The mechanisms of vertebrate seasonal time measurement were a mystery for a long time, but recent comparative studies have uncovered the photoperiodic signal transduction cascades in birds, mammals, and fish. These studies reveal the universality and diversity of photoperiodic mechanisms. That is, the molecules involved are conserved, while the tissues responsible for these mechanisms are different in different species (Nakane and Yoshimura, Front Neurosci 8:115, 2014). It is well established that the circadian clock is involved in photoperiodic time measurement. However, the underlying mechanism that defines the photoinducible phase or critical photoperiod (i.e., how organisms measure day length using a circadian clock) is at the heart of photoperiodic time measurement, and this question remains to be answered by future studies.
Ikegami, K., & Yoshimura, T. (2017). Molecular mechanism regulating seasonality. In Biological Timekeeping: Clocks, Rhythms and Behaviour (pp. 589–605). Springer (India) Private Ltd. https://doi.org/10.1007/978-81-322-3688-7_28