Hydramoeba

1. Hydramoeba hydroxena (Entz, 1912) (ref. ID; 3687, 7615, 7692) reported author and year? (ref. ID; 7606)
   Syn; Amoeba hydroxena Entz, 1912 (ref. ID; 3687)

Hydramoeba hydroxena (Entz, 1912) (ref. ID; 3687, 7615, 7692) reported author and year? (ref. ID; 7606)

Synonym

Diagnosis

Descriptions

• Light microscopy: When moving in dishes or on slides, these amoebae usually had a monopodial (limax) form which might put out a new pseudopodium laterally to change direction. Locomotion involved a gentle antero-lateral bulging, with a hyaline cap which might soon be largely filled in by the advancing granuloplasm; there was usually a narrow hyaline border along the sides. An almost constant feature was the finely morulate uroid. As reported by other workers (e.g., Entz 1912), the size of the amoebae varied greatly, with a length of 105 to 250 um in the few measured; larger and smaller cells did not differ other than in size. The nucleus (17-23 um in its greatest diameter) was oval or round. It was bounded by an easily discernible nuclear envelope, always with a layer of irregularly shaped, block-like bodies separated by a narrow clear space from the inner surface of the nuclear envelope. No central body (endosome, karyosome, nucleolus) was ever seen, though there was occasionally a very faint suggestion of possibly solid material. Although the majority of amoebae appeared to be uninucleate, occasional binucleate cells and one with three nuclei were seen. Among the cytoplasmic inclusions were a contractile vacuole and many crystals, the larger one distinctly bipyramidal. Food vacuoles could sometimes be identified, and ingested nematocysts were seen. These features have been described by either observers. Amoebae on two disintegrating hydras contained zoochlorellae from the endodermal cells of the host. No cysts were identified. (ref. ID; 7692)
• Electron microscopy: The cell surface was covered, outside the plasma membrane, with a filamentous coat which was densest near the plasma membrane. The most regular part of it extended, in the form of fine filaments, about 25 nm, but the coat as whole continued about 50 nm above the plasma membrane. Tangential sections suggested a hexagonal arrangement of the fibrous material. Comparison with the surface of Amoeba protes, fixed by the same method and resembling results obtained by other workers (e.g., Flickinger 1973, 1974), shows that the filaments of the latter are even more tangled in appearance than those of H. hydroxena as well as thicker and longer. In all sections the profile of the oval or elliptical nucleus was irregular, with many projections, and the nuclear envelope consisted only of the two closely applied membranes, with the perinuclear cisterna between them, and frequent nucleopores. Although occasional strands of the very fine fibrous material just beneath the nuclear membrane were oriented more or less parallel to the membrane, this was by no means a regular observation, and the fibrous material did not differ from that found elsewere in the nucleus. At a minimum distance of about 0.5 um from the inner surface of the nuclear envelope was a layer of presumed nucleolar material, about 1.6 um thick at its narrowest points, usually thicker. The gaps in this layer gave it the appearance of a ring of irregular blocks, as described in the light microscopic literature. This is the 'peripheral chromatin' of Entz (1912) or 'ectocaryosomal chromatin' of Reynolds and Threlkeld (1929). In none of the sections was there any suggestion of a central 'nucleolus' (Entz 1912) or 'endosome' (Reynolds and Threlkeld 1929; Beers 1964) as described in preparations fixed and stained for light microscopy. At most, a few randomly distributed, small, loose clumps could be found amongst all the fibrous and granular material medial to the thick, densely staining layer of presumed nucleolar material. Mitochondria were oval to elongate in profile, up to 2.5 um long, and with the tubular, branching cristae which are usual in Gymnamoebia. Golgi bodies of the usual dictyosome structure were numerous and widely distributed, with no particular location. Each contained up to six flattened saccules with expanded ends, as well as associated vesicles; definite convex and concave faces could be distinguished only in a few. The rough endoplasmic reticulum included both longish, more or less flattened cristernae and more open, shorter ones. Short, elongate and more rounded or elliptical vesicles apparently constituted the smooth endoplasmic reticulum. Within the digestive vesicles, particularly the smaller, less advanced ones, inclusions from the hydra tissues could often be distingushed, including nematocysts and even zoochorellae. The numerous lipid bodies were often oval or elliptical, with larger ones sometimes constricted in the middle; the large diameter of oval and elliptical ones was about 1.0 to 1.5 um. Ito's (1949) comparison of his light microscopical observations of living and fixed amoebae suggests the presence of lipid bodies. Although many crystals were seen with the light microscope and ghost crystals had been seen in sections of Polychaos fasciculatum prepared by the same method (Page and Baldock 1980), crystals could not be identifed with certainty in these sections. Perhaps some of the smaller vesicles which appeared empty or had unidentifiable contents were crystal vesicles, since it was generally an amorphous surface coat rather than the crystal itself that was preserved in P. fasciculatum. Contractile vacuoles, seen in several sections, were bordered by a sponginome of many small vesicles. Microfilaments were seen in some sections. More unusual was the observation of what were almost certainly cytoplasmic microtubules, seen in a number of places. (ref. ID; 7692)

