Myxobolus branchiostegi, Xiong & Zhao & Chen & Wang & Peng & Cui & Yang, 2025

Myxobolus branchiostegi sp. nov.

Type host.

Branchiostegus argentatus Cuvier, 1830 ( Perciformes , Malacanthidae ).

Type locality.

Xiamen ( 24°41'52"N, 118°10'59"E), East China Sea, China (Fig. 1 View Figure 1 ) GoogleMaps .

Infection site.

Gallbladder.

Date of sampling.

23 July 2017.

Prevalence.

Of 28 B. argentatus specimens examined, six were infected (21.4 %).

Type materials.

Syntypes (mounted in glycerine-alcohol-formalin; accession numbers XM 2017072306 ) were deposited in the collection centre of the Animal Biology Key Laboratory of Chongqing Education Commission of China, Chongqing Normal University, Chongqing, China.

Etymology.

The specific name branchiostegi is derived from the generic name of its host, Branchiostegus argentatus .

Description.

Vegetative stages were not observed. The mature myxospores are oval in shape, with blunt anterior and posterior ends, in both valvular and sutural views (Fig. 2 View Figure 2 ). Two equal-sized pyriform polar capsules are arranged in an inverted V-shape in the upper half of the myxospore, with their anterior third closely appressed to each other and containing a polar tubule coiled 4–6 turns. The sporoplasm is single and occupies the entire intracapsular cavity beneath the polar capsules, with a small, round nucleus visible in the lower half of the spore. The morphological measurements of mature myxospores (n = 25) are as follows: spore length 9.21 ± 0.52 (8.3–9.8) μm, spore width 7.40 ± 0.60 (6.2–8.0) μm, polar capsule length 4.40 ± 0.34 (3.7–4.8) μm, polar capsule width 2.58 ± 0.17 (2.3–2.9) μm.

Remarks.

Morphological comparison with congeners shows that M. branchiostegi sp. nov. most closely resembles Myxobolus arrabonensis Cech et al. 2015 , Myxobolus cerveirensis Rocha et al. 2019 , Myxobolus labrosus Rocha et al. 2019 , Myxobolus squamalis Iversen, 1954 ( Polley et al. 2013) and Myxobolus urinaris Rocha et al. 2019 (Table 1 View Table 1 ). The new species, together with M. cerveirensis , M. squamalis and M. urinaris , possesses ellipsoidal spores with bluntly rounded anterior and posterior ends in valvular view; however, their dimensions differ subtly. Compared with M. branchiostegi sp. nov. (9.21 ± 0.52 µm in length, 7.40 ± 0.60 µm in width), M. cerveirensis has slightly shorter and narrower spores (8.1 ± 0.2 µm × 6.8 ± 0.2 µm), M. squamalis slightly shorter, but wider spores (9.00 µm × 8.60 µm) and M. urinaris slightly longer and wider spores (10.0 ± 0.3 µm × 8.2 ± 0.3 µm). In contrast, M. arrabonensis and M. labrosus differ more markedly: the former has shorter and wider spores (8.7 ± 0.6 µm × 7.8 ± 0.34 µm), described as “ short ellipsoidal ” ( Cech et al. 2015), while the latter has longer and wider spores (10.0 ± 0.2 µm × 8.1 ± 0.3 µm) with an oval outline ( Rocha et al. 2019). Distinct sutural markings are visible in M. cerveirensis and M. labrosus , whereas they are absent in M. branchiostegi sp. nov. Ecologically, the similar species differ in host and infection site: M. arrabonensis infects the gill filaments of Chondrostoma nasus Linnaeus, 1758 ; M. cerveirensis the intestine of Chelon ramada Risso, 1827 ; M. labrosus the urinary bladder of Chelon labrosus Risso, 1827 ; M. squamalis the scales of Oncorhynchus mykiss Walbaum, 1792 ; and M. urinaris the urinary bladder of Mugil cephalus Linnaeus, 1758 . In contrast, the new species parasitises the gallbladder of B. argentatus . Their geographical ranges are likewise distinct: M. branchiostegi sp. nov. is known from China, whereas M. arrabonensis occurs in Hungary, M. cerveirensis , M. labrosus and M. urinaris in Portugal and M. squamalis in the United States (Table 1 View Table 1 ).

In addition to the above congeners, Myxobolus iwagiensis Kawano et al. 2025 , which exhibits the highest sequence similarity (92.0 %) to M. branchiostegi sp. nov. in the GenBank database, was also included in the morphological comparison. Compared with the present species, M. iwagiensis possesses noticeably larger spores in all measured dimensions, including spore length, spore width and the length and width of the polar capsules (Table 1 View Table 1 ). The two polar capsules in M. branchiostegi sp. nov. are equal in size, whereas those of M. iwagiensis are unequal. Ecologically, the two species also differ markedly in host and infection site: the new species was discovered in the gallbladder of B. argentatus collected from the East China Sea, whereas M. iwagiensis was reported from the peripheral nerves and beside the gill-arch cartilage of Oryzias latipes Temminck & Schlegel, 1846 , the species being obtained from a brackish reservoir in Japan.