Developmental changes of thyroid hormones in sheepshead minnow, Cyprinodon variegatus

The sheepshead minnow is widely used in ecotoxicological studies and such investigations have begun to focus on potential disruption of the thyroid axis. However, normal levels of thyroxin (T4) and 3,5,3’-triiodothyronine (T3) and their developmental patterns are unknown. This study set out to determine the profiles of whole-body thyroxin (T4) and 3,5,3'-triiodothyronine (T3) levels during the development of sheepshead minnow from embryo to juvenile and adults. Couples of three females and two males were placed in breeding chambers designed for this experiment. More than 1000 eggs were collected and maintained in seawater. Embryos were selected under a dissection microscope and placed in incubation dishes (50 per dish) at 25°C. On day 8, embryos hatched and larvae were transferred to 1L beakers. For one week after hatching, larvae were fed on artemias and from 8 to 30 days post-hatch they were fed on flaked fish food. Embryos were sampled on day 0, 2, 4, 6 post-fertilization and larvae and juveniles were sampled every three days from day 0 to 28 days post-hatch. The pooled samples were taken from several incubation dishes and divided in three replicate batches of 30 individuals. Enzyme-linked immunoassay were used and validated for analysis of T4 and T3 after extraction from whole fish. At each sampling point 5 individuals were placed in formalin fixative for histology. Length and body mass were measured. Hatching success, gross in vivo observations, thyroid hormone levels and histology data will be determined and discussed in the framework to characterize the profiles of thyroid hormone levels during the development of sheepshead minnow from embryo to adult. This study establishes a baseline for thyroid hormones in sheepshead minnows, which will be vital for the understanding of thyroid hormone functions and in future studies of thyroid toxicants in this species