New Myxozoan Parasite Found Lurking in the Intestinal Wall of Barbel Chub in China

Deep in the freshwater systems of Hunan Province, China, a microscopic parasite has been quietly colonizing the intestinal wall of a fish of growing aquacultural importance. The discovery, published in Zoosystematics and Evolution, marks the first record of any myxosporean parasite infecting the barbel chub Squaliobarbus curriculus in this region and adds a new branch to one of the most speciose lineages of animal parasites on Earth. These parasites target tissues ranging from gills and skin to intestine and liver, and while many species are considered harmless, some are responsible for devastating diseases in aquaculture.

Modern taxonomy has therefore shifted toward an integrative framework that combines morphology, host and tissue specificity, and molecular sequence data, allowing researchers to separate cryptic species that look nearly identical under the microscope. The new study exemplifies this approach, synthesizing evidence from spore morphometrics, histopathology, and ribosomal DNA phylogenetics to establish the new species on firm ground. The survey took place between June and December 2024 in the Zi River near Xinhua County in Loudi City, Hunan Province, an area whose river systems and Dongting Lake support a documented fish fauna of 218 species.

Yet myxosporean surveys in this province remain scarce, with only two prior studies, one conducted in the last century and another covering just twelve fish species. Thirty-four barbel chub specimens, weighing between 316.5 and 498.5 grams, were collected using gill-nets and examined comprehensively upon arrival at the laboratory. Infection was found in 11 of the 34 fish examined, a prevalence of 32.35 percent.

Numerous whitish plasmodia, ellipsoidal in shape and measuring up to 2 millimeters long and 1.5 millimeters wide, were distributed throughout the intestinal wall. No infections were detected in other organs, and despite the extensive plasmodial burden, infected fish showed no abnormal behavior compared with healthy individuals. Upon rupturing the plasmodia, mature myxospores were released and observed.

These spores were round in the valvular view and lenticular in the sutural view, measuring 8.8 micrometers long, 8.3 micrometers wide, and 3.8 micrometers thick on average. Two equal, pyriform polar capsules sat at the anterior end, each containing a polar tubule coiled with four to five turns. The spores lacked a mucous envelope and intercapsular appendix but possessed three to four distinct posterior folds, a combination of traits that distinguishes the new species from its relatives.

Histopathological examination revealed that the plasmodia developed within the circular muscle layer of the host intestine, a tissue site frequently favored by intestinal myxozoans. Expanding plasmodia compressed adjacent muscle fibers and became surrounded by a thin connective tissue capsule, yet the infection elicited no observable inflammatory response. No muscle fiber atrophy, necrosis, mucosal damage, epithelial desquamation, or obstruction of the intestinal lumen was detected.

The pronounced tropism for muscular tissue, while not fully understood, may be linked to the relatively high availability of nutrients and metabolic resources in this tissue, which could favor parasite proliferation. To characterize the parasite molecularly, researchers extracted genomic DNA from ethanol-preserved plasmodia and amplified a partial sequence of the small subunit ribosomal DNA using universal myxosporean primers. The resulting sequence, 1,731 base pairs in length, was deposited in GenBank under accession number PV785691.

Phylogenetic reconstruction based on a dataset of 43 SSU rDNA sequences, rooted with Zschokkella nova as the outgroup, yielded two primary clades, A and B, with highly similar topologies from both Maximum Likelihood and Bayesian Inference methods. Notably, the new species did not cluster with other Myxobolus species known to infect the intestine, such as Myxobolus huasaensis, Myxobolus gayerae, and Myxobolus kiuchowensis. This pattern suggests that intestinal tropism may not provide a strong phylogenetic signal in this group, in contrast to site specificity at organs such as the gills, where species infecting similar tissues often cluster together.

The analysis also revealed structured patterns linking myxozoan evolution to morphology, host phylogeny, and biogeography. Subclade I predominantly comprises species with ellipsoidal myxospores infecting cyprinid and leuciscid hosts in Hungary and Portugal, while Subclade II consists of species with ellipsoidal spores infecting these host families across a broader Holarctic range including Europe and North America. Subclade III, by contrast, is primarily composed of species from China and India infecting cyprinid hosts, exhibiting a mix of spore shapes including ellipsoidal, pyriform, and round forms.

This diversity suggests that spore morphology within this lineage is evolutionarily labile and may have undergone multiple transitions, reinforcing the conclusion that myxozoan evolution is shaped by a combination of ecological, morphological, and biogeographic factors rather than by any single trait. The discovery of Myxobolus curriculus sp. nov. enriches the known myxosporean fauna of Hunan Province and establishes the first record of any myxosporean parasite in barbel chub from this region. The barbel chub, a widely distributed cyprinid of growing aquacultural interest due to its disease resistance, including potential resistance to grass carp hemorrhage disease, had only approximately 11 myxosporean species reported globally before this study.

Subject of Research: Myxozoan parasite taxonomy and biodiversity

Article Title: Morphological and molecular characterizations of Myxobolus curriculus sp. nov. ( Myxozoa , Myxobolidae ) infecting the intestine of Barbel chub Squaliobarbus curriculus in China

Article References: Yuan, S., Liu, X., Geng, X., Liu, W., He, M., Li, D., Xiang, J., & Dai, W. (2026). Morphological and molecular characterizations of Myxobolus curriculus sp. nov. (Myxozoa, Myxobolidae) infecting the intestine of Barbel chub Squaliobarbus curriculus in China. Zoosystematics and Evolution, 102(3), 807-815. https://doi.org/10.3897/zse.102.191790

Image Credits: AI Generated

DOI: 10.3897/zse.102.191790

Keywords: Myxozoa, Myxobolus, parasitology, fish parasites, taxonomy, China, aquaculture, biodiversity, Morphological, molecular, characterizations, curriculus