Rhizamoeba Page, 1972 (ref. ID; 2618 original paper)
Class Tubulinea: Order Leptomyxida: Family Leptomyxidae (ref. ID; 6789)

See Ripidomyxa

[ref. ID; 2618]
Amoebae in locomotion monopodial, elongate, subcylindrical, advancing by steady flow with occasional cytoplasmic eruptions. Anterior hyaline cap present or absent. Uroidal holdfast often present at posterior end or around periphery, formed by retraction of amoeba from points of attachment and composed of tapering, sometimes branched, filaments not borne on a terminal bulb. Reproduction asexual, by fission. Disintegration of nucleolus during mitosis. (ref. ID; 2618)
Type species; Rhizamoeba polyura (ref. ID; 2618)

[ref. ID; 7606]
Rhizamoeba is easily distinguishable from Trichamoeba on the basis of uroidal structure, frequent occurrence of a flattened form, eruptive activity, and nuclear structure (Page 1972) as well as ultrastructure (Page 1980). Rhizamoeba was originally established for marine organisms, but unpublished investigations have shown that species also occur in fresh water and that locomotion occurs in other forms besides the limax form. (ref. ID; 7606)


Rhizamoeba polyura Page, 1972 (ref. ID; 2618 original paper, 3847)
Description; Actively locomotive amoebae with hemispherical or somewhat truncate anterior end, tapering toward posterior end, which may bear ragged group of filaments trailing from hyaline, sometimes flattened base. Occasional eruptions of hyaline cytoplasm back along side followed by cessation of locomotion. Resting form lens-shaped or irregular, often anchored by holdfast of hyaline filaments on one or several areas of periphery. Majority uninucleate but often two or several nuclei. Nucleus rounded, spindle-shaped, or comet-shaped; chromosomal material apposed to one side of central nucleolus, which may be cup-shaped. No contractile vacuole. Nucleolus and nuclear membrane disintegrate during mitosis. No cysts known. When an amoeba is advancing steadily, the cytoplasmic granules often flow without hindrance to the anterior tip, though a hyaline cap may form, especially when the amoeba hesitates or changes direction. If locomotion continues, this hyaline area is often invaded by granular cytoplasm, the granules intruding first at one point, then occupying the entire anterior end in a manner that suggests the breaking of a barrier between granular and hyaline regions. The flow of granules from the posterior region often gives the endoplasmic region a somewhat fibrous appearance because of the orientation of granules in files suggesting rather narrow passages through the central axis of the amoeba. As in other limax amoebae (e.g., Hartmanella limax), there is a transverse speed gradient, granules in the central axis overtaking more slowly advancing ones on either side. The locomotive rates of 10 amoebae ranged from 98-272 um/min. Although normal advance did not involve the more or less regular alternation of antero-lateral eruptions characteristic of vahlkampfiid amoebae, violent eruptions of hyaline cytoplasm at the anterior end did occur. However, in such instances the hyaline cytoplasm ran back along one side of the granular mass, and locomotion halted; the hyaline cytoplasm often continued to run around the periphery and back toward the front on the other side, producing an irregular form. Eruptions of hyaline material from the posterior end running forward along one side were sometimes associated with resumption of locomotion by a stationary cell. These amoebae sometimes gave other indications of internal pressure, e.g., the formation of constrictions which might divide them into two or more rounded masses. A fairly large minority of locomotive amoebae had a trailing mass of hyaline filaments, which were often up to 10-12 um long. These filaments, which in moving cells formed a uroid, arose by retraction of the main mass from points where the cell had attached to the substratum. They were never seen to be actively put out from the cell surface. They were never borne on a spherical, bulb-like posterior end. In attached cells, the position and extent of the holdfast filaments were related to the shape of the cell, which might be lenticular or irregularly lobed. The uroidal or holdfast filaments often branched once, or a fairly large cluster might arise from the same flattened, hyaline base. They tapered rapidly to a sharp point. When the amoeba resumed locomotion, they were pulled loose, lengthening somewhat at first but eventually being resorbed into the main mass. Many actively locomotive amoebae without trailing filaments had a short, narrow, hyaline tail region of the kind seen in some other species. One very common form of stationary cell resembles superficially the active form of the genus Flabellula in being spread out in a fan-like manner with a hyaline edge around the broad extremity of the fan. These extended fan-shaped or reniform cells were especially common on coverglasses left in moist chambers for hours. However, the hyaline region was only a narrow edge rather than an extensive veil as in Flabellula, and these forms of R. polyura, though changing shape, were not engaged in locomotion to any great degree and often had extensive holdfasts. Some such forms had two or three fan-like regions, each with a narrow, hyaline edge. When they resumed locomotion, they gradