Description of Ellipsomyxa prima n. sp. in the gallbladder of Gambusia yucatana (Cyprinodontiformes: Poeciliidae) from freshwater springs in the Yucatán Peninsula, Mexico.

The Celestún Lagoon is located on the coast of the Yucatán Peninsula in southern Mexico. The peripheral margins of the lagoon are exposed to a constant influx of water from freshwater springs, where a wide diversity of freshwater fishes occurs. Simultaneously, these freshwater springs are mixing with marine water from the sea and brackish water from the lagoon. Gambusia yucatana is an endemic poeciliid freshwater fish of this coastal region, with no current parasitological records. In December 2023, 24 specimens from two freshwater springs near the Celestún Lagoon were examined for myxozoan infection. Disporic plasmodia containing mature myxospores floating freely within the bile were detected in all fish. The myxospores were ellipsoidal in the valvular and sutural views, measuring 9.5 ± 0.6 μm length and 6.5 ± 0.5 μm width. Two pyriform polar capsules discharging on opposite sides, 3.2 ± 0.4 μm length and 2.3 ± 0.2 μm width, and 5-6 polar filament coils. Polar tubule 31.3 μm long and coiled 5-6 times. The partial sequences of 18S rDNA and 28S rDNA displayed a similarity of ≤ 96.4% and ≤ 85.2% to all other available sequences from the genus Ellipsomyxa. Phylogenetic analyses revealed that the novel species named Ellipsomyxa prima n. sp. is closely related to other congeneric Ellipsomyxa parasites. This study describes the first parasite in G. yucatana and the first Ellipsomyxa species reported in a fish of the order Cyprinodontiformes and the family Poeciliidae.