A team of scientists from the Hebrew University of Jerusalem in Israel has uncovered the secrets behind the chemical and biological processes that dictate how certain embryos choose the timing of their emergence into the world, based on optimal environmental factors. So, how did the research unfold? The scientists studied the eggs of the aquarium fish known as zebrafish (Danio rerio). They discovered that the release of thyrotropin-releasing hormone (TRH) from the embryo is crucial for producing enzymes that dissolve the eggshell. “Hatching is the most significant event in the life cycle of species,” the researchers noted in their report.
According to the scientists, fish carefully calculate the timing of their hatching to coincide with favorable conditions that enhance survival during the early stages of life. Different species of fish employ a variety of strategies and triggers for hatching. For instance, zebrafish typically wait for daylight, while clownfish and halibut prefer darkness. The California grunion, on the other hand, waits for the waves to wash it into the sea.
Why does this happen? The authors of the study identified the mechanisms behind this process. In zebrafish, TRH enters the hatching gland through the bloodstream, guided by a neural circuit that forms just before hatching and disappears immediately afterward, as reported by Science Alert. The researchers also examined the Japanese rice fish, or medaka (Oryzias latipes), a distant relative of the zebrafish. Although the evolutionary paths of medaka and zebrafish diverged around 200 million years ago, both species exhibit the same hatching process, triggered by TRH despite differences in hatching glands, types of enzymes, and embryonic periods.
In humans and other mammals, TRH helps regulate key biological processes, including heart rate and metabolic speed. Notably, the same neurohormone is utilized differently in fish, which may also hint at divergent evolutionary paths. Now, the researchers aim to delve deeper into the hatching process in zebrafish, as well as explore the similarities and differences that may exist in other aquatic species with varying hatching approaches.
Global warming is another area for future research. Scientists need to understand how rising temperatures might influence fish decisions about hatching, a mechanism that has evolved over hundreds of millions of years. The findings of this study were published in the journal Science.