Dynamic Selenium Assimilation, Distribution, Efflux, and Maternal Transfer in Japanese Medaka Fed a Diet of Se-enriched Mayflies

Selenium (Se) trafficking in oviparous species remains understudied and a major source of uncertainty in developing sound Se regulations. Here, we utilized Se-75 to follow Se through a simulated natural food chain (water, periphyton, mayflies (Centroptilum triangulifer), fish (Japanese medaka)). We specifically examined Se assimilation efficiency, tissue distribution, efflux rate, and maternal transfer in medaka. Selenium assimilation efficiency (AE) averaged 63.2 +/- 8.8% from mayfly diets and was not affected by mayfly [Se] across a dietary range of 5.6-38.7 mu g g(-1) (dry wt). However, AE decreased significantly as mayfly larva size increased. Efflux rate constants (k(c)) were consistent between reproductively inactive (0.066 d(-1)) and spawning females (0.069 d(-1)). Total Se loss rate constant (k(c+egg); efflux and egg deposition) was 0.17 d(-1) in spawning females. Interestingly, medaka appeared to rapidly shuttle Se to their eggs directly from their diet via the ovary, as opposed to mobilization from surrounding tissues, resulting in dynamic egg [Se] that was more attributable to recent dietary Se ingestion than female whole body [Se] in this asynchronous spawning fish. Spawning strategy likely plays a large role in the process of fish egg Se deposition and requires further attention to understand risk and toxicity of Se to fish.