Development of thyroid abnormalities in mummichogs, Fundulus heteroclitus, from a polluted site

Previous studies showed that adult and 6-month-old young-of-year Fundulus heteroclitus from a polluted site have enlarged thyroid follicles and (in adults) higher thyroxine levels compared to fish from an unpolluted reference site. This studyʹs purpose was to discover when in development these differences begin to appear. For 15–20 mm fish, the average follicle size of the polluted population was significantly larger than that of the reference population, similar to the pattern found in older fish. However, for the smaller size (12–14.5 mm), the average follicle size of fish from the polluted site was significantly smaller. Although not statistically significant, radioimmunoassay measurements of thyroxine levels were consistent with follicle differences: in larger sizes (20–25 and 15–20 mm) fish from the polluted site had higher thyroxine levels but the smaller size (12–14.5 mm) had lower levels. Therefore, the thyroid abnormalities seen in adults begin when the fish are approximately 15–20 mm long. However, the follicle size reversal for the 12–14.5 mm range may also indicate abnormality at that developmental stage.