Insights into the trophic ecology of Etmopterus molleri, one of the smallest shark species: a multi-tracer analysis

Deepwater sharks represent nearly half of all known shark species but aspects of their biology and ecology remain poorly understood, largely due to challenges associated with their complex life cycle, habitat, and extreme environmental conditions. The slendertail lanternshark Etmopterus molleri (Whitley, 1939) is one of the smallest deepwater sharks, and information on many aspects of its ecology remains scarce or non-existent to date. Here, we used a multi-tracer approach that included stomach content analysis, stable isotopes analysis (δ 13 C and δ 15 N), and total mercury (THg) measurements in muscle and liver tissues to characterize diet and trophic ecology at 3 different life stages—juvenile, subadult, and adult. Stomach content analysis revealed that juveniles primarily consumed euphausiids, while subadults and adults shifted to mesopelagic teleosts, crustaceans, and cephalopods. Ontogenetic shifts in dietary habits were linked to increases in δ 15 N and THg levels, and an increase in isotopic niche width. These findings suggest that E. molleri undergoes a progressive transition to higher trophic levels and adopts a more generalist diet as it matures. Combined with pronounced differences in niche overlap among life stages, this suggests intraspecific trophic partitioning of prey and habitats. However, further studies are needed to confirm this observation. Our study provides the first comprehensive assessment of the trophic ecology of E. molleri, highlighting the importance of using multiple tracers to understand the feeding strategies of deepwater sharks.