Myxobolus opsaridiumi, Lekeufack-Folefack, Guy Benoit, Tchoutezo-Tiwa, Armandine Estelle, Fomena, Abraham & Mansour, Lamjed, 2021

Lekeufack-Folefack, Guy Benoit, Tchoutezo-Tiwa, Armandine Estelle, Fomena, Abraham & Mansour, Lamjed, 2021, Myxobolus opsaridiumi sp. nov. (Cnidaria: Myxosporea) infecting different tissues of an ornamental fish, Opsaridium ubangiensis (Pellegrin, 1901), in Cameroon: morphological and molecular characterization, European Journal of Taxonomy 733, pp. 56-71 : 59-64

Myxobolus opsaridiumi sp. nov.

urn:lsid:zoobank.org:act:F3105EFD-A1BC-483F-8FC8-5D07FC9B82F0

Figs 1–4 ; Table 1

Etymology

The specific epithet is related to the host genus name.

Type material

CAMEROON • infected skin, muscle and spleen of Opsaridium ubangiensis with plasmodia; Centre Region , Anga River, Yaounde ; deposited in parasitological collection of the Zoology Department Museum, College of Science, King Saud University, Saudi Arabia; Myxospsar/12/2018 .

Taxonomic summary

Type host

Opsaridium ubangiense Pellegrin, 1901 (Cyprinidae) .

Infected tissues

Skin, muscles and spleen.

Prevalence

54.7% (288 parasitized fish out of 526 examined).

Vegetative stages

Ovoid, spherical or ellipsoid plasmodia, variable in size, measuring from 0.3 mm to 2.5 mm in length and 0.2 mm to 1.5 mm in width.

Description of myxospores ( Fig. 1 )

Mature spores were ovoid to subspherical in frontal view and lenticular in lateral view ( Fig. 1 A–B). The valves were relatively thick, without edge markings. Intercapsular processes were absent. The spore size was 10.7 ± 0.14 (10–11.5) µm long, 9 ±0.15 (8–10) μm wide and 6.2± 0.7 (5.6–7.2) μm thick. The two ovoid polar capsules were equal in size, converging and opening together at the anterior end of the same pore ( Fig. 1A , C–D). They measured 5± 0.07 (4.3–6.0) μm in length and 2.7± 0.07 (2.2–3.0) μm in width. Polar filaments were coiled from 5 to 7 turns perpendicular to the longitudinal axis of the polar capsules ( Fig. 1D ). A sporoplasm containing an iodinophilous vacuole of varying shape and size filled the entire space below the polar capsules ( Fig. 1A ).

Clinical finding and histopathology

Based solely on gross observation of the fish, no signs of disease were observed. Parasitized fish harbored cysts on skin, muscles and spleen. On skin, white cysts up to 2 mm long were collected from the body flanks of some fish ( Fig. 2A ). Sections revealed that plasmodia developed in the connective tissue of the dermis beneath the underside of scales ( Fig. 2B ). Plasmodia were flattened and surrounded by a thin membrane and an internal endoplasm comprising a loosely defined matrix containing developed spores ( Fig. 2B ).

Some plasmodia were spotted within muscle cells ( Fig. 2C ). Plasmodia were spindle-shaped, centrally located in the cell and not surrounded by a visible wall. No evidence of inflammation or immune-cell recruitment was seen. The integrity of myofibrils within the infected fibers showed some degree of lysis, with partial loss of myofibrillar details and striations ( Fig. 2 C–D). These lesions were observed close to plasmodia. Mature spores were scattered in the cytoplasm of infected cells ( Fig. 2D ).

Infected spleens had plasmodia of up to 2.5 × 1.5 mm ( Fig. 3A ). They were white, isolated or clustered ( Fig. 3B ). Some infected spleens were heavily infected and plasmodia were randomly distributed in the whole organ. In these cases, abnormal enlargement of the spleen was evident ( Fig. 3C ). Histological sections revealed that, for moderately infected spleens, cysts were either fixed to the external region of the organ ( Fig. 3D ) or completely implanted within it ( Fig. 3E ). Development of cysts in the spleen was asynchronous ( Fig. 3 D–F). Atrophy of the adjacent splenic cells surrounding the cyst was likely due to mechanical compression ( Fig. 3 F–G). Each plasmodium was surrounded by a wall of a monolayer of flat cells ( Fig. 3 F–G). The central part of the plasmodium was occupied by fully mature spores, with initial stages of development visible in the periphery ( Fig. 3G ).

Phylogenetic position

Partial SSU rDNA sequences obtained fromdifferent organs were 100% identical.The consensus sequence of 1667 base pairs was submitted to GenBank with the accession number MN497413 . This sequence did not match any publicly available myxozoan sequence. The sequence with the highest nucleotide similarity, at 91.8%, was for Myxobolus haichengensis Chen, 1958 (GenBank entry KY965936 ), which reportedly infects the gills of Abbottina rivularis (Basilewsky, 1855) . Similarity with M. dibombensis Folefack et al., 2019 , a species we recently sequenced from Labeobarbus batesii (Boulenger, 1903) in Cameroon, was only 88.6%.

The phylogenetic position of the newly sequenced species was analyzed with maximum likelihood and Bayesian inference methods. Both methods produced an identical topology. Myxobolus opsaridiumi sp. nov. occurs in a large clade that includes species infecting cyprinids ( Fig. 4 ). The new species exhibits the highest phylogenetic affinity with M. haichengensis , Myxobolus sp. (accession number MG253819 ) from the fins of Capoeta tinca (Heckel, 1843) off Anatolia, and M. squamae Keysselitz, 1908 infecting the skin of the common barbel Barbus barbus (Linnaeus, 1758) .

Figures

Fig. 1. Photomicrographs of Myxobolus opsaridiumi sp. nov. infecting skin, muscle and spleen of Opsaridium ubangiensis (Pellegrin, 1901). A. Fresh myxospores in frontal view. B. Fresh myxospore in lateral view. C. Giemsa-stained myxospores. D. Diagrammatic drawing of a mature myxospore.

Fig. 2. Photomicrographs of plasmodia of Myxobolus opsaridiumi sp. nov. developing on Opsaridium ubangiensis (Pellegrin, 1901). A. Plasmodium development on the skin. B. Histological section stained with hematoxylin and eosin showing plasmodium situated in the dermis. C. Plasmodium developing within muscle fibers (hematoxylin and eosin). D. Higher magnification of a plasmodium from the muscle fibers.

Fig. 3. A–C. Photomicrographs of plasmodia of Myxobolus opsaridiumi sp. nov. affecting a spleen of Opsaridium ubangiensis (Pellegrin, 1901). A. Spleen harbouring large plasmodia. B. Whitish plasmodia isolated from each other (black arrows) or arranged in grape-like clusters (white arrow). C. Spleen completely filled with plasmodia. – D–G. Histological sections stained with hematoxylin and eosin of spleens of O. ubangiensis infected with plasmodia of M. opsaridiumi sp. nov. D. Plasmodia implanted on the external region of the spleen. E. Asynchronous development of plasmodia within the spleen. F. Mechanical compression of the cells adjacent to the cysts. G. Higher magnification of plasmodia showing each surrounded by a wall and full of myxospores. Abbreviation: P = plasmodium.

Fig. 4. Maximum likelihood phylogenetic tree based on the SSU rDNA sequences showing the position of Myxobolus opsaridiumi sp. nov. (in bold) and related species. Accession numbers and infected tissues are listed adjacent to the species names. Numbers at the nodes represent Bayesian posterior probabilities and ML bootstrap percentages. Kudoa thyrsites (Gilchrist, 1924) was used as the outgroup.