For corals, gardening’s a matter of life and death. Corals compete with algal seaweeds for space, and many types of seaweed release chemicals that are toxic to corals, act as carriers for coral diseases and boost the growth of dangerous microbes. These dangers require close contact—the seaweed poisons won’t diffuse through the water, so they need to be applied to the corals directly. And that gives the corals an opportunity to save themselves. When they sense encroaching seaweed, they call for help.

This alliance between corals and gobies is vital for more than just these partners. Acroporans include species like the staghorn and elkhorn corals, which are reef lynchpins. Because of their fast growth and vast, branching