| Sacramento splittail, a native cyprinid in the San Francisco Estuary, has declined in numbers and distribution range in the last three decades. Despite the well documented life history of splittail, fragmentary descriptions in its early life stages made it difficult to determine the appropriate stages and endpoints for toxicity tests. Meanwhile, the potential impact of elevated selenium (Se) levels on prey items of splittail has raised serious concerns but no investigations have been conducted to address Se contamination in association with the changes in splittail populations in the estuary. The goals of this study were to systematically document the early life stages of splittail, and to investigate the effects of Se exposure in embryo, juvenile and adult splittail in laboratory conditions.; The early development of splittail from the onset of fertilization to metamorphosis was characterized following the staging system for zebrafish (Chapter 1). Splittail have meroblastic cleavage and developmental stages generally similar to those of other cyprinids (e.g., zebrafish). Splittail's developmental rate, however, differs from zebrafish primarily due to differences in definitive somite numbers and rate of somitogenesis.; One-day old splittail embryos at 11-somite stage were exposed to 0.0, 5.0, and 15.0 mg 1-1 sodium selenite solutions for 24-h under static conditions (Appendix B). The result showed that a short term exposure to selenite solutions can significantly induce musculoskeletal deformities, and a dose range between 0--5 mg 1-1 sodium selenite may be appropriate for a future exposure test. In the juvenile (Chapter 2) and adult splittail (Chapter 3), dietary exposure is the major route of Se intake. The 9-month chronic dietary exposure of juveniles showed that the dietary Se concentrations ≥26.0 mug g-1 induced mortalities, liver lesions, and inhibited growth. The result suggests that chronic exposure to 6.6 mug Se g-1 diet could reduce the fitness and survival of the young. Splittail adults appear to be more tolerant to high dietary Se concentrations compared to juveniles. No mortality was observed in treatment groups fed diets with Se levels at 17.3, 33.0, and 70.1mug g-1) during the 24 week exposure, and only growth in weight was affected in the highest treatment group. The tissue Se distribution was similar to other fish with the highest Se concentration in kidney, followed by liver, ovary, muscle and plasma. Perhaps the most significant finding in this study was that Se concentrations above 17.3 mug g -1 in food and 6.5 mug g-1 in ovaries could potentially induce atresia in adult splittail, which could significantly reduce the recruitments. This finding suggests that a similar Se level (15.0 mug g-1) in the invasive overbite clam which is an important food item of splittail may raise serious concerns on current conditions of wild splittail populations in the San Francisco Estuary. |
