# Changes in Gonadal Sex Differentiation, Digestive Enzymes, and Growth-Related Hormone Contents in the Larval and Juvenile Black Scraper, Thamnaconus modestus

## TL;DR

This study tracks how hormones, digestive enzymes, and sex differentiation change in black scraper fish during early development, offering insights for breeding and growth.

## Contribution

The study provides new insights into the hormonal and enzymatic changes during early development in the black scraper fish.

## Key findings

- Gonads in black scraper fish differentiate into ovaries or testes by 60 days post-hatching, likely due to increased estrogen and testosterone.
- T4 levels are consistently higher than T3, suggesting a more significant role for T4 in development.
- IGF-I and GH levels correlate with development, indicating their joint role in promoting fish growth.

## Abstract

Sex hormones, digestive enzymes, and growth-related hormones play important roles in the early phases of marine fish including sexual differentiation, growth, and development. This study aimed to understand these changes in the larval and juvenile black scraper, Thamnaconus modestus. 17β-estradiol (E2) and testosterone (T) contents were detected, as well as the activities of digestive enzymes and the levels of Triiodothyronine (T3), thyroxine (T4), growth hormone (GH), and insulin-like growth factor-I (IGF-I). The results suggest that the gonads had differentiated into ovaries or testes at 60 dph in T. modestus, perhaps due to increased estrogen and androgen levels, respectively. The activity of six digestive enzymes could be detected in newly hatched larvae, suggesting that the larvae already have the ability to digest food. Furthermore, T3 and T4 could be detected as early as at 0 dph, and the T4 content was always much higher than that of T3 in each stage, indicating that T4 may play more important roles than T3. The levels of IGF-I and GH are positively correlated with development in fish, suggesting that IGF-I and GH jointly promote fish growth. This study may be beneficial to promote the artificial reproduction and healthy growth in T. modestus.

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.