A multi-taxon evaluation of tropical-subtropical boundary using tree-of-life eDNA across Taiwanese waters

Understanding whether biogeographic boundaries are consistent across different marine taxa is essential for effective biodiversity monitoring and ecosystem management. In Taiwan, a boundary between tropical and subtropical marine communities has been described primarily based on hard coral and fish assemblages, yet it remains unclear whether this boundary is detectable across broader components of the marine metazoan taxa. Here, we use multi-marker eDNA metabarcoding across 10 coastal locations spanning a latitudinal gradient from northern to southern Taiwanese waters to evaluate whether the previously-defined biogeographic break is reflected in other taxonomic groups. Phylum-specific analyses revealed distinct tropical assemblages for multiple taxa, including cnidaria (hard corals), mollusca, arthropoda (copepods), chordata (tunicates), echinodermata and bryozoa with putative transitional communities at Penghu Islands. ZOTU richness showed clear latitudinal gradients for many metazoan phyla: hard corals (scleractinia) and several benthic groups increased towards the south, whereas copepod richness was highest in northern waters. Indicator species analyses identified that copepods dominate in the north and scleractinian corals in the south. Collectively, our results demonstrate that the tropical-subtropical boundary in Taiwanese waters exerts a potential structuring effect on multiple marine taxa. This study highlights the value of broad-scale eDNA metabarcoding for detecting biogeographic transitions. It provides a multi-taxon baseline for long-term monitoring across one of the most dynamic marine boundaries in the Western Pacific.