Pseudomonas plecoglossicida infection, which causes visceral white spot disease, is a significant and economically devastating disease in aquaculture. In this study, we investigated the impact of bacterial infection on the protein composition of exosomes derived from the surface mucus of the hybrid grouper Epinephelus fuscoguttatus♀ × E. lanceolatus♂. Two hundred healthy fish were randomly separated into challenge and control groups. Fish from the challenge group received 10³ CFU/g of the bacterial pathogen P. plecoglossicida via intraperitoneal injection, while sterile PBS was used as a negative control. After injection, the mucus was collected and the exosomes were extracted for proteomic analysis. The results of proteomic analysis revealed that P. plecoglossicida infection significantly increased the levels of innate immune proteins, including lysosomal and peroxisomal proteins, within the exosomes. Furthermore, the CAD protein was found to play a pivotal role in the protein interaction networks involved in the response to P. plecoglossicida infection. Intriguingly, we also observed a significant increase in the levels of metal-binding proteins within the exosomes, providing important evidence of nutritional immunity on the surfaces of the fish hosts. Notably, several proteins, such as plasma kallikrein, Annexin A5, eukaryotic translation initiation factor 3 subunit M, and S-methyl-5-thioadenosine phosphorylase, exhibited a remarkable increase in abundance in exosomes after infection. These proteins show promising potential as noninvasive biomarkers for the diagnosis of visceral white spot disease. The study contributes to the understanding of the host response to P. plecoglossicida infection and may aid policymakers in implementing appropriate intervention measures for effective risk management of this devastating disease.

In this study, we conducted molecular identification and complete mitochondrial genome annotation of isopod parasites isolated from mud crabs (Scylla paramamosain) in the China Sea. The specimen was identified as Portunion sinensis, a recently described species of the family Entoniscidae. P. sinensis is characterized by an anterior thorax positioned vertically and posterior ventral processes directed backwards; the female’s marsupium is formed by the close adhesion of oostegites to the host membrane; the first oostegite is differentiated into three parts; the pleopoda have well-developed lamellar and pleural lamellae; and two ventral ovarian processes are present along with a pair of dorsal processes. The complete mitochondrial genome of the parasite is 14,603 bp in length, containing 35 genes (13 protein-coding genes, 20 transfer RNAs, and 2 ribosomal RNAs) as well as a putative control region (CR) located between the cob and nad1 genes. This study represents the initial characterization of the mitochondrial genome of P. sinensis. These findings provide molecular validation of its status as an independent species and lay the groundwork for the future identification of parasites within the genus Portunion. These data provide a necessary molecular reference for future research into the systematics and evolutionary history of decapod-infesting parasites.