Barramundi (Lates calcarifer), a catadromous teleost of significant and growing commercial importance, are reported to have limited fatty acid bioconversion capability and therefore require preformed long-chain PUFA (LC-PUFA) as dietary essential fatty acid (EFA). In this study, the response of juvenile barramundi (47·0 g/fish initial weight) fed isolipidic and isoenergetic diets with 8·2 % added oil was tested. The experimental test diets were either devoid of fish oil (FO), and thus with no n-3 LC-PUFA (FO FREE diet), or with a low inclusion of FO (FO LOW diet). These were compared against a control diet containing only FO (FO CTRL diet) as the added lipid source, over an 8-week period. Interim samples and measurements were taken fortnightly during the trial in order to define the aetiology of the onset and progression of EFA deficiency. After 2 weeks, the fish fed the FO FREE and FO LOW diets had significantly lower live-weights, and after 8 weeks significant differences were detected for all performance parameters. The fish fed the FO FREE diet also had a significantly higher incidence of external abnormalities. The transcription of several genes involved in fatty acid metabolism was affected after 2 weeks of feeding, showing a rapid nutritional regulation. This experiment documents the aetiology of the onset and the progression of EFA deficiency in juvenile barramundi and demonstrates that such deficiencies can be detected within 2 weeks in juvenile fish.

Calreticulin (CRT) is a Ca2+-binding molecular chaperone in the endoplasmic reticulum. We cloned and characterized the CRT gene in an important marine food fish species Asian seabass (Lates calcarifer). The full-length DNA of the CRT gene was 2194 bp, including a complete open reading frame encoding 420 amino acid residues, a 113 bp 5′-untranslated region and an 818 bp 3′-untranslated region. The CRT gene contained nine exons and eight introns covering a total of 2772 bp genomic DNA from the start to stop codon. Ten single nucleotide polymorphisms (SNPs) were detected in introns and an exon in six individuals collected from five different locations. The CRT gene was assigned to linkage group 4 of the linkage map of Asian seabass. Quantitative real-time PCR revealed that the CRT gene was highly expressed in liver at the age of 1, 3 and 7 months under normal conditions, whereas its expression in liver reduced sharply after 0.5 to 2 h cold challenge at 16°C, and then increased slowly. A preliminary association analysis showed a significant (P < 0.001) association between the SNP6 in the CRT gene and the mortality after cold challenge at 16°C. Our results suggest that the CRT gene is associated with cold tolerance of Asian seabass and further investigation will be necessary to illustrate the underlying mechanisms.