Tank colour and dietary arachidonic acid affects pigmentation, eicosanoid production and tissue fatty acid profile of larval Atlantic cod (Gadus morhua)

We determined the effect of dietary arachidonic acid (20:4n − 6, ARA) and tank colour (green or grey) on the performance of larval Atlantic cod from 2-28 days post-hatch (dph) using a two-factor, fully orthogonal design. Performance of larvae was assessed by evaluating growth, whole body fatty acid (total and polar lipid) profile, whole body eicosanoid (prostaglandins (PG) E and F) production and pigmentation. Rotifers were enriched on one of four experimental emulsions containing graduated ARA concentrations, and fed to 6 replicate tanks (3 each of green and grey colour) of larvae. Tank colour had no significant effect on larval growth or survival at 28 dph. Significant differences in growth (length and dry weight) were detected among diets, although no direct correlation was recorded between growth and dietary ARA. The whole body fatty acid profile from the total and polar lipid fractions of larvae was directly influenced by the diet. Diet did not influence pigmentation, but larvae held in green tanks had significantly elevated pigmentation on both their head and body compared to those held in grey tanks. The interaction between diet and tank colour had a significant effect on total PGE concentration, while total PGF was not influenced by diet or tank colour. We propose that larvae adapt to their background tank colour by changes in pigmentation (melanophore) dispersal modulated by an elevation of circulating α-melanocyte stimulating hormone, which in turn influences total PGE production. To our knowledge this is the first study to demonstrate a relationship between pigmentation and eicosanoids in larval fish, although the biochemical pathways and mechanisms remain poorly understood.