Fish Can Choose Their Own Birth Timing—Here’s How

Fish embryos in many species can control their hatching timing, effectively selecting their own birthdays. A study from Hebrew University of Jerusalem has uncovered the chemical and biological processes enabling this precise control, showing how embryos align their hatching with optimal environmental conditions.

Researchers focused on zebrafish (Danio rerio) and discovered that the thyrotropin-releasing hormone (Trh) plays a pivotal role. Released by the embryo, Trh triggers the production of enzymes that dissolve the egg wall, allowing hatching to occur. “Hatching is a critical event in the life history of oviparous species,” the researchers explain. “Timing it with favorable conditions improves survival in early life stages.“

Different fish species use unique strategies for hatching. Zebrafish typically hatch at daylight, clownfish and halibut prefer darkness, and California grunion wait for ocean tides. In zebrafish, Trh is delivered to the hatching gland via a neural circuit that forms just before hatching and disappears shortly after.

Evolutionary Conservation of Trh-Based Hatching Mechanisms Across Species

The researchers also examined medaka (Oryzias latipes), a distantly related species, finding the same Trh-based hatching process despite differences in their biology. This suggests that the mechanism is evolutionarily conserved, even after 200 million years of divergence. Using immunostaining techniques, they observed transient Trh circuits in both species, which vanish after hatching.

While in humans and mammals Trh regulates key functions like heart and metabolic rates, its role in fish hatching highlights its evolutionary adaptability. Understanding these mechanisms further could provide insights into other aquatic species with varied hatching strategies.

Looking ahead, scientists aim to explore how climate change might affect this delicate timing. As temperatures rise, understanding how embryos adapt their hatching decisions—fine-tuned over millions of years—will be crucial for preserving species.

“It would be fascinating to test how conserved Trh’s role is across species and study variations in hatching circuits among those with different strategies,” the researchers note.

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