This study investigated the characteristics of the digestive system in adult Japanese mantis shrimp (Oratosquilla oratoria), which is a species of carnivorous crustacean, with a focus on morphological and histological analysis. The digestive system of O. oratoria includes the mouthparts, the esophagus, cardiac stomach, pyloric stomach, midgut, hindgut, anus and hepatopancreas. The histological structure of each organ is composed of the mucosal epithelial layer, submucous layer, muscularis, and outer membrane. Besides, the foregut is covered with a diverse chitinous layer. The labrum is densely populated with minor salivary glands. The mandibular-gastric mill apparatus and evenly arranged bifurcated setae are observed in the cardiac stomach. The secondary filter of the pyloric stomach is subtly intricate, with fine spicules and seta plate. The midgut, being the longest segment of the digestive tract and accounting for 59.39% of body length, has villi covered with dense microvilli. The hindgut also forms villi, but the height of the villi (695.96 μm) is 3.20 times that of the midgut (217.41 μm). The hepatopancreas encircles the entire pyloric stomach, midgut, and hindgut. The hepatosomatic index is approximately 3.83%.

To understand the changes in gonadal sex differentiation, digestive enzyme activity, and growth-related hormone levels in the larval and juvenile black scraper, Thamnaconus modestus, continuous sampling was conducted from 0 to 91 days post-hatching (dph). 17β-estradiol (E2) and testosterone (T) levels, six digestive enzymes, as well as T3, T4, GH, and IGF-I were detected. The results showed that oogonia or spermatogonia was observed at 60 dph. During the sex differentiation to female or male, both E2 and T levels significantly increased (p < 0.05), suggesting that E2 and T may induce the sex differentiation to female or male in T. modestus, respectively. The amylase activity from 0 to 35 dph showed a slow upward trend, which may be due to the transition from endogenous to exogenous nutrition at this time. From 12 to 25 dph, alkaline protease activity significantly decreased (p < 0.05), while acid protease levels significantly increased (p < 0.05), suggesting that as organs in the digestive system continue to develop, acid protease plays an important role. T3 and T4 could already be detected at 0 dph, and the T4 content was always much higher than T3 throughout the stages, indicating that T4 may play more important roles than T3. Additionally, the changes in IGF-I and GH content followed a trend of an initial increase, a subsequent decrease, and then an increase, ultimately showing an overall upward trend. These results indicate that T4, IGF-I, and GH play crucial roles in growth and development in the juvenile fish. In conclusion, the results of this study provide useful information for growth, artificial reproduction, and sex regulation in T. modestus.