L-tryptophan supplementation decreases cannibalism by megalopae of the Chinese mitten crab, Eriocheir sinensis H. Milne Edwards, 1853 (Decapoda: Brachyura: Varunidae), on stage-5 zoeae in culture

Abstract Cannibalism by the larvae of the Chinese mitten crab (Eriocheir sinensis H. Milne Edwards, 1853) has seriously hindered the economic benefits of crab farmers. We investigated the effects of L-tryptophan (L-Trp) nutrition regulation on serotonin (5-HT), digestive physiology, and cannibalism of zoea stage 5 (Z5) by the megalopa (M) of E. sinensis. Different gradient concentrations of L-Trp were used to soak the live bait Artemia nauplius (AN) for feeding the Z5 and M. The cannibalism of Z5 by M was observed, and the rate of cannibalism was quantified. The results showed that L-Trp supplementation significantly reduced the frequency of cannibalism (P < 0.05). Physiological and biochemical analysis showed that after feeding AN for 12 h, the activities of the enzymes trypsyn and lypase were significantly increased by L-Trp supplementation; the highest amylase activity was observed in Diet 2 (P < 0.05). After feeding AN for 12 h, the 5-HT level in M was significantly lower in Diet 2 than in Diet 1, whereas significantly higher levels were observed in Diet 3 and Diet 4 (P < 0.05). Compared with Diet 1, the mRNA expression levels of 5-HT1BR and 5-HT7R were significantly upregulated in Diet 2 (P < 0.05). After 24 h of feeding, the 5-HT level in M was significantly higher in Diet 2 and significantly lower in Diet 4 than in Diet 1 (P < 0.05), whereas the mRNA expression levels of 5-HT1BR, 5-HT2BR, and 5-HT7R were significantly downregulated in all L-Trp-supplemented groups (P < 0.05). L-Trp supplementation reduced cannibalism of M on Z5 larvae within the 24 h experimental period and altered digestive enzyme activities, whole-body 5-HT levels, and the expression of 5-HT receptor-related genes in M. The study provides a reference for the healthy breeding of E. sinensis larvae.