This is the first report on ultrastructural features and a partial
SSU rRNA gene sequence of a microsporidian exclusively infecting
hepatopancreatic tubule epithelial cells of the black tiger shrimp P.
monodon. Its unique ultrastructural features linked it to the family
Enterocytozoonidae. Based on its distinction from the microsporidian
genera Nucleospora and Enterospora in the family and on its
similarity to the single species in the genus Enterocytozoon, we assigned
the new microsporidian infecting P. monodon to the genus
Enterocytozoon. We believe that its occurrence in an invertebrate
host and its relatively low SSU rRNA sequence identity with E. bieneusi
justify its assignment to a new species within the genuThe name E. hepatopenaei is proposed for the new species with
justification based on ultrastructural characters that conform to
the unique features of the family Enterocytozoonidae but distinguish
it from other species in the family, i.e., E. beiniusi that also
develops in the host-cell cytoplasm and species in the genera Ent-cycle stages such as diplokaryotic forms and gametes might occur
in an alternative host, and that shrimp could constitute such a host.
However, the fact that the spores in P. mondon are also monokaryotic
and that they develop in a manner similar to those of E. bieneusi
rather than developing from diplokaryotic or meiotic nuclei
(Flegel and Pasharawipas, 1995) argues against this possibility. In
addition, work on possible sources of E. bieneusi in human infections
has implicated birds and mammals rather than shrimp, and
fresh water sources rather than marine (Dowd et al., 1998; Didier
et al., 2004; Lobo et al., 2006). All this information argues in favor
of our contention that E. hepatopenaei is distinct from E. bieneusi.The argument for distinction of the two species is further supported
by the 16% difference in identity for compared fragments
of their SSU rRNA genes. A rough estimation of the sequence identity
cutoff for a species is approximately 97% (Stackebrandt and
Goebel, 1994) so that 84% identity is well outside the range of what
would be considered reasonable for con-specific isolates. Indeed,
the fact that N. salmonis and E. bieneusi are in different genera
and differ by 85%, might suggest that the hepatopancreatic microsporidian
in P. monodon should also be placed in a new genus.
Thus, our proposal for a new species within the genus Enterocytozoon
rather than a new genus may be viewed as conservativ