choriogenins, and estrogen receptor esr1 -but not esr2a -in the liver within 72 hrs, and that these transcriptional changes return to pre-exposure levels within 12 days of the termination of 4-NP or E2 exposure. In sum, these findings validate the use of mRNA levels for several estrogen-responsive genes as accurate biomarkers for short-term 4-NP exposure in the arrow goby. In Chapter 2, I evaluated the effects of 4-NP and E2 exposures on the osmoregulatory ability of C. ios. I exposed adult arrow gobies to 4-NP (10 μg/L or 100 μg/L) or E2 (50 ng/L) for 14 days, and then transferred the fish from a 33 ppt salinity (control) environment to either 20 ppt, or 5 ppt conditions. Whole body water content was then measured, and the relative mRNA levels for the ion channels