Spatial and host-specific structuring in symbiont community composition of an endemic Hawaiian octocoral, <i>Sarcothelia edmondsoni</i> (Verrill 1928)

Coral reefs are increasingly threatened by climate-induced bleaching, yet some taxa-like the Hawaiian endemic octocoral <i>Sarcothelia edmondsoni</i>-exhibit notable stress tolerance. This study investigates whether distinct color morphotypes (blue and brown) of <i>S. edmondsoni</i> maintain stable or flexible symbiont associations that might underlie this resilience. Using high-throughput ITS2 sequencing and SymPortal analyses, we characterized Symbiodiniaceae communities across morphotypes on three Hawaiian Islands. Assemblages were overwhelmingly dominated (>99%) by <i>Symbiodinium</i> (Clade A), particularly <i>S. tridacnidorum</i> (ITS2 type A3), with blue morphotypes consistently hosting more diverse symbiont profiles. Dinoflagellate community composition varied significantly by morphotype and location, with no ITS2 profile shared across all morphotype-island combinations. Bray-Curtis analyses revealed strong ecological structuring, while UniFrac (a measure of evolutionary relatedness) indicated phylogenetic similarity, suggesting intragenomic or ecotypic divergence within a conserved lineage. Morphotype-specific associations may reflect environmental adaptation or host-symbiont specificity. The greater symbiont diversity in blue morphotypes, coupled with the lack of profile overlap among sites, points to fine-scale host-symbiont structuring shaped by local environmental conditions. These results demonstrate that <i>Sarcothelia edmondsoni</i> hosts morphotype- and location-specific Symbiodiniaceae communities within a conserved lineage, revealing fine-scale ecological structuring and potential symbiont ecotypes that may contribute to this species' resilience across variable reef environments. This study supports previous findings that symbiont community structure is shaped by the combined influence of host specificity and local environmental conditions.