Taxonomy, distribution, and habitat associations of Gambierdiscus from coastal waters off Queensland, Australia

Ciguatera poisoning is the most prevalent non-bacterial seafood illness globally, with an estimated 10,000 to 50,000 cases reported annually. It is caused by consumption of seafood (mainly fish) that have accumulated ciguatoxins produced primarily by benthic dinoflagellates of the genus Gambierdiscus. Ciguatera poses a significant public health and fisheries challenge in tropical and subtropical regions. Although these dinoflagellates are widespread across tropical and subtropical marine ecosystems, their ecology, taxonomy, and environmental drivers remain poorly understood in many regions, limiting the capacity to monitor and predict ciguatera risk. This study uses DNA metabarcoding (targeting the V8-V9 regions of the 18S rRNA gene) to investigate the diversity, spatial distribution, and ecological associations of Gambierdiscus species across multiple coastal sites in Queensland, Australia. Results revealed the distribution of known species (G. carpenteri, G. holmesii) and several potential new phylotypes, notably a genetically distinct group (Clade II GBR) that may represent an undescribed lineage. Importantly, this study provides the first confirmed detection of G. holmesii, a known ciguatoxin producer, in Hervey Bay and Gladstone, extending its known geographic range from offshore to inshore reefs and coastal habitats of the Great Barrier Reef. This study is also the first documented record of any Gambierdiscus taxa in the Gladstone coastal region. Taxa composition was influenced by macroalgal host, and shallow coral reef habitats supported the highest species diversity and detection frequency, suggesting their role as ecological hotspots. Canonical correspondence analysis (CCA) identified associations between species and macroalgal hosts, including a strong link between G. holmesii and Cladophora sp., while G. carpenteri showed no specific host preference. Overall, this study provides ecological characterisation of Gambierdiscus communities across diverse tropical and subtropical habitats in Queensland, contributing to regional ciguatera risk assessment and global efforts to understand the ecology and distribution of benthic toxic dinoflagellates.