All animals rely on a unique community of microbes to help digest food. This is especially true for plant-eating animals, which need a complex mix of gut bacteria, also known as microbes, to digest the tough plant material they eat. However, when it comes to herbivorous animals in the sea, like some fish, we know much less about the role microbes play in helping to digest food. Marine algae, better known as seaweeds, are unlike land plants in many ways, so herbivorous fish species likely have unique ways of digesting them. Therefore, we investigated the gut microbes in several herbivorous fishes in the Red Sea to see how they help fish digest algae. Surprisingly, we found that even though these fish had unique gut microbe communities, all were made up by varieties of one type of giant bacteria that appears to have evolved to help them digest their favorite algae!