In recent years, a growing number of studies sought to examine the composition and the determinants of the gut microflora in marine animals, including fish. For tropical tuna, which are among the most consumed fish worldwide, there is scarce information on their enteric bacterial communities and how they evolve during fish growth. In this study, we used metabarcoding of the 16S rDNA gene to (1) describe the diversity and composition of the gut bacteriome in the three most fished tuna species (skipjack, yellowfin and bigeye), and (2) to examine its intra-specific variability from juveniles to larger adults. Although there was a remarkable convergence of taxonomic richness and bacterial composition between yellowfin and bigeyes tuna, the gut bacteriome of skipjack tuna was distinct from the other two species. Throughout fish growth, the enteric bacteriome of yellowfin and bigeyes also showed significant modifications, while that of skipjack tuna remained relatively homogeneous. Finally, our results suggest that the gut bacteriome of marine fish may not always be subject to structural modifications during their growth, especially in species that maintain a steady feeding behavior during their lifetime.
Until recently, jellyfish have been ignored as an important source of food, due to their low nutritional value. Here, quantitative PCR was used to detect and quantify the DNA of the jellyfish Aurelia coerulea in the gut contents of commercially important fish species from the Thau Lagoon. Individuals from five fish species were collected during two different periods: the bloom period, when the pelagic stages of A. coerulea are abundant, and the post-bloom period, when only the benthic stagepolypsis present in the lagoon. The DNA of A. coerulea was detected in the guts of 41.9% of the fish analysed, belonging to four different species. The eel Anguilla anguilla and the seabream Sparus aurata were important jellyfish consumers during the bloom and post-bloom periods, respectively. These results provide new insights on the potential control of jellyfish populations and on jellyfish importance as a food source for exploited fishes. Highlights ► DNA of the jellyfish Aurelia coerulea was detected by qPCR in the guts of four commercially important fish species. ► All the individuals of European eel analysed had jellyfish DNA in their guts. ► Gilthead Sea bream likely indirectly consumed jellyfish polyps, by preying on their settling substrates (bivalves). ► Jellyfish might be a non-negligible important food source for commercially exploited fish species.
Identifying ontogenetic changes in jellyfish diet is fundamental to understand trophic interactions during their life cycle. Scyphomedusae blooms exert major predation pressure on plankton communities, although their role in ecosystems has long been misrepresented. This study assesses seasonal and ontogenetic changes in the diet of the scyphomedusa Rhizostoma pulmo, one of the largest yet overlooked Mediterranean jellyfish. Medusae gut contents (n = 127) were collected during one year in Bages Sigean lagoon, southern France. Results show that the diet composition differs from the availability of prey in the environment with contrasting preferences along ontogeny. Calanoid (70%) and harpacticoid (45.8%) copepods were the most frequent prey and the major carbon contributors for small medusae (bell diameter < 15 cm). In contrast, ciliates (43.5%) were the most frequent prey for large organisms (> 15 cm), which obtain most of their carbon intake from ciliates and fish eggs (20.9%). The overall impact on micro and mesozooplankton showed that small medusae consume 5% of the copepods daily standing stock, while large medusae consumed 8% of ciliates daily standing stock. Our results stress that R. pulmo display different trophic pathways along its life cycle, firstly interacting with the classical food web, and shifting afterwards to a greater interaction with the microbial loop.
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