Extraocular Photoreception Controls nocturnal hatching via a neuroendocrine pathway in False Clownfish

Hatching is the transition from embryo to larva in oviparous animals. Birds, reptiles, and insects rupture the eggshell mechanically, whereas fish and amphibians digest the egg envelope using hatching enzymes. When and where larvae emerge influences their fitness. Many species regulate hatching timing using environmental cues, such as light, oxygen availability, and physical stimuli. In fish, hatching is driven by a neuroendocrine event that triggers hatching enzyme secretion; however, the mechanisms controlling this event are unknown. Using the false clownfish Amphiprion ocellaris, which hatches synchronously within 1-2 h after sunset, we showed that the role of thyrotropin-releasing hormone (TRH) as a hatching hormone, previously established in zebrafish and medaka, extended to clownfish, where TRH activated Ca2+; signaling in hatching gland cells through its receptor trhra, reinforcing the TRH-Ca2+-hatching enzyme axis as a conserved core module of teleost hatching. In clownfish, this module was additionally placed under upstream control by extraocular photoreception because dark-induced hatching persisted in eyeless embryos. This photoreceptive system was most sensitive to green light (550 nm), and several opsins with absorbance maxima in the corresponding middle- to long-wavelength range were expressed outside the eyes of pre-hatching embryos. Coupling extraocular photoreception to a conserved neuroendocrine module placed a critical life-history transition under stringent photic control, releasing larvae only in complete darkness, an adaptation that may have underpinned the success of this lineage in predator-rich coral reefs.