Comparative analysis of the effect of nano-copper and Cu2O treatment on Cryptocaryon irritans tomonts in large yellow croaker (Larimichthys crocea) farming

Cryptocarya irritans is an obligate parasite causing the "white spot disease." This disease often leads to the death of a large number of cultured marine fish. This study aimed to compare the effects of cuprous oxide (Cu2O) and nano-copper on the morphology and hatchability of tomonts to investigate the effects of copper derivatives on C. irritans tomonts. The potential mechanistic difference was also explored using the transcriptome. The results showed that Cu2O seriously damaged the internal cell structure of tomonts, and the hatching rate of the tomonts in the Cu2O-treated group was lower than that in the control group within 10 days. On the contrary, the effect of nano-copper on tomonts was not significant. The cytoplasm of tomonts treated with nano-copper dissolved, the internal structure was slightly loose, and the hatching rate of tomonts was even slightly higher than control group within 5 days. In the transcriptome analysis, 850 differentially expressed genes (DEGs), including 332 upregulated genes and 518 downregulated genes, were identified in the Cu2O-treated groups. However, only 56 DEGs, including 34 upregulated genes and 22 downregulated genes, were identified in the nano-copper-treated groups. This indicated that the effect of Cu2O on tomonts was greater than nano-copper. In the enrichment pathway analysis, DEGs in the Cu2O-treated group were mostly enriched in the ribosome and apoptosis path-ways. Further analysis showed that genes (RPL18e, RPLP0, RPL10, RPL17, RPL26, RPL35, and RPL37) related to cell stress and defense were significantly upregulated and those (P53 and PKA) related to cell proliferation and development were downregulated, indicating that the killing effect was mainly mediated by different ribosomal and structural proteins. However, the DEGs (KCNMA1, CACNA1C, CNGA1) in the nano-copper-treated group were enriched in the insulin secretion and cAMP signaling pathways, which were activate part of tomonts' innate immune response, completely different from those in the Cu2O-treated group. The results provided more in-formation on the mechanism of killing parasites by copper derivatives.