Morphological, ultrastructural, and molecular description of Unicapsula fatimae n. sp. (Myxosporea: Trilosporidae) of whitespotted rabbitfish (Siganus canaliculatus) in Omani waters

Abstract

Investigations regarding the parasite fauna of wild whitespotted rabbitfish (Siganus canaliculatus) Park, 1797 revealed white, spherical, loosely attached cysts measuring 896 (375-1406) μm in diameter in the inner endothelial wall of the esophagus and stomach. Mature spores inside these cysts corresponded to the original description of spores belonging to the genus Unicapsula Davis, 1924. Unicapsula fatimae n. sp. spores were 6.23 (5.60-6.60) μm in length and 6.80 (6.12-7.39) μm in width. The length of large polar capsule was 2.62 (2.18-2.97) μm and width was 2.65 (2.32-2.90) μm, and the extended large polar capsule filament length was 15.50 (11.71-19.99) μm. Transmission electron microscope images of the plasmodia revealed a complex cyst structure that was unique among other Unicapsula spp. Ultrastructural details of the host-parasite interface and developmental stages of a species from the Unicapsula genus are described for the first time. Histology of an infected esophagus revealed some abnormalities and changes in the host tissue around the infection site, including hypertrophy of host esophagus epithelial cells and hyperplasia of host glandular tubules. The parasite presented here has been added to the genus Unicapsula using comparative morphological analysis and ultrastructural investigations supported by 18S small subunit ribosomal DNA molecular analysis.

Keywords: Arabian Sea; Esophagus; Molecular; Myxozoa; Oman; Siganidae; Unicapsula.

Fig. 1

Line drawings depicting mature spores of Unicapsula fatimae n. sp. frontal view (a) and apical view (b). Scale bars a and b = 5 μm

Fig. 2

a Heavily infected esophagus of Siganus canaliculatus collected from Muttrah local fish market showing numerous Unicapsula fatimae cysts (>100 cysts detected). b Close-up of a portion of the infected esophagus showing the variable sizes of the cysts and several “empty” cysts. c Magnified portion of the esophagus showing two full cysts (asterisk) and two “empty” cysts (arrow heads). Scale bar 3 mm for b and 500 μm for c

Fig. 3

a Mature spores of Unicapsula fatimae. b Some mature spores of U. fatimae with visible rudimentary polar capsules (arrows). c Apical view of U. fatimae spores with extruded polar filament. d Extruded polar filament of U. fatimae tapering to the anterior portion and with double turns (arrow heads). Scale bar for all images =5 μm

Fig. 4

a SEM image of the apical pole view of a mature spore, showing the position of the large functional polar capsule and two rudimentary polar capsules immediately below it. The capsulogenic cells bearing the rudimentary polar capsule can be seen as two protrusions that take a leaf shaped form. The sutural lines form a Y shape on both the apical pole and basal pole view (b), dividing the three valves equally. Scale bar for a, b = 1.0 μm, 2.0 μm and 1.0 μm, respectively

Fig. 5

a Semi-thin section through a cyst showing the division of the cyst complex into several layers, the endoplasmic region (EN), ectoplasmic region (EC), peripheral membrane (arrows), connective tissue (CT), and host tissue (HT). b Ultrathin section of the plasmodia showing a close-up of the plasmodia and host interface with details of the peripheral membrane which is located between the ectoplasmic region and the connective tissue wall; note the host cell (asterisk) with the nucleus (HN) trapped inside the membrane and the several arms or web- like structure which possibly could be pinocytotic channels (PiC). c Details of the ectoplasmic region with several single nucleus GC generative cells, PS Pansporoblast, mi mitochondria, lipid droplets, and young spores. d Young spore with a developing polar capsule or CP capsular perimordium of the large polar capsule. Furthermore, note the two structures (arrows), which appear to be perimordium of the RCP rudimentary polar capsules. Scale bar for b, c, d = 1.0, 2.0, and 1.0 μm, respectively

Fig. 6

a Section through a mature spore showing the large polar capsule and the two rudimentary polar capsules with what appears to be a rudimentary polar filament. b Section through a mature polar capsule with the filaments showing 2 and half turns and the opening of the polar capsule. c Section through the sporoplasm of some mature spores showing the two adjacent nuclei. d Close-up of the sporoplasm showing the two nuclei and indicating the second membrane which could be a secondary sporoplasm (arrow heads) within the main sporoplasm. Scale bars: a and d = 0.2 and c = 0.5 μm

Fig. 7

a Gross morphology of Unicapsula fatimae n. sp. cyst showing part of the host esophageal tissue, the stalk (peduncle) by which the parasite is attached on to the host tissue and the spherical plasmodia that is surrounded by host tissue (white arrow head). b Histological section through U. fatimae host complex showing the structure of the esophagus tissue near the infection site command position of the stalk structure and cyst complex. c Close-up of the cyst complex showing the hypertrophic folded host esophagus epithelial cells (HE) and glandular tubules (GT) that comprise the stalk formation. d shows the formation of an abnormal structure (asterisk) within the submucosal region of the esophagus. Scale bars: a = 500 μm, b = 600 μm, and c = 400 μm

Fig. 8

Maximum likelihood topology of 20 histozoic marine myxosporean SSU rDNA sequences using PhyML. Tree shows the phylogenetic relationship between the species of Unicapsula based on the available Unicapsula and the closest matches of 18s rDNA sequences available on GenBank NCBI. Numbers at the nodes represent bootstrap support values; nodes with no numbers are fully supported