Head stabilization behavior and underlying neural circuit mechanisms discovered in fish without a "neck"

Postural control is a fundamental behavior for most animals, and head stability in particular plays a crucial role in achieving stable sensory input, accurate spatial perception, and efficient motor output. Tetrapods, including humans, possess a "neck" — a structure that separates the skull from the trunk skeleton — and stabilize their head in space by contracting and relaxing neck muscles in response to body tilt. This reflexive mechanism is known as the vestibulo-collic reflex. Fish, by contrast, have their skull directly connected to the trunk skeleton and therefore lack a morphological "neck." Whether these neckless animals possess a head stabilization behavior equivalent to the tetrapod vestibulo-collic reflex was unknown.

A research group led by Takumi Sugioka (Researcher), Masashi Tanimoto (Assistant Professor), and Shin-ichi Higashijima (Professor) at the Exploratory Research Center on Life and Living Systems (ExCELLS) / National Institute for Basic Biology (NIBB), National Institutes of Natural Sciences, collaborated with Dr. Herwig Baier and Dr. Tod R. Thiele from the Max Planck Institute in Germany. Using larval zebrafish — a model organism with a relatively simple body plan well-suited for neural circuit research — the team first conducted detailed behavioral observations. They found that the degree of trunk flexion changes in accordance with the angle of body tilt. This trunk flexion shifts the orientation of the head toward horizontal, effectively stabilizing the head in space.

Next, the group succeeded in identifying the neural circuits and muscles involved in trunk flexion through activity measurements and cell ablation experiments during tilting. Although head stabilization behavior in fish — which lack a morphological neck — had not been previously characterized, this study demonstrates for the first time that fish also possess such behavior. Because fish lack a "neck," the muscles involved cannot be called "neck muscles." Nevertheless, the neural circuit underlying this behavior shares multiple structural and neuronal features with the mammalian vestibulo-collic reflex circuit. These findings suggest that the trunk flexion observed in fish may represent an evolutionarily ancestral mechanism of the vestibulo-collic reflex. This research is expected to illuminate the origins of the vestibulo-collic reflex and may open new avenues for understanding the evolutionary emergence of the vertebrate neck.