This research reveals that certain environmental bacteria are essential for the development of a reproductive organ in female Hawaiian bobtail squid. Without these specific bacteria, the organ fails to develop properly or doesn’t form at all. This is the first known case where bacteria are absolutely required for an organ to form in animals. Impacts on everyday life: – Demonstrates how environmental microbes can be crucial for normal animal development – Provides insights into how beneficial bacteria influence organ formation and function – Helps understand the importance of maintaining natural marine bacterial communities – Could lead to new understanding of how microbes influence human development – May inform strategies for protecting marine species and their essential bacterial partners