Seahorses are increasingly regarded as a promising farming object suitable for recirculating aquaculture systems (RASs) due to their high economic value. However, reports on the large-scale farming of seahorses in RASs are rare, and some key parameters, such as stocking densities, are still unclear. In the present study, we employed the lined seahorse (Hippocampus erectus), for which large-scale farming has been achieved, to determine the suitable stocking density for three different-sized juveniles in RASs. The three different-sized juveniles had body heights of 4.0, 7.0, and 9.0 cm, and their test density gradients were 1.0, 0.8, 0.6, and 0.4 inds/L; 0.6, 0.5, 0.4, and 0.3 inds/L; and 0.4, 0.3, 0.2, and 0.1 inds/L, respectively. The juveniles were cultivated for one month, and then their survival, growth, and plasma cortisol and brain serotonin contents (two stress-related indicators) were analyzed. The results show that, regardless of the size of the juveniles, a high density can inhibit growth and trigger stress responses. In addition, for small- (4.0 cm) and medium-sized (7.0 cm) juveniles, a high density can also exacerbate size heterogeneity and cause death. Taking into account the welfare and yield of farmed seahorses, the present study suggests that the suitable stocking densities for 4.0, 7.0, and 9.0 cm juveniles in RASs are 0.6, 0.4, and 0.2 inds/L, respectively.

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.

Waterborne chemical cues mediate reproductive interactions in decapod crustaceans, but interspecific variation in these cues remains poorly characterized. We compared behavioral responses and exploratory chromatographic profiles among four Lysmata species with contrasting social organizations: L. amboinensis, L. boggessi, L. debelius, and L. pederseni. Intermolt euhermaphrodite-phase (EP) focal shrimp were tested with conspecific and heterospecific postmolt EP-conditioned water, uridine-5′-triphosphate (UTP), uridine-5′-diphosphate (UDP), and seawater control. Conspecific conditioned water generally elicited the strongest responses, whereas response magnitude and type differed among species. UTP elicited the defined approach and follow response only in L. boggessi. UDP elicited no behavioral response in any species. Exploratory HPLC of one pooled composite conditioned-water sample per species revealed peaks at 17.01 and 24.29 min, with retention characteristics consistent with UDP and UTP standards, respectively. Two additional peaks at approximately 30.81 and 41.10 min were visually apparent in the pooled chromatograms of L. amboinensis, L. debelius, and L. pederseni. These results demonstrate interspecific differences in behavioral responsiveness and identify chromatographic features that warrant future testing with structural chemical identification.

Ectotherms can elevate their body temperature in response to infection by seeking warmer environments, a phenomenon known as behavioral fever. This adaptive response, widely documented in fish, activates immune defenses and improves survival. To explore an eco-friendly approach for managing Vibrio-induced enteritis in lined seahorse (Hippocampus erectus) aquaculture, we investigated whether Vibrio harveyi infection triggers behavioral fever and enhances immune function. Seahorses were intraperitoneally injected with V. harveyi (1 × 10⁷ cfu/fish) and placed in a thermal gradient tank (19–31 °C), allowing free movement between chambers. Challenged seahorses exhibited a significant preference (p < 0.05, 1.31-fold) for warmer zones compared to unchallenged controls, whereas no such difference (p > 0.05) was observed in a constant temperature (25 °C) tank, confirming behavioral fever. Furthermore, fevered seahorses showed significantly elevated plasma cytokine levels (PGE2, IL-1β, IL-6, and TNF-α; p < 0.05), which normalized (p > 0.05) to baseline levels, except for TNF-α, compared to unfevered individuals. In kidney tissue, challenged seahorses expressing behavioral fever exhibited gene expression levels (tnf-α, il-6, ifn-g, and il-10) similar to unchallenged controls (p > 0.05) but significantly lower (p < 0.05) than those kept at constant temperature. These findings suggest that behavioral fever in H. erectus modulates core temperature to regulate cytokine release and immune-related gene expression. This study provides foundational insights for developing practical, non-invasive strategies to mitigate enteritis in seahorse aquaculture through thermal behavior manipulation.