Locating sound sources such as prey or predators is critical for survival in many vertebrates. Terrestrial vertebrates locate sources by measuring the time delay and intensity difference of sound pressure at each ear^1,2,3,4,5. Underwater, however, the physics of sound makes interaural cues very small, suggesting that directional hearing in fish should be nearly impossible⁶. Yet, directional hearing has been confirmed behaviourally, although the mechanisms have remained unknown for decades. Several hypotheses have been proposed to explain this remarkable ability, including the possibility that fish evolved an extreme sensitivity to minute interaural differences or that fish might compare sound pressure with particle motion signals^7,8. However, experimental challenges have long hindered a definitive explanation. Here we empirically test these models in the transparent teleost Danionella cerebrum, one of the smallest vertebrates^9,10. By selectively controlling pressure and particle motion, we dissect the sensory algorithm underlying directional acoustic startles. We find that both cues are indispensable for this behaviour and that their relative phase controls its direction. Using micro-computed tomography and optical vibrometry, we further show that D. cerebrum has the sensory structures to implement this mechanism. D. cerebrum shares these structures with more than 15% of living vertebrate species, suggesting a widespread mechanism for inferring sound direction.

定位猎物或捕食者等声源对于许多脊椎动物的生存至关重要。陆地脊椎动物通过测量每只耳朵的时间延迟和声压强度差来定位声源^1,2,3,4,5 。然而，在水下，声音的物理特性使得耳间线索非常小，这表明鱼类的定向听觉几乎是不可能的⁶ 。然而，定向听觉已在行为上得到证实，尽管几十年来其机制仍然未知。人们提出了几种假设来解释这种非凡的能力，包括鱼类进化出对微小的耳间差异极其敏感的可能性，或者鱼类可能将声压与粒子运动信号进行比较^7,8 。然而，实验挑战长期以来一直阻碍着明确的解释。在这里，我们在最小的脊椎动物之一透明硬骨鱼大脑 Danionella中凭经验测试了这些模型^9,10 。通过有选择地控制压力和粒子运动，我们剖析了定向声惊吓背后的感觉算法。我们发现这两种线索对于这种行为都是不可或缺的，并且它们的相对相位控制着其方向。利用微型计算机断层扫描和光学振动测量，我们进一步表明大脑D. cerebrum具有实现这种机制的感觉结构。大脑D. cerebrum与超过 15% 的现存脊椎动物共享这些结构，这表明推断声音方向的广泛机制。