Locomotor Activity and Concentration of Thyroid Hormones in Migratory and Sedentary Juvenile American Eels

Abstract

We compared spontaneous locomotor activity and thyroidal activity between juvenile yellow American eels Anguilla rostrata caught climbing waterfalls (migratory) and juvenile yellow American eels captured in an estuary (sedentary). Migratory fish were three times more active than sedentary fish during the first day of the experiment, both in natural light–dark conditions and in constant darkness. Migratory fish also had a mean concentration of plasma thyroxine (21.6 ng/mL) twice as high as that of sedentary fish (9.9 ng/mL). Mean concentrations of plasma triiodothyronine differed little between migratory fish (4.3 ng/mL) and sedentary fish (4.8 ng/mL). These results suggest that the thyroid gland is associated with migration of juvenile yellow American eels because no other life history transition that could involve the thyroid was occurring in the migratory individuals.