In the 1950’s, the shipping industry began painting ships with antifouling chemicals called triorganotins (TBTs). Though outlawed in the U.S., TBTs are used illegally, leach from old paint, and persist for decades in sediment. Ingestion and exposure to TBTs causes inhibited growth and development in a wide range of marine invertebrates. Despite the emerging importance of the Holobiont theory, little information examining how TBTs may induce dysbiosis and/or facilitate inoculation by new and/or harmful microbes into the bodies of marine invertebrates exists. Echinoderms are ideal model organisms to study the effects of TBTs on host development because they undergo complex life cycles, including a larval stage especially vulnerable to predation and to relatively low concentrations of TBTs. We are using transcriptomics and 16s rRNA sequencing to understand how Strongylocentrotus purpuratus larvae benefit from their microbiomes and how ontogeny and/or toxins affect the holobiont. In collaboration with Cabrillo Marine Aquarium, we are rearing S. purpuratus larvae with ecologically relevant doses of TBT. Same life-stage individuals will be sampled in batches of 10 ensuring sufficient microbial DNA and sufficient host RNA. We will sample larvae through time and track morphometrics and mortality rates. Corresponding microbiomes and host transcriptomics will be examined. We predict that the microbiome of S. purpuratus will shift in species composition across developmental stages since microbiomes vary with morphology and diet within echinoderm host species. Additionally, we expect TBTs will disrupt the metabolic pathways maintaining host health and host-microbial interactions in S. purpuratus because toxins affect echinoderm growth and development. Because S. purpuratus are a keystone species, host-microbial research will serve as an indicator for the health and resilience of the rocky intertidal.