A new microsporidian species, Enterocytozoon hepatopenaei sp. nov., is described from the hepatopancreas
of the black tiger shrimp Penaeus monodon (Crustacea: Decapoda). Different stages of the parasite
are described, from early sporogonal plasmodia to mature spores in the cytoplasm of host-cells. The multinucleate
sporogonal plasmodia existed in direct contact with the host-cell cytoplasm and contained
numerous small blebs at the surface. Binary fission of the plasmodial nuclei occurred during early plasmodial
development and numerous pre-sporoblasts were formed within the plasmodium. Electron-dense
disks and precursors of the polar tubule developed in the cytoplasm of the plasmodium prior to budding
of early sporoblasts from the plasmodial surface. Mature spores were oval, measuring 0.7 1.1 lm and
contained a single nucleus, 5–6 coils of the polar filament, a posterior vacuole, an anchoring disk attached
to the polar filament, and a thick electron-dense wall. The wall was composed of a plasmalemma, an electron-
lucent endospore (10 nm) and an electron-dense exospore (2 nm). DNA primers designed from
microsporidian SSU rRNA were used to amplify an 848 bp product from the parasite genome (GenBank
FJ496356). The sequenced product had 84% identity to the matching region of SSU rRNA from Enterocytozoon
bieneusi. Based upon ultrastructural features unique to the family Enterocytozoonidae, cytoplasmic
location of the plasmodia and SSU rRNA sequence identity 16% different from E. bieneusi, the
parasite was considered to be a new species, E. hepatopenaei, within the genus Enterocytozoon.