals potentially harmful to amphibians have been found in the most remote montane lakes to highly urban streams. Most amphibians spend their embryonic and larval periods in aquatic habitats before metamorphosing into the terrestrial environment. Characteristics of the breeding site can have important effects on metamorphosis and early life-stage survival, and aquatic contamination is believed to have re- duced amphibian abundance and species richness at some field sites [1,2]. Amphibians have been found in wastewater treat- ment wetlands, farm ponds, mining sites, and countless other contaminated habitats. Amphibians are unlikely to escape en- vironmental degradation because they have high breeding-site fidelity and low mobility. Laboratory tests have been crucial for understanding direct effects of contaminants on amphibians. However, these studies rarely examine indirect effects [3] and may greatly overesti- mate [4] or underestimate [5] responses that would occur in natural conditions because they tend to be acute (usually less than 7 d), maintained at a constant exposure concentration, and involve a single treatment variable (contaminant concen- tration) and route of exposure (across the skin or gills from water). Mortality can increase significantly when the exposure is chronic [5] or an additional stressor is present [6]. Dietary uptake of contaminants also can be important, but is difficult to manipulate realistically in the laboratory. Alternatives to traditional testing are needed to determine more accurately amphibian responses in contaminated habitats and routes of uptake. Mesocosms such as cattle tanks provide the benefits of a more realistic aquatic environment while still maintaining relatively controlled conditions [7]. Numerous investigators * To whom correspondence may be addressed (sjames@usgs.gov). have used cattle tanks to study amphibian ecology [7,8] and ecotoxicology [3,7]. The addition of litter (dead leaves or grass), plankton, and periphyton to cattle tanks subject to nat- ural climatic conditions allows contaminants to partition into several mediums and incorporates environmental complexity and fluctuation. Contamination may act both directly on larval amphibians through dermal and oral uptake, and indirectly by altering the aquatic community [3]. Unfortunately, the inter- pretation of cattle tank study results can be more difficult than those obtained in the laboratory. One of the most toxic metals amphibians encounter is cad- mium. Aquatic habitats become polluted with Cd from terres- trial runoff, aerial deposition, and the release of effluent di- rectly into water bodies. The chronic criterion for Cd as set by the U.S. Environmental Protection Agency is 0.15 g/L (at 50 mg/L hardness; [9]), but concentrations may exceed 200 g/L in some areas [10]. Cadmium is more bioavailable in soft water [11], so species such as amphibians that breed in rain-fed pools may be at increased risk. Larval amphibians reared in Cd-contaminated water can experience reduced growth [11–13] and survival [11–14]. However, Cd also can have hormetic effects [5]. Significant Cd uptake by tadpoles from water can occur within 24 h [15] and may increase with the length of exposure [15] and aqueous Cd concentration [12,13]. Frog tadpoles reared outdoors in small aquatic me- socosms containing leaf litter, sand, algae, and zooplankton dosed once with 100 g Cd/L contained an average of 2.2 g Cd/g whole body wet weight after three weeks, and only 25% survived to metamorphosis [14]. Larval amphibians collected from contaminated field sites have possessed in excess of 13 g Cd/g whole body dry mass [16], and those exposed in the laboratory can withstand at least 60 g Cd/g [13]. Cadmium added to the water column can partition into periphyton and plankton [17], polluting and altering the food