Metabarcoding confirms the opportunistic foraging behaviour of Atlantic bluefin tuna and reveals the importance of gelatinous prey

Abstract

Studies of the diet, feeding habits and trophic activity of top marine predators are essential for understanding their trophodynamics. The main direct method used for such studies thus far has been morphological inventories of stomach contents. This approach presents limitations such as missing gelatinous prey, which are usually digested too quickly to be detectable. Here, we analysed the stomachs of 48 Atlantic bluefin tuna (Thunnus thynnus, approximately 15 to 60 kg, including juveniles and adult fishes) collected from the Mediterranean Sea through the metabarcoding of two gene regions (cytochrome c oxidase subunit I (COI) and the ribosomal 18S-V1V2 region). The identified prey taxa and their relative read abundances (RRAs) estimated using COI results were in line with the findings of morphologically based inventories simultaneously performed on the same set of tuna samples. In both cases (and with the same rankings), the prey taxa included anchovy (Engraulis encrasicolus, here detected in more than 80% of samples, RRA = 43%), sardine (Sardina pilchardus, also approximately 80%, RRA = 30%), sprat (Sprattus sprattus, approximately 66%, RRA = 8%), mackerel (Scomber colias, approximately 44%, RRA = 7%) and cephalopods (approximately 15%, RRA = 1.4%). Another striking result was the detection, based on 18S (with which vertebrates were detected as the most abundant group, RRA = 61.6%), of a high prevalence and diversity of gelatinous organisms (RRA = 27.1%), including cnidarians (6.7%), salps (11.7%), and ctenophores (8.7%), the latter increasing with the size of the predator. These results thus support the hypothesis of the role of gelatinous prey in the diet of Atlantic bluefin tuna, suggesting that this species is even more generalist and opportunistic than previously thought. This study further confirms that DNA metabarcoding can be a powerful tool for assessing the diet and trophodynamics of top marine predators.

Feces DNA analyses track the rehabilitation of a free-ranging beluga whale

“…Changes in diet composition occur frequently by region, depth, season and size of the beluga 32 – 34 . Although passive or secondary (ie preys of preys) predation may explain part of the invertebrates found in these inventories, results here are in line with previous morphological studies, and suggest a broader and more diverse range of prey are consumed, including many small or soft-bodied species 10 . This was most prominent during the second period of monitoring, when Hvaldimir enlarged his geographical range.…”

“…While absolute biomass cannot be reliably estimated using metabarcoding approaches, several recent studies have shown that in comparison to other methods, relative read abundance can be used to estimate relative biomass for vertebrate as well as invertebrate feces/stomach content within one experiment 3 , 10 , 36 . Here, the high relative abundance of reads assigned to mussels ( Mytilida sp.…”

“…Several studies have now indicated that relative read abundance of fecal DNA is representative of stomach content, and offers reliable qualitative and semi quantitative results 3 , 7 – 9 . For our study, two gene regions were chosen; mitochondrial cytochrome c oxidase subunit I (COI), which allows species-specific detection, especially of fish, and ribosomal 18S which supports the general broad range detection of invertebrates as potential prey 10 .…”

Back Into the Wild: Feces DNA Analyses Track the Return of a Beluga Whale Back to Its Natural Environment

“…Changes in diet composition occur frequently by region, depth, and season 8,9 . Results here, from the concomitant use of COI and ribosomal 18S in metabarcoding analyses build upon this, and suggest a broader and more diverse range of prey are consumed, including many small or soft-bodied species 10 . Taxa detections using 18S-V1 include Acanthocephala, Annelida (Polychaeta), Arthropoda (Malacostraca, Maxillopoda), Ctenophora, Cnidarian (Hydrozoa, Myxozoa), Echinodermata (Ophiuroidea, Holothuroidea, Echinoidea), Gastrotricha, Mollusca (Bivalvia, Gastropoda), Nematoda, Platyhelminthes (Cestoda, Monogenea, Trematoda), Porifera (Calcarea, Demospongiae), Rotifera (Monogononta), Tunicata (Appendicularia, Ascidiacea).…”

“…Relative read abundance can be linked to relative biomass for vertebrates as well as invertebrate feces/stomach content 2,10,11 . Here, the high relative abundance of mussels (Mytilida sp.…”