Molecular diet study of Antarctic toothfish, Dissostichus mawsoni

Abstract

The feeding ecology of marine organisms contributes to our understanding of phenomena like predation, habitat preferences, and population dynamics and provides insights into energy transfer within and between ecosystems, including trophic levels and food webs. Most diet studies have been made through a direct observation of prey items, which is primarily dependent on the morphological characteristics of each species. Although morphological analysis has traditionally been used to examine stomach contents, it is inadequate in identifying prey items at the species level. With the recent advancements in next generation sequencing technology, DNA metabarcoding has been introduced as an alternative approach to the identification of species that cannot be discriminated by morphological analysis. Because DNA metabarcoding can provide improved spatial, temporal, and taxonomic resolution, it has been recently applied to dietary studies to understand feeding ecology, especially in marine ecosystems. The Antarctic toothfish, Dissostichus mawsoni, is a species of the family Nototheniidae that is endemic to Antarctica. As a top predator, this species is an ecologically and economically important fishery resource in the Antarctic Ocean that deserves further study. In this thesis study, we describe the emergence of dietary DNA-based metabarcoding analysis as an innovative tool to identify the feeding ecology of D. mawsoni and enable the long-term monitoring of the Antarctic ecosystem. The first part of the thesis aims to investigate the feeding ecology of Antarctic toothfish by year and subarea using metabarcoding analysis. To this end, we collected the stomach contents of 1774 D. mawsoni fish from area 88 (Bellinghausen–Amundsen sector, Ross Sea sector) and area 58.4 (Western Pacific Ocean sector) from 2016 to 2019. A total of 48 species belonging to 7 classes, 16 orders, and 27 families were obtained from these stomach contents. On average, 99.52% of the species were fish (Actinopteri and Elasmobranchii), while only 0.38% were cephalopods. We also investigated the biodiversity of prey by regions and years using cluster analysis and nMDS based on the Bray– Curtis index. The cluster analysis revealed that the composition of the prey was clearly related to the subarea rather than the year. To determine whether the composition of the diet reflected a geographical difference, the second part of our study consisted of analyzing the diets of D. mawsoni in the research block in subareas 88.1 and 88.3, which were collected between 2019 and 2021 using metabarcoding analysis. The clustering analysis of the metabarcoding results obtained from the various subareas also revealed that their prey compositions were clearly related to geographical proximity and depth. We then used quantitative PCR (qPCR) on important prey species to quantify the individual prey items more accurately. The results showed that the main prey species, M. caml and M. whitsoni, had significant spatial differences along the Antarctic Circumpolar Current. We propose molecular diet analysis through metabarcoding and qPCR of the stomach contents of D. mawsoni as one of the most efficient methods to monitor the ecosystem stability of each Antarctic research area. However, in order to establish a more reliable molecular monitoring system for marine ecosystems, additional studies should be conducted keeping in mind various oceanic features; these studies will deepen our understanding of the changes in the feeding ecology of marine ecosystems, including the Antarctic Ocean, which is inhabited by D. mawsoni.