Sponges are important aquatic invertebrates that inhabit many different freshwater and marine habitats. I investigated the diversity and specificity of sponge-associated microbial communities under different methodological and environmental aspects in this thesis. In summary, temperate and tropical sponge-associated microbial communities exhibit a significant sponge species specificity. Furthermore, these microbial community patterns are consistent among high taxonomic ranks, which indicates that long-term host-specific processes may have shaped the relationships between sponges and microbial symbionts. In addition, sponge-associated microbes possess significant variations in community structure within their particular hosts along local depth gradients, and are thus potentially influenced by environmental factors
