# Dietary Astragalus Polysaccharides Can Improve the Immune Capacity and Reproductive Performance of the Lined Seahorse (Hippocampus erectus)

## TL;DR

Adding Astragalus polysaccharides to the diet of lined seahorses improves their immunity and reproductive success, helping address issues in aquaculture.

## Contribution

This study is the first to demonstrate the immune-boosting and reproductive-enhancing effects of Astragalus polysaccharides in seahorses.

## Key findings

- Dietary Astragalus polysaccharides increased juvenile seahorse survival, growth, and immune markers.
- APS improved intestinal microbiota diversity and reproductive performance in seahorse broodstocks.
- Newborn seahorses from APS-fed broodstocks had increased body height and faster mating readiness.

## Abstract

In aquaculture, due to the high stocking density, undiversified food, and monotonous environment, the decline in the immune function of cultured animals is a common issue. Therefore, some immunostimulants are usually used to improve the immunity and disease resistance of cultured animals. Astragalus polysaccharides (APSs) have been used as immunostimulants in aquaculture for more than 20 years, and their immune-promoting effects have been well-verified in a large number of cultured animals. Seahorses are widely cultivated in China, and recently also face the issue of decline in immune capacity. Up until now, there has been no report of APS application in this animal. In order to test whether APSs would also have immune-promoting effects on seahorses, we conducted several investigations in this study. The results showed that dietary APSs could significantly improve the growth, survivorship, plasma immune levels, and intestinal microbiota diversity of seahorse juveniles as well as the reproductive performance of seahorse broodstocks. These results are valuable for the healthy breeding of seahorses.

Seahorse (Hippocampus spp.) is popular in the markets of traditional Chinese medicine, aquarium, and curio. In order to protect wild stocks and still meet the market demand, China attempted the large-scale cultivation of seahorses in the early 21st century and achieved it in the 2010s. However, in recent years, two new issues have gradually emerged in Chinese seahorse cultivation. One is that the juveniles are prone to disease during diet conversion, and the other is that the reproductive performance of broodstocks is significantly reduced. With the aim to provide some measures that can alleviate these issues, in the present study, we used lined seahorse (Hippocampus erectus, a species widely cultured in China) as the experimental subject and Astragalus polysaccharides (APSs) as the immunostimulant to test whether APSs could improve the immune-health status and reproductive performance of seahorses. The measured indices for reproductive performance included ovarian lipid content, assessment time required before mating for paired male and female seahorses, mating success rate, brood size, and newborn body height. The results showed that for juveniles during diet conversion, their body weight, survival rate, plasma immunocytokine contents (interleukin-2, interferon-α, and immunoglobulin M), and alpha diversity indices (Simpson and Pielou’s-e) of the intestinal microbiota were significantly higher than those of the control group after dietary APSs. For broodstocks, compared with the control group, the expression of lipid substances in the ovary was significantly upregulated, the assessment time was significantly shortened, and the body height of their newborns was significantly increased in the APS group. These results demonstrate that APSs could indeed improve the immune-health status and reproductive performance of seahorses, providing guidance for addressing existing issues in seahorse cultivation.