Researchers crack the code of how fish embryos actively control their hatch timing

Dr. Matan Golan of the Hebrew University of Jerusalem and the Agricultural Research Organization—Volcani Institute led a team of researchers who uncovered how fish embryos determine the ideal moment to hatch.

Their discovery, published in Science, reveals an active mechanism by which these embryos control a critical life-or-death decision, offering new insights into biology and evolution.

The process of hatching is a pivotal moment for all egg-laying species. Emerging too soon or waiting too long can mean certain death for a newborn animal, unprepared to navigate the challenges of the outside world. From breathing to evading predators, survival hinges on perfect timing. Remarkably, this timing is dictated by the embryo itself—but until now, the mechanism behind this decision remained unknown.

The researchers discovered that fish embryos initiate hatching through a signal from their brain: a neurohormone called Thyrotropin-Releasing Hormone (TRH).

TRH travels via the bloodstream to a specialized gland, triggering the release of enzymes that dissolve the egg wall, allowing the embryo to break free. This critical neural circuit for hatching forms just before the event and disappears shortly thereafter. Without TRH, embryos are unable to release the enzymes, resulting in their death inside the egg.

This discovery uncovers a previously hidden neural circuit that governs one of the most crucial life-stage transitions and demonstrates how fish embryos, far from being passive, possess the ability to actively control their own hatching process, a key to their survival.

The findings have significant evolutionary implications, as they reveal the long-sought neuronal mechanism controlling hatching in the largest group of living vertebrates. Looking ahead, the researchers plan to explore how TRH and other neuroendocrine factors influence hatching in other species.

In addition to its evolutionary insights, this research underscores the remarkable ability of embryos to make decisions that directly affect their survival, offering a deeper understanding of the intricate interplay between neurobiology and environmental adaptation.