A fish organ, called the saccus vasculosus works as internal clock to detect seasonal differences in day length, reports research in Nature Communications.
In mammals, the sun light is captured by the eyes and the information transmitted to the part tuberalis of the pituitary gland, which relays seasonal information to the brain. This gland acts as a seasonal timer, letting the brain know whether it is day or night, differentiating short from long days, and hence capable to distinguish between seasons. As fish do not have a part tuberalis it has been unclear whether they possess an organ able of function as seasonal sensor.
Takashi Yosimura and colleagues demonstrate the clock function of the saccus vasculosus in a study in masu Salmon by detecting the capacity of this organ to respond to photoperiodic signals, such as variations in day length. They observe the expression of known genes regulating seasonal reproduction, and known light receptors similar to those found in eyes or the pineal gland. They also observe that isolated saccus vasculosus respond to changes in light, and that fish without saccus vasculosus lose their ability to respond to light changes.