Molecular and microbial differentiation of Xestospongia testudinaria in Singapore

The giant barrel sponges (Xestospongia spp.) belong to a pan-global species complex with evidence suggesting they could encompass up to 9 cryptic species. In this study, we leveraged molecular and microbial techniques to investigate giant barrel sponges (X. testudinaria) from Singapore in relation to their placement within this species complex. Twenty-four giant barrel sponges from three sites were sequenced with mitochondrial (CO1) and nuclear (ATP6) DNA markers, identifying 6 distinct haplotypes belonging to 4 of the proposed barrel sponge species. Analysis of the sponge microbiomes was achieved with 16S rRNA gene amplicon sequencing. Sponge microbiome composition by did not differ by reef site, a pattern frequently observed in coral microbiomes across Singapore. However, there was significant differentiation in microbiome composition by host genetics consistent with the proposed species boundaries. Our results support previous findings of microbiome differentiation in co-occurring genetic haplotypes of barrel sponges from the Caribbean. Together these studies underline the potential for ecological partitioning based on genetic haplotype that could contribute to cryptic speciation within the barrel sponge species complex.