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Speaker: Babett Guenther
Title: Metabarcoding reveals the importance of gelatinous prey for Atlantic bluefin tuna and confirms their opportunistic foraging behaviour
Synopsis: Climate change leads to an increasing abundance of gelatinous zooplankton worldwide, whereas their interactions within marine food webs are insufficiently understood. Several commercially important fish species such as bluefin tuna (Thunnus thynnus) are suggested to prey on them. The main direct method used for studies of the diet and feeding habits thus far has been morphological inventories of stomach contents, which are missing gelatinous prey because of quick digestion rates. Here, we analysed the stomachs of 48 Atlantic bluefin tuna (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), sardine (Sardina pilchardus), sprat (Sprattus sprattus), mackerel (Scomber colias), and cephalopods. Another striking result was the detection, based on 18S (with which vertebrates were detected as the most abundant group), of a high prevalence and diversity of gelatinous organisms (RRA = 27.1%), including cnidarians, salps, and ctenophores, 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. We further confirmed that DNA metabarcoding can be a powerful tool for assessing the diet and trophodynamics of top marine predators and better understanding the role of gelatinous zooplankton.
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