Sponges often host large amounts of symbionts, mainly represented by cyanobacteria and dinoflagellates. Recent findings show a widespread presence of symbiotic diatoms living inside antarctic demosponges. In this paper, the invasion by large populations of the diatom Melosira sp. into specimens of the hexactinellid sponge Scolymastra joubini is reported. SEM analyses support the hypothesis that the embedded living diatoms have a negative impact on sponge tissues, leading to degenerative processes.