Mummichogs (Fundulus heteroclitus) from a polluted site, Piles Creek (PC), differ in behavior and thyroid status from fish collected from a reference site, Tuckerton (TK). This dissertation explores when thyroid differences appear in the developing fish, whether thyroid differences could cause behavior differences, and whether exposure to lead could cause thyroid follicle size differences.

To find out when thyroid differences appear, thyroid follicles were measured histologically and thyroid hormone levels by radioimmunoassay. PC mummichogs develop oversized thyroid follicles in their first summer of life, at 15-20 mm long. At 20-30 mm long, their T3/T4 ratio is significantly lower than that of TK fish, reflecting the follicle differences, although their hormone levels are not significantly different. The changes in thyroid status of PC fish occur after behavioral differences have already appeared and therefore cannot have caused them. However, the significant difference in T4 found in unfertilized eggs may influence early behavioral differences seen in these two populations.

To investigate the effects of thyroid status on behavior, yearlings and young-of-year from both populations were exposed to T3 and propylthiouracil (thyroid suppressor). Spontaneous activity and prey capture ability were measured. T3 exposure produced the expected increase in spontaneous activity for TK fish, but the increase was not consistent among tests, ages, and exposure times. T3 exposure did not produce significant differences in PC fish. Propylthiouracil did not produce as strong a response in spontaneous activity as T3 exposure, and the response was opposite of the expected decrease: activity increased in both populations, although the TK response was more consistent. Therefore, both thyroid hormone and thyroid suppressor were associated with behavioral changes similar to differences in the field populations, but responses were not entirely predictable and TK fish responded more often as expected.

PC sediments have high concentrations of lead, which has been shown to affect thyroid in fish. To examine the effects of lead, lab-raised mummichogs were exposed to four concentrations of lead for three months beginning post-hatch. Thyroid follicle sizes and follicle number were unaffected at concentrations that previously produced behavioral changes. However, severe skeletal deformities developed which correlated with lead concentration.