For intact daphnids reared in circumstances of controlled trace element exposure, one consequence of insufficient zinc (Zn) is an increased demand on the animal's pool of available selenium (Se). This demand is manifested by the type of cuticle deterioration associated with Se deficiency and by a depression of reproduction. In the presence of 25 parts per billion (ppb) Zn, 1 ppb Se eliminates these symptoms. In the absence of detectable Zn, 5 ppb Se eliminates overt cuticle damage and substantially increases reproduction. A shortening of life span resulting from Zn deprivation is not ameliorated by Se addition. We suggest that the interplay between Zn and Se concentrations reflects an underlying interplay between interdependent, but distinct, metabolic pathways; i.e., (for Se) glutathione peroxidase and (for Zn) Cu,Zn-superoxide dismutase-each offering protection against free radical damage. Because they are not necessarily localized in a given tissue, the key to recognition of such subtle, complex trace nutrient interactions has been use of intact animals in circumstances of control previously attainable only in tissue cultures.Little specific information concerning the trace element requirements of zooplankters exists. For example, although more than 100 eukaryotic enzymes have been shown to depend on zinc (Zn) (1), no certain proof that Zn is required by a Cladoceran has been published. This reflects more the limitations of the state of the art in animal culture than an absence of common eukaryotic enzymes in zooplankters. Extensive observation of healthy animals under explicit chemical conditions is necessary for determination of trace element requirements, and such observation has not been possible with zooplankters until recently (2).The significance of trace elements in the structuring of aquatic communities has long been acknowledged (3, 4). Yet, in the absence of secure data, only reasonable speculation about the requirements of a specific organism for a specific trace element or the effects on community structure of such requirements has been possible. There is great diversity among the trace element matrices found in various fresh waters. Of particular interest have been the influences on community structure of idiosyncratically low or high concentrations of single elements (5) or of sets of elements (6). As phytoplankton culture became more sophisticated and suitable chelators (7) fostered more qualitative and quantitative control of trace elements in cultures, absolute requirements for most of the trace elements currently identified as essential by nutritionists were recognized as essential for various phytoplankters (8). Zooplankton work has lagged behind.Reflecting the generalized absence of basic proofs of trace element essentiality for zooplankters, our original intent had been simply to withdraw Zn from both the food and ambient water and to produce symptoms of severe deficiency for the first time in a daphnid culture. However, a complex response, involving an induced selenium (Se) defi...

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