Scanning Electron Microscopy
The phylum Haplosporidia is described as being a group of parasitic protists that form uni-nucleated spores without polar capsules or polar filaments; the sporoplasm is contained by a spore wall that has an orifice at one pole (Perkins, 1990, 1996). There are three genera- Minchinia Labbé 1896, Haplosporidium Caullery and Mesnil 1899, and Urosporidium Caullery and Mesnil 1905.
Presently, only three genera- Haplosporidium, Minchinia and Urosporidium -are recognized with certainty in the phylum Haplosporidia. Urosporidium is distinguished by an internal diaphragm covering the spore orifice rather than an external hinged operculum as in both Minchinia and Haplosporidium. The distinction between Haplosporidium and Minchinia has been the subject of much debate and definitions of the two genera are still unsettled.
Within the last two decades, electron microscopy has increased our understanding of spore morphology, but has not reduced the classification confusion. With electron microscopy, a variety of tubular, filamentous or ribbon-like strands of material can be observed on the external spore surface and are referred to throughout the literature as ornamentation (Perkins, 1996). This ornamentation is structurally diverse within the phylum. Differentiation between Minchinia and Haplosporidium has been based on the presence and type of spore projections. Unfortunately descriptions and terminology throughout the literature have been vague (McGovern and Burreson, 1990). In the past, authors have used a variety of terms in the description of ornamentation or in some cases have not included descriptions at all. This inconsistency has led both to misinterpretation and to problematic placement of species. The problem is exacerbated because the ornamentation of H. scolopli, the type species of Haplosporidium, is unknown. Caullery and Mesnil (1905) illustrate H. scolopli spores that appear to have extensions in some drawings, but not in others. Both Sprague (1963) and Perkins (1996) argue that H. scolopli spores do not have extensions, but Ormières (1980) argues that they do have extensions. It is difficult to establish a definition for the genus Haplosporidium when the status of spore ornamentation of the type species is unknown.
Sprague (1970) suggested that presence or absence of tails or filaments be used as the basis for distinguishing between Minchinia and Haplosporidium species. Ormières (1980) was the first to consider the structural origin of the ornamentation and distinguished between extensions of the spore wall and extensions of epispore cytoplasm. Ormières (1980) defined Minchinia spores as possessing tails (extensions of epispore cytoplasm) and Haplosporidium spores as possessing filaments (extensions of the spore wall that persist after degradation of epispore cytoplasm). Perkins (1990, 1991), however, grouped species with spores with extensions visible with a light microscope, irrespective of whether they originate from the spore wall or in epispore cytoplasm, in the genus Minchinia, and species with spores lacking such extensions in the genus Haplosporidium.
In the most recent review of the phylum Haplosporidia (Perkins, 1996), the three genera have been defined as follows:
- The genus Haplosporidiumis characterized by oval spores possessing an operculum covering the orifice, which gives the spore a slightly flattened look on one end when seen through a light microscope. Spores are wrapped with filaments formed in the epispore cytoplasm. Under this definition there are fifteen named Haplosporidium species found in freshwater oligochaetes and molluscs and in marine molluscs, crustacea, urochordates, echinoderms, and polychaetes.Haplosporidium nelsoni (marine, in oysters)
- The genus Minchinia is characterized by species having spores of the same general shape as those of Haplosporidium except ornamentation is organized into prominent extensions which are visible with light microscopy; developmental origin of the extensions (i.e., whether derived from epispore cytoplasm or the spore wall) is not considered. Nine named species are parasites of marine polychaetes and molluscs.
Minchinia teredinis (marine, in shipworms)
Minchinia tapetis (marine, in limpets)
Minchinia pickfordae (freshwater, in snails)
- In the genus Urosporidium, spores are either oval or spherical and also have an anterior orifice. In contrast to the former two genera, the orifice in Urosporidium is covered by a tongue of wall material tucked inside the aperture. One or more characteristic extensions of epispore cytoplasm are supported by bands or ribbons of cytoplasmic material. There are seven named species that mostly are found as hyperparasites of trematode sporocysts or metacercariae, or of juvenile nematodes.Urosporidium crescens (marine, hyperparasite of trematodes in blue crabs)