GabyScientists at the Hebrew University of Jerusalem have mapped the chemical and biological signals that let some embryos pick the right time to hatch based on environmental cues. So how did they figure it out? The team studied the eggs of the aquarium fish zebrafish (Danio rerio) and found that embryos release thyrotropin-releasing hormone (TRH), which triggers production of enzymes that dissolve the eggshell. “Hatching is the most significant event in the life cycle of species,” the study says.
Fish time their hatching to match conditions that boost survival in early life. Different species use different strategies and triggers: zebrafish typically wait for daylight, while clownfish and halibut prefer darkness. The California grunion times its hatching to waves that wash it into the sea.
What controls that timing? The researchers traced the mechanism. In zebrafish, TRH travels through the bloodstream to the hatching gland, guided by a neural circuit that forms just before hatching and then disappears immediately afterward, Science Alert reported. The team also studied the Japanese rice fish, or medaka (Oryzias latipes), a distant relative of zebrafish. Although their evolutionary lines split about 200 million years ago, both species use TRH to trigger hatching despite differences in hatching glands, enzyme types, and embryonic periods.
In humans and other mammals, TRH helps regulate processes such as heart rate and metabolic rate. The same neurohormone is used differently in fish, which could reflect divergent evolutionary paths. The researchers plan to probe the zebrafish hatching circuit further and compare it with other aquatic species that hatch in different ways.
They also want to know how rising temperatures from global warming might affect when fish choose to hatch—a timing mechanism shaped over hundreds of millions of years. The findings were published in the journal Science.
