This thesis describes the genetic and prokaryotic diversity of giant barrel sponges. Although it was originally believed that the giant barrel sponges in the Atlantic Ocean and the Indo-Pacific Ocean were two different species, our analyses showed that there are several species in both ocean basins. Even more remarkable, is that each species is most closely related to a species in the other ocean basin. According to our analysis, there are three giant barrel sponge species in the Atlantic Ocean, and, at least, six in the Indo-Pacific Ocean.

Sponges house a large and diverse community of microorganisms. It is generally believed that these microorganisms play a central role in the physiological processes in the host. It appears that the region in which a giant barrel sponge occurs mainly determines the composition of their microbial community. However, if you look at a smaller scale within a region, for example within one reef or around an island such as Curaçao, it appears that not only is the geographical location important, but the host species to which an individual belongs also plays an important role.