Four replicated groups of sea bass (Dicentrarchus labrax) larvae were fed diets containing an extra-high level of highly unsaturated fatty acids (HUFA) (XH; 3·7 % EPA+DHA), a high level of HUFA (HH; 1·7 %), a low level of HUFA (LH; 0·7 %) or an extra-low level of HUFA (XLH; 0·5 %) from day 6 to day 45 (experiment 1; XH1, HH1, LH1, XLH1). After a subsequent 1-month period feeding a commercial diet (2·7 % EPA+DHA), the capacity of the four initial groups to adapt to an n-3 HUFA-restricted diet (0·3 % EPA+DHA; R-groups: XH2_R, HH2_R, LH2_R, XLH2_R) was tested for 35 d. Larval dietary treatments had no effect on larval and juvenile survival rates. The wet weight of day 45 larvae was higher in XH1 and HH1 (P < 0·001), but the R-juvenile mass gains were similar in all treatments. Δ-6-desaturase (Δ6D) mRNA level was higher in LH1 and XLH1 at day 45 (P < 0·001), and higher in LH2_R and XLH2_R, with a significant increase at day 118.Concomitantly, PPARα and PPARβ mRNA levels were higher in XLH1 at day 45, and PPARβ and γ mRNA levels were higher in XLH2_R at day 118, suggesting possible involvement of PPAR in stimulation of Δ6D expression, when drastic dietary larval conditioning occurred. The low DHA content in the polar lipids (PL) of LH1 and XLH1 revealed an n-3-HUFA deficiency in these groups. Larval conditioning did not affect DHA content in the PL of R-juveniles. The present study showed (i) a persistent Δ6D mRNA enhancement in juveniles pre-conditioned with an n-3 HUFA-deficient larval diet, over the 1-month intermediate period, and (ii) brought new findings suggesting the involvement of PPAR in the Δ6D mRNA level stimulation. However, such nutritional conditioning had no significant effect on juvenile growth and lipid composition.

Determining the lipid content and fatty acid (FA) composition of aquatic organisms has been of major interest in trophic ecology, aquaculture, and nutrition for over half a century. Although protocols for lipid analysis are well-described, their application to aquatic sciences often requires modifications to adapt to field conditions and to sample type. Here, we present the current state of knowledge of methods dedicated to both marine and freshwater lipid analyses, from sampling to data treatment. We review: (i) sample preservation, storage and transport protocols, and their effects on lipids, (ii) lipid extraction, separation of polar and neutral lipids, derivatization, and detection methods, and (iii) available tools for the statistical analysis of FA data. We provide recommendations for best practices in field situations and advocate for protocol standardization and interlaboratory calibration.

Plastic pollution in marine ecosystems constitutes an important threat to marine life. For vertebrates, macro/microplastics can obstruct and/or transit into the airways and digestive tract whereas nanoplastics (NPs; < 1000 nm) have been observed in non-digestive tissues such as the liver and brain. Whether NPs cross the intestinal epithelium to gain access to the blood and internal organs remains controversial, however. Here, we show directly NP translocation across the intestinal barrier of a fish, the European seabass, Dicentrarchus labrax, ex vivo. The luminal side of median and distal segments of intestine were exposed to fluorescent polystyrene NPs (PS-NPs) of 50 nm diameter. PS-NPs that translocated to the serosal side were then detected quantitatively by fluorimetry, and qualitatively by scanning electron microscopy (SEM) and pyrolysis coupled to gas chromatography and high-resolution mass spectrometry (Py-GC-HRMS). Fluorescence intensity on the serosal side increased 15-90 min after PS-NP addition into the luminal side, suggesting that PS-NPs crossed the intestinal barrier; this was confirmed by both SEM and Py-GC-HRMS. This study thus evidenced conclusively that NPs beads translocate across the intestinal epithelium in this marine vertebrate.

Greenland halibut (Reinhardtius hippoglossoides), especially juveniles, are frequently found in severely hypoxic areas (18%–25% saturation) of the St. Lawrence Estuary. We investigated the tolerance of this species to hypoxia and evaluated the consequences of low oxygen levels on metabolic capacity. At 5 °C, juveniles had a higher critical oxygen threshold than adults (15% versus 11% saturation), indicating that they were less tolerant to hypoxia. Severe hypoxia (19% saturation) did not affect the juveniles' standard metabolic rate but significantly reduced (by 55%) their maximum metabolic rate compared with normoxia. Consequently, the aerobic scope was reduced by 72% in hypoxia compared with normoxia. In juveniles, severe hypoxia increased the duration of digestive processes. The decrease in aerobic scope in hypoxia and the determination of critical oxygen threshold at a saturation level close to actual field dissolved oxygen values strongly suggest that juveniles from the St. Lawrence Estuary are living at the edge of their metabolic capacity. Consequently, the growth and distribution of Greenland halibut could be affected if there are further declines in dissolved oxygen availability.