Mitigating effect of dietary tryptophan on intra-cohort cannibalism in tiger puffer (Takifugu rubripes) with clipped or intact teeth

This study was to investigate the effect of dietary tryptophan supplementation on the growth, survival, cannibalism and 5-HT metabolism in tiger puffer (Takifugu rubripes) with clipped or intact teeth. The X diet contained 5.45 g/kg tryptophan based on tryptophan requirement of tiger puffer reported in our previous study. The other three diets were supplemented with tryptophan at levels of two- (2×), four- (4×) and eight (8×) times the tryptophan concentration of the X diet. All four experimental diets were randomly used in fish with clipped teeth (the X, 2×, 4× and 8× groups), meanwhile the X and 8× diets were used in fish with intact teeth (the T-X and T-8× groups). Tiger puffer (14.16 ± 0.04 g mean weight) were reared in 18 tanks, with 45 fish per tank, for 50 days. Compared with fish fed the X diet, eight-fold tryptophan requirement significantly decreased the growth of fish with clipped teeth, but had no significant effect on the growth of fish with intact teeth. Treating fish with clipped teeth resulted in significantly higher survival rates regardless of dietary tryptophan level, meanwhile dietary tryptophan supplementation (the T-8× group) could significantly improve survival rate when intact teeth were left. Although bite marks on fish body skin still observed after taking the measures of teeth clipping or dietary supplemental tryptophan, the number of bite marks was reduced by using both measures. The results of caudal fin bite showed that, graded II and III of caudal fin bite were observed in all fish with intact teeth. However, graded I and II of caudal fin bite were predominantly found in fish with clipped teeth, and that the percentage of graded I were significantly increasing while the percentage of graded II were significantly decreasing with the increase of dietary tryptophan level. A higher level of dietary tryptophan significantly decreased the concentration of brain 5-HIAA and the ratio of 5-HIAA/5-HT in the case of fish with intact teeth. The expression of tph2 and ddc involved in 5-HT synthesis in the brain of fish with clipped teeth was significantly up-regulated with the increase of tryptophan level in the diets, while the expression of sert, which controls the reuptake of 5-HT in the synaptic cleft, increased first and then decreased. In addition, the results of gene expression of 5-HT receptors in the brain exhibited that supplementation of dietary tryptophan resulted in a trend of down-regulation of htr1b expression, whereas leaving fish teeth intact resulted in a significant up-regulation of htr1a expression. In conclusion, although tryptophan enriched diets had a weaker effect on reduced cannibalism than teeth clipping for tiger puffer, it was still effective in mitigating intra-cohort cannibalism based on survival rates, bite marks and caudal fin bites. The reason was that supplementation of dietary tryptophan modulated 5-HT synthesis, its availability of the synaptic cleft, and its specific receptors in the brain. In addition, it was worth noting that an excess of dietary tryptophan (eight-fold dietary requirement) significantly inhibited growth performance of tiger puffer with clipped teeth.