Dietary supplementation with arachidonic acid alters the stress response of gilthead seabream (Sparus aurata) larvae

Gilthead seabream (Sparus aurata L.) larvae were fed Artemia nauplii enriched with a low or high level of arachidonic acid (ArA, 20:4n-6; 1.5 and 7.5 mg ArA/g dw, respectively) and their response to two stressors was determined. Larvae of 28 and 50 days post-hatch (DPH) were subjected to 90 s of air exposure, where the remaining 28 DPH fish were fed for another 4 days and exposed to a decrease from 42 ambient salinity to 25 over 1 h. After feeding on the ArA-enriched Artemia, the larvae of both age groups exhibited elevated ArA levels, decreased EPA levels and decreased EPA/ArA and DHA/ArA ratios. At 32 DPH, the high ArA larvae demonstrated better growth than cohorts fed the lower ArA diet, but this effect did not occur in the 50 DPH larvae. Feeding the high ArA Artemia strongly reduced the cortisol response after air exposure of both 28 and 50 DPH larvae. The low ArA larvae responded to the salinity decrease with an initial increase in cortisol followed by a gradual decline below basal levels, whereas the high ArA larvae responded with a drop followed by a return to basal levels. After the onset of the salinity decrease the whole-body Na+, K+-ATPase activity increased in larvae of both dietary treatments. After 24 h, the ATPase activity had decreased in the low ArA larvae, but remained elevated in the high ArA larvae, coinciding with the osmoregulatory role of cortisol. Together with the salinity decrease, the whole-body sodium content dropped in the high ArA larvae, while no marked change occurred in the low ArA larvae. ArA appears to be involved in the regulation of cortisol synthesis in a divergent way: dietary supplementation with ArA down-regulated the cortisol response after air exposure, while it up-regulated basal cortisol synthesis after salinity change and enhanced the osmoregulatory balance accordingly.