Effect of KK-42 on the carapace ultrastructure in Macrobrachium nipponense during postmolt

In order to further investigate the effect of KK-42 on cuticle structure, the juvenile prawns (Macrobrachium nipponense) in postmolt stage during which the formation of exocuticle gradually ends but that of endocuticle starts, were employed to study the structures of carapace exocuticle and endocuticle using the observation of paraffin section by scanning electron microscopy (SEM). Healthy intermolt M. nipponense with body length of (3.5±0.1) cm were randomly divided into two groups. The M. nipponense were soaked for 1 min in KK-42 solution at a concentration of 1.95×10⁻⁴ mol/L (treatment group) or 0 mol /L (control group), respectively. The carapaces of M. nipponense at 1.5, 3.0, 6.0 and 12.0 h after molting were obtained to be used for ultrastructural observation. To observe directly the surface structure of endocuticle, the carapace obtained at 6.0 h after molting was scraped gently with an anatomical knife to remove tissues on the inner surface of carapace, and then examined using SEM. The results showed that the carapace was only composed of the epicuticle and exocuticle at 1.5 and 3.0 h after molting. The number of exocuticle lamellae derived from KK-42 treatment group rose significantly at 1.5 and 3.0 h after molting, and the thickness of exocuticle increased by 72.07% and 38.67%, respectively, compared with the corresponding control group. At 6 and 12 h after molting, the loose lamellae in exocuticle tended to be dense, and there was no significant difference in thickness to be found between two groups. The number of endocuticle lamellae with a loose structure, being only one at 6.0 h, separately increased to two in control group and three in treatment group at 12.0 h after molting. In addition, pore canals within the endocuticle presented different sizes and head-tail orientation. No significant structural change was observed in the endocuticle after KK-42 treatment. The results reveal that KK-42 has a significant effect on the carapace ultrastructure of juvenile M. nipponense during postmolt, and accelerate the formation rate of the exocuticle as well as endocuticle.