Comprehensive Analysis of Cyan Soft-Carapace and Red Hard-Carapace Color Variants in Procambarus clarkii: Muscle Quality, Carapace Pigmentation, and Tissue-Specific DGAT2 mRNA Expression

Crayfish with cyan soft-carapace and red hard-carapace differ considerably in market value. To investigate the causes underlying this difference in shell color, we selected 120 healthy Procambarus clarkii of similar size (9–10 cm), equally divided by shell color and sex. Muscle quality, pigment deposition, and DGAT2 mRNA expression were compared between the two shell-color types. The results showed that: (1) Muscle hardness was significantly higher in red hard-carapace crayfish than in cyan soft-carapace crayfish (p < 0.05), whereas no significant difference in muscle elasticity was observed among groups (p > 0.05). (2) A significant interaction between shell color and sex influenced crude fat and ash content (p < 0.05). Crude fat content was higher in red hard-carapace crayfish, and ash content was highest in male red hard-carapace crayfish, significantly exceeding that in male cyan soft-carapace crayfish (p < 0.05). (3) Moisture and crude protein content were not significantly affected by the interaction between shell color and sex (p > 0.05), with no notable differences across groups. (4) The contents of astaxanthin, lutein, and β-carotene in the shell were significantly influenced by the interaction between sex and shell color (p < 0.05). Astaxanthin was lowest in female cyan soft-carapace crayfish, and lutein was highest in male red hard-carapace crayfish, and β-carotene showed no significant differences. (5) DGAT2 mRNA expression was highest in hepatic tissue across all groups (p < 0.05). In gonads, expression was higher in ovaries of cyan soft-carapace crayfish and in testes of red hard-carapace crayfish (p < 0.05). (6) DGAT2 mRNA expression in the inner membrane, muscle, and intestine was significantly affected by the interaction between shell color and sex (p < 0.05), with specific expression patterns varying among tissues and groups. These findings indicate that shell color and sex interact to influence nutritional composition, carotenoid profile, and gene expression, providing insights into the mechanisms of body color formation and its physiological correlates.