Description of Triangulamyxa psittaca sp. n. (Myxozoa: Myxosporea), a new parasite in the urinary bladder of Colomesus psittacus (Teleostei) from the Amazon River, with emphasis on the ultrastructure of plasmodial stages

A fish-infecting myxosporean was found in the urinary bladder of the teleostean Colomesus psittacus, collected from the Amazon River, Brazil. Specimens were sampled in three different periods: May and June, with water temperature ranging from 18–23ºC; August, with water temperature ranging from 24–28ºC; and November and December, with water temperature ranging from 29–32ºC. Upon observation, several fish displayed abnormal behaviour, consisting of erratic movements, and mortality was recorded among them. Necropsy of all sampled fishes revealed hypertrophy of the urinary bladder only among specimens previously displaying the irregular behaviour. Microscopic analysis of this organ confirmed the parasitic infection, resulting in the observation of spores floating free in the urine, and numerous plasmodia attached to the epithelium of the urinary bladder. Light and ultrastructural studies allowed recognition of the spores and plasmodia morphological characteristics. Coelozoic plasmodia were polysporic with varying organizational structure, according to the sampling period. Spores were equilaterally triangular with rounded ends in valvar view, measuring 8.8 ± 0.4 μm (n = 30) in length and 8.4 ± 0.5 μm (n = 30) in width, and displaying a ridge surface pattern. Two polar capsules were observed in the anterior end of the spores, measuring 3.1–3.2 μm in diameter. The spores were morphologically identified as belonging to the recently described genus Triangulamyxa. Further observation and comparison to the morphological features described for Triangulamyxa amazonica, the only other species within this genus, allowed us to conclude our parasite as a new species, herein named Triangulamyxa psittaca sp. nov. from the Amazon River, Brazil. Also, three different stages were distinguished in the plasmodium evolution, based on the observed morphological features at the three sampling periods. Fish sampled during May and June displayed small plasmodia (up to ~ 15–20 μm long), containing early stages of sporogenic development. Fish sampled during November and December presented larger plasmodia (up to ~ 850 μm long), which appeared flattened against and lining the urinary bladder epithelial cells and contained the later stages of sporogenic development, including some mature spores. Fish sampled during August presented plasmodia displaying intermediate morphological features between those observed in infected fish from the other sampling periods. Several immature and mature spores were among the different developmental stages. The parasite-host interface evolution is described throughout the different observed stages, with emphasis on the formation of septate junctions. Considering several previous reports, as well as the different environmental conditions during the sampling periods, the plasmodium development here described appears to be influenced by environmental factors, namely water temperature.