Sponges represent ecologically important models to understand the evolution of symbiotic interactions of metazoans with microbial symbionts.
are commonly found in sponges, but their potential adaptations to a host-associated lifestyle are largely unknown. Here, we present three novel sponge-associated thaumarchaeal species and compare their genomic and predicted functional features with those of closely related free-living counterparts. We found different degrees of specialization of these thaumarchaeal species to the sponge environment that is reflected in their host distribution and their predicted molecular and metabolic properties. Our results indicate that
may have reached different stages of evolutionary adaptation in their symbiosis with sponges.