The aim of this experiment was to determine the optimal initial food for hatchling cuttlefish and to investigate the influence of dietary composition on the growth, survival, and nutritional composition of cultured juvenile cuttlefish, Sepia pharaonis. Six experimental food groups were designated: Artemia nauplii, Calanus sinicus, frozen Hyperacanthomysis brevirostris, Ampithoe valida, H. brevirostris, and subadult Artemia. The results showed that survival, growth body biochemical composition of juvenile cuttlefish were significantly affected by experimental diets (P < 0.05). The optimum initial food was H. brevirostris, yielding a growth rate as high as 6.39%/d and survival rate reaching 81%. Growth rate was significantly positively correlated with dietary protein, Lys, Met, Phe, Iie, Leu, Trp, Arg, Gly, Eicosapentaenoic acid (EPA), Docosahexaenoic acid (DHA), and 16:0 (P < 0.05). Survival was significantly positively correlated with dietary protein, Lys, Met, Phe, Val, Thr, Iie, Leu, Trp, Arg, Gly, EPA, DHA, and 16:0 (P < 0.05). The dietary protein, lipid, Met, Val, Thr, Leu, 18:0, and EPA were prone to accumulation within the body of juvenile cuttlefish (P < 0.05). These results demonstrate that juvenile cuttlefish exhibited the best growth rates and survival when fed a diet that supplied high-protein, low-fat, and larger quantities of Lys, Met, Phe, Val, Thr, Iie, Leu, Trp, Arg, Gly, EPA, DHA, and 16:0.

The aim of this study was to provide a reference value for the safe regulation and control of ammonia nitrogen in the aquaculture of Sepia pharaonis. The effects of acute and chronic toxicity of ammonia on the cuttlefish, S. pharaonis, were tested experimentally using juvenile S. pharaonis. The results showed that the half‐lethal concentration (LC50 ) values of ammonia nitrogen in juvenile S. pharaonis with a body weight of 6.52 ± 0. 23 g at 24, 48, 72, and 96 h were 31.72, 25.77, 23.33, and 18.33 mg/L, respectively, and the corresponding un‐ionized ammonia nitrogen (UIA‐N) concentrations were 1.66, 1.35, 1.22, and 0.96 mg/L, respectively. Compared with the control, the survival rate, specific growth rate, and feed intake of juvenile S. pharaonis declined significantly, and the feed conversion ratio and hepatosomatic index increased significantly at 56 d after exposure to >1 mg/L ammonia nitrogen. Juvenile S. pharaonis should be maintained at a concentration of ammonia nitrogen of no more than 1 mg/L (UIA‐N is 0.056 mg/L) in culture, and removing harmful nitrogenous wastes from the seawater is critical in maintaining cuttlefish culture.

To investigate the ammonia detoxification pathways of the cuttlefish, Sepia pharaonis, the effects of environmental ammonia nitrogen exposure (control, 1, 3, and 6 mg/L) on nitrogen metabolism were quantified in the hemolymph, liver, and gills. The levels of glutamine synthetase, glutamate dehydrogenase, and arginase activities and ammonia, glutamine, and urea concentrations in the hemolymph, liver, and gills significantly increased upon exposure to 3 and 6 mg/L ammonia nitrogen and exhibited a dose‐dependent relationship with the ammonia exposure concentration. These results suggest two main pathways of metabolic ammonia detoxification in S. pharaonis exposed to ammonia in this study: (1) conversion of ammonia to urea, which is stored temporarily or excreted, via the ornithine‐urea cycle and (2) conversion of ammonia to glutamine, which can be stored in the body or used for other anabolic processes.