Fine-scale differences in feeding ecology of three tropical euphausiid species revealed using 18S V9 metabarcoding and stable isotope analysis

Euphausiids critically link higher and lower trophic levels in the marine ecosystem; however, their trophic role at lower latitudes with more complex food web structures is often overlooked. Here, we examined the detailed gut content composition and trophic levels of 3 representative euphausiid species in the tropical Indian and Pacific Oceans: Euphausia diomedeae , Hansarsia gracilis , and Thysanopoda tricuspidata , using a combined 18S V9 metabarcoding with stable isotope analysis approach. All species were omnivorous but exhibited fine-scale niche partitioning and different trophic plasticity. E. diomedeae is an opportunistic generalist, feeding on Pelagomonadales pelagophytes, chlorophytes, Prorocentraceae dinoflagellates, Metridinidae copepods, and Sphaeronectidae hydrozoans. Its wide diet range and adaptability to environmental food availability make it the most abundant and widely distributed species at low latitudes. H. gracilis exhibited the highest degree of carnivory, thriving mainly on Sphaeronectidae and Eucalanidae and displaying a significantly higher trophic level than E. diomedeae and T. tricuspidata . This has also allowed its co-existence with E. diomedeae across a broad geographical range. T. tricuspidata specifically preyed upon Gymnodiniaceae, Peridiniaceae/Peridiniopsidaceae dinoflagellates, Telonema, and Tintinnidae and showed remarkable efficiency in utilizing phytoplanktonic prey. Its possible preference for Bacillariaceae was particularly satisfied by the increased diatom abundance in the equatorial region, supporting its local dominance. These results confirm the trophic role of euphausiids as primary/secondary consumers in the extended food chain at low latitudes. This knowledge enhances our understanding of energy flow and food web dynamics in low-latitude oceans, contributing to wider ecological and environmental research.