Ultrastructure of two microsporidians Inodosporus sp. and Myospora sp. co-infecting muscles of the Amazon River prawn Macrobrachium amazonicum (Heller, 1862)

Abstract

In the present study, we describe a co-infection of two microsporidians within the abdominal muscle of the Amazon River prawn Macrobrachium amazonicum from Brazil, detected through light and transmission electron microscopy and histopathological analysis. Two spore types and respective early developmental stages (meronts, sporonts, and sporoblasts) appeared grouped in numerous adjacent clusters among the muscle fibers of the host. Spores were initially divided into two morphotypes, Sp. 1 and Sp. 2, based on differences in shape, size, and internal organization. The Sp. 1 spores are pyriform to ovoid (4.3 ± 0.3 × 3.7 ± 0.4 µm in size) and resided in groups of eight within a sporophorous vesicle. Sp. 1 spores had bilayered walls and long tape-like external filaments with irregular morphology and size, forming a complex contiguous membranous system attached to the spore wall. The umbrella-like anchoring disc of the spores was in continuity with an anisofilar polar tube arranged in 7‒8 (rarely 9) coils. The Sp. 2 spores were rod-like in shape (3.1 ± 0.5 × 0.8 ± 0.1 µm in diameter) and the polar tube had 7‒8 coils. Their merogonic and sporogonic stages occurred within the sarcoplasm of the muscle cells in close contact with the myofibrils. Based on ultrastructural organization of the sporogonic stages, the Sp. 1 and Sp. 2 morphotypes probably belong to the genera Inodosporus and Myospora, respectively.

Keywords
Crustacea; Microsporidia; muscle infection; spore dimorphism