Shifts in food composition of Antarctic krill (Euphausia superba) enhance coexistence with the pelagic tunicate (Salpa thompsoni)

Antarctic krill (Euphausia superba) is a keystone species in the Antarctic marine ecosystem. The pelagic tunicate (Salpa thompsoni) interacts with krill in diverse aspects, which imposes krill distribution and even survival, especially under the context of changing climate. Understanding the diet of both species during autumn—a season with shifting food composition and availability—is critically understudied. Therefore, this study aims to investigate the diet composition and trophic ecological niches of krill and salps in the Bransfield Strait (BS) during autumn in 2022 using stomach content, fatty acid (FA), and stable isotope analyses. Stomach content analysis revealed that krill primarily consumed copepods, while salps primarily consumed diatoms. FA analysis confirmed significant differences between the FA markers of diatoms and copepods for each species, aligning with stomach content analyses. Stable isotope analysis also indicated low overlap of trophic niches. By consuming more copepods, krill may reduce direct dietary competition with the less prevalent salps during autumn. Although the feeding strategy (active or passive) exerted by krill is warranted to further investigate, the differentiated feeding pattern occupied by two species promotes their coexistence.