The effect of thyroid hormone and thyroid hormone blocker on visual pigment shifting in juvenile coho salmon ( Oncorhynchus kisutch)

Abstract

Juvenile coho salmon were exposed to either dim-light (9.1×10 −7 lx)/5°C water or bright-light (2.6×10 −6 lx)/15°C water conditions to affect retinal dominance in porphyropsin or rhodopsin, respectively. The administration of thyroid hormone over a 5-week period resulted in the rhodopsin-dominated retinae of the fish in the bright-light/15°C tanks to shift to porphyropsin dominance whereas the fish in the dim-light/5°C tanks had their porphyropsin-dominated retinae exhibit a small, but, significant increase in the relative proportion of rhodopsin. When methimazole, an anti-thyroid drug, was administered for 4 weeks to fish in dim-light/5°C that were transferred to bright-light/15°C tanks, the shift to rhodopsin dominance was significantly less than in nontreated controls. Similarly, 6 weeks of methimazole treatment of fish in bright-light/15°C that were transferred to dim-light/5°C tanks exhibited significantly less porphyropsin than in nontreated controls. Methimazole treatment to nontransferred fish showed no effect on visual pigment levels. The response of the retinae to thyroid hormones and the impairment of response when treated with thyroid hormone blocker suggests that during smoltification, changes in visual pigment composition may be, in part, attributed to alteration in thyroid function.