THE CRITICAL ROLES OF DHA IN FEEDING AND ITS REGULATION OF KEY GENES IN THE LARVAE OF ATLANTIC BLUEFIN TUNA Thunnus thynnus

A major factor influencing high mortality during Atlantic bluefin (ABFT) larval rearing may be poor eye development, which reduces the efficiency of prey capture leading to poor growth, survival and potential lethal tank wall collisions. The retinal membranes of the larval BFT eye are particularly rich in docosahexaenoic acid (DHA; 22:6n-3) broadly hinting at its pivotal role in photopigment function and the phototransduction cascade. In contrast, an IOLR study demonstrated a marked 59.8% decrease in whole body DHA from hatching to 2 dph first feeding larvae together with concurrent reductions in other PUFA as well as SAT and MONO fatty acids to satisfy the larva's high energy demands.  Nevertheless, our group found a significant (P<0.05) dietary DHA dose-dependent effect on larval tissue DHA accumulation, ingestion rate of enriched rotifers (Brachionus rotundiformis) (Fig.1a) and retinal opsin abundance in 2-14 dph ABFT (Fig.1b). On the other hand, DHA is involved in the expression of a plethura of other genes modulating key physiological processes that are developmentally determined, particularly in neural tissue. In order to elucidate further the range of roles of DHA in larval BFT, transcriptome analysis was performed on extracted cDNA in a separate study that analyzed the effect of graded levels of rotifer DHA on different age (5, 10,15 dph) BFT larvae. The highest number of genes differentially expressed by DHA in 5, 10 (Fig. 2) and 15 dph larvae were those in cellular processes (225, 242 and 64 genes, respectively) that modulated neurotransmitter levels, axon and dendrite development, chemical synaptic transmission and retinal development. The results and and their relevance to the mariculture of ABFT are discussed.