Remarks

• Comparison with previous morphological reports. In most respects our obsevations agree with the light-microscopical reports of earlier workers, certainly to the extent of confirming that all workers have been examining organisms of the same species. The normal form of H. hydroxena when moving on a flat, inanimate substratum is monopodial (limax). The 'verrucosa' forms of earlier authors are simply the irregular shapes, often with short projections or bumps, assumed by many free-living amoebae in contact with food organisms, motionless on the substratum, or floating. The 'proteus' forms are further most part amoebae which are changing direction or at least not advancing steadily in one direction. This study demonstrates conclusively the absence of an 'endosome' or central nucleolus, which was undoubtedly an artifact of the fixation procedures for light microscopy and was in fact seldom pictured as a compact body like that found in many medium-sized and small amoebae. The results of Beers (1964) demonstrate Feulgen-positive material in the centre of a nucleus but not, as he thought, in a body corresponding to that found in many nuclei. (ref. ID; 7692)
• Comparison with free-living amoebae. Hydramoeba has often been considered to resemble Amoeba proteus, though Entz (1912) also sggested affinities with other amoebae. Only a few electron-microscopical characters have so far proved useful in the taxonomy of Gymnamoebia. Surface structure is one of the most helpful. The surface structure of H. hydroxena, with its long, loose, rather untidy filaments, resembles that of A. proteus (Pappas 1959; Flickinger 1973, 1974), Chaos carolinense (Pappas 1959; Daniels 1973; Flickinger 1973), and P. fasciculatum (Page and Baldock 1980), all classified in the family Amoebidae; however, the filaments composing it are shorter than in those species. Flickinger (1974) did not find such a filamentous coat on Polychaos dubium, also a member of the Amoebidae. A similar coat, though with the filaments more tidy than those of A. proteus in most preparations (except those of Pappas 1959), is found on Thecamoeba proteoides (Page 1978), which appears to be intermediate between Amoebidae and Thecamoebidae though classified in the latter. It is interesting that a hexagonal configuration of the filamentous material is hinted at in Fig.10 of Flickinger (1974) and is found at least on other family of Gymnamoebia (Page 1980). The structure of the nuclear envelope varies within the family Amoebidae. Amoeba proteus and Polychaos dubium have an internal fibrous lamina of the 'honeycomb' pattern (Flickinger 1974), while C. carolinense has a morphologically simpler lamina (Pappas 1959; Flickinger 1973), and P. fasciculatum has none (Page and Baldock 1980). The border-line organism Thecamoeba proteoides has an internal fibrous lamina similar to that of C. carolinense. It should be noted that the profiles of the nuclei of both H. hydroxena and P. fasciculatum are the most irregular, undoubtedly because of the lack of such a supporting lamina. Arrangements of the nuclear material in numerous pieces, usually around the periphery, are common but not universal in the Amoebidae and are also found in the Thecamoebidae. The cytoplasmic organelles in general resemble those of the Amoebidae, but few internal organelles are distinctive among the various families of the order Amoebida. Although a more complete study of the mitotic pattern, some details of which were described by Reynolds and Threlkeld (1929), is desirable, it is certainly not promitotic and seems similar enough to that of A. proteus (Liesche 1938) not to separate the two species into different families. One problem in classifying H. hydroxena is its limax locomotive form. Within the Amoebidae, a consistently monopodial form is characteristic only of Trichamoeba Fromentel, 1874, though monopodial forms occur at times in other members of the family including A. proteus and C. carolinense. The validity of the genus Trichamoeba is sometimes questioned because described species have been observed only in field samples or mixed material derived from such samples and might be individuals of a usually polypodial species. Doubt has therefore existed about the possibility of a monopodial genus of Amoebidae. For that reason, Hydramoeba must also be compared with Saccamoeba, the genus other than Trichamoeba which Hydramoeba most resembles in its limax form. Saccamoeba belongs to the family Hartmannellidae. But any similarity is superficial. All known species of Saccamoeba have a nucleus with a distinct central nucleolus, the chromosomal material occurring as a continuous, the peripheral layer of fine material, and their characteristic villous uroid is easily distingished from the morulate uroid of Hydramoeba (Page 1976). Furthermore, unpublished electron micrographs of Saccamoeba limax show that it lacks the filamentous coat of Hydramoeba and does not have numerous dictyosomes. (ref. ID; 7692)