Marine sponges are essential for the survival and productivity of coral reefs, and host a distinct community of microorganisms. These microbes can shape sponge phenotype by mediating important developmental stages and their mode of transmission can have profound effects on the adaptability of the sponge. However, the characterisation of each microbial taxa throughout the life cycle of a sponge remains challenging, with many sponge species hosting over 3,000 distinct microbial species. Ianthella basta, an abundant reef sponge in the Indo-Pacific, has recently emerged as a model organism for sponge symbiosis research, harbouring only three dominant symbionts. Here, we successfully spawned Ianthella basta in indoor aquaria, characterised its mode of reproduction, and used 16S rRNA gene amplicon sequencing, fluorescence in situ hybridisation, and transmission electron microscopy to characterise the microbial community throughout its life cycle. We confirmed I. basta as being gonochoric and showed that the three dominant symbionts, that make up more than 90 percent of the microbiome, are vertically transmitted from the mother to the offspring, suggesting an obligate relationship between these microbes and their host.