Molecular detection and microbiome differentiation of two cryptic lineages of giant barrel sponges from Conch Reef, Florida Keys

The giant barrel sponge, Xestospongia muta , represents a dominant member of Caribbean reef communities. Recent microsatellite data have revealed the presence of two genetic clusters of X. muta in a monitored population on Conch Reef, Florida Keys, with a reduced abundance of one cluster among the largest individuals. Tracking changes to populations over time and their ecological significance requires rapid identification of each genetic cluster and subsequent studies of biological differences between clusters. Here, we show that single-gene barcoding detected the same intraspecific genetic variation within X. muta from Conch Reef as microsatellite data, with mitochondrial gene sequences (cytochrome c oxidase subunit I, I3-M11 partition) from 54 individuals corresponding to 4 known haplotypes within the two genetic clusters. Remarkably, mapping these haplotypes to barrel sponges worldwide revealed positioning on opposite ends of a global network, despite their sympatric occurrence. Further, we investigated whether differences in symbiotic microbial communities could be detected between the two clusters using next-generation (Illumina) sequencing of 16S rRNA gene amplicons. Both clusters exhibited highly diverse microbial communities, with 12,185 total OTUs spanning 38 bacterial and 3 archaeal phyla, but significant differences in microbial community structure (PERMANOVA; p < 0.001) and diversity (Shannon diversity index; p < 0.01) were detected between the two clusters. As sponges typically exhibit interspecific, but not intraspecific, variability in microbial communities, these findings within a sympatric population provide additional support for ecologically relevant cryptic species of X. muta .