Sponges are one of the oldest lineages of animals on Earth and play key roles in shaping marine ecosystems. They are diverse, with more than 9600 species known to science, and come in a wide range of shapes, sizes, and colours. Sponges are, furthermore, known to host diverse communities of microbial symbionts, which play important roles in their physiology and ecology. In the present study, we sampled prokaryotic communities f...