Use of the microalgae Tisochrysis lutea in a "green water" system increases the absorption area in the intestine of larvae of the seahorse Hippocampus reidi Ginsburg, 1933.

The study aimed to investigate the influence of different microalgae used in green water culture on the intestinal morphology of the seahorse Hippocampus reidi during the first 15 days of life. Four treatments were tested in triplicate, consisting of two microalgae (Tisochrysis lutea-ISO and Chaetoceros muelleri-CHO), used either individually (TISO and TCHO) or combined (TIC 1:1) in larval rearing water, and a treatment without microalgae (TWM). Larvae (6.68 ± 0.55 mm) were fed copepods (Parvocalanus crassirrostris; 2 ind mL^-1) and rotifers (Brachionus rotundiformis; 8 ind mL^-1) from the first to the seventh day, and from the eighth day onwards, with Artemia sp. nauplii (2 ind mL^-1). For the analysis of intestinal histomorphometry, histological analysis of the tissue was performed on day 0 (initial) and after 7 and 15 days. The results demonstrate a significant increase in weight and weight gain of the larvae on the 15th day in the treatments with the addition of the microalgae T. lutea, whether alone or combination with other microalgae, compared to TWM. In relation the intestinal histomorphometry of larvae, the ANCOVA revealed that, on the 7th and 15th day, only the treatments with microalgae significantly affected the surface area of the intestinal villi. The intestinal histomorphometry of larvae from TISO showed higher values of villus height (85.29 ± 1.23 µm²) compared to TWM (69.42 ± 2.82 µm²) and TCHO (76.92 ± 2.23 µm²) (P < 0.05) but did not show significant differences compared to TIC (80.127 ± 2.08 µm²), which reflected on the villi surface area. Therefore, larvae from TISO (3603.43 ± 151.31 µm²) and TIC (3550.29 ± 120.99 µm²) showed significantly higher values of this parameter compared to TWM (3026.79 ± 147.64 µm²) and TCHO (3069.09 ± 126.18 µm²) (P < 0.05). In conclusion, the protocol for H. reidi larvae rearing in a green water system with the addition of the microalgae T. lutea resulted in an increase in the intestinal absorption area, resulting in improved performance of the larvae.