The fish pituitary directly responds to daylength and drives seasonality.

Seasonal timing is important for many critical life history events of organisms, and annual changes in daylength provide a reliable seasonal cue. In birds and mammals, photoperiod-driven seasonality is caused by changes in pituitary thyroid stimulating hormone (TSH), brain deiodinase (DIO) and triiodothyronine (T₃), but it is unclear if a similar mechanism exists in fish. We used anadromous Atlantic salmon that migrate downstream and enter the ocean in spring to examine photoperiod signaling in fish. Manipulations of diurnal and circannual photoperiod cycles indicate that pituitary tshβb and brain dio2b transcription are long day signals that precede endocrine and physiological changes necessary for seawater entry. Exposure to long days caused increased dio2b transcription and T₃ levels throughout the brain, and intracerebral T₃ treatment resulted in elevated levels of plasma GH that drives migration physiology. Significant ambient light levels reach the pituitary, and transcription of non-visual opsins were found in the pituitary. The isolated (in vitro) pituitary directly responds to increased daylength with elevated tshβb transcription, the first time this has been shown in any animal. The results indicate that photoperiod-driven seasonality in fish is driven by direct light stimulation of the pituitary, expanding the known mechanisms of photoperiod responses in vertebrates.