Fireworms (Hermodice carunculata) are a reservoir and potential vector for Anthozoan-infecting apicomplexans
Anthozoans (Phylum: Cnidaria) are a diverse class of sessile marine organisms, where many play critical roles as habitat-forming species with a wide geographic and environmental range, from warm shallow-water tropical coral reefs to cold-water ecosystems including Mediterranean coralligenous reefscapes. They also present a complex ecosystem in their own right as intricate holobionts made up microbes from all domains of the tree of life, where many are known to play significant roles in host health and fitness. The corallicolids are a recently described clade of apicomplexan parasites of Anthozoans whose abundance has been shown to influence the thermal tolerance of habitat-forming corals. Considering corallicolids have been detected in 70% of anthozoan genera from tropical and cold marine ecosystems, these microeukaryotes may be essential to our understanding of coral reef health and conservation under the current trajectory of global oceanic heating. Despite these large-scale implications, the literature remains unclear about corallicolid biology and ecology. Apicomplexans are known to often have a closed life cycle, requiring multiple hosts with minimal exposure to the environment. To date, corallicolids have only been documented in anthozoan hosts with no known secondary/reservoir hosts or vectors. Here we provide evidence for the high abundance of distinct corallicolid sequences recovered from bearded fireworms (Hermodice carunculata) across multiple years and locations in tropical reef habitat located off Curacao. We suspect that these sequences represent different life stages of the parasite and that H. carunculata serves as a vector of corallicolids between anthozoan hosts through its feces, where we find a high abundance of the parasites. These findings not only expand our understanding of the ecological interactions within coral reef ecosystems but also highlight the potential role of host-associated parasites in shaping the resilience of reef habitats. Future research into the ecology of corallicolids, their life cycle, how they influence coral thermal tolerance, and the role of vectors like H. carunculata in other marine environments may provide critical insights into mitigating the impacts of ocean warming and preserving biodiversity in these vulnerable ecosystems.