3.4.2. Midgut

Table 3 shows the index values of the midgut tissues of coral trout in different water flow velocity groups, indicating significant differences among the groups. There was no significant difference in coral trout's VL among the 1 bl/s water flow velocity group and the control group (*p* > 0.05), but it was significantly higher than in other water flow velocity groups (*p* < 0.05). Compared with the control group and other velocity groups, the MS value of coral trout in the 2 bl/s velocity group was the highest (*p* < 0.05), and those in the 1 bl/s velocity group and 2 bl/s velocity group were significantly higher than in the control group and 2.5 bl/s velocity group (*p* < 0.05). The VT of coral trout was the widest in the 1 bl/s velocity group and the narrowest in the 2 bl/s velocity group. The numbers and sizes of goblet cells decreased with the increase in water flow velocity (Figure 5).


**Table 3.** Effect of water flow velocity on the index of midgut tissue in coral trout.

The values are average ± standard deviation of four replications (*n* = 6). Values with different letters show significant differences among the groups (*p* < 0.05), and those with no letters show no significant difference (*p* > 0.05).

**Figure 5.** The effect of water flow velocity on the midgut structure of coral trout. (**a**) Hydrostatic control group: muscular thickness (MS), villus length (VL), and villus thickness (VT) were uniform, and there were many goblet cells (thick arrows); (**b**) 1 bl/s water flow velocity group: compared with the control group, the muscular layer was thicker, the villus length (VL) was longer, the villus thickness (VT) was not significantly different, and the number of goblet cells (thick arrows) was greater, but the size was relatively smaller; (**c**) 2 bl/s water flow velocity group: the muscular layer was thicker than in the control, the numbers and sizes of goblet cells (thick arrows) were significantly reduced, and some columnar epithelial cells in the mucosa layer were separated from the mucosa layer; (**d**) 2.5 bl/s water flow velocity group: muscular thickness (MS) was uniform, the number of goblet cells (thick arrows) was relatively small and the size was small.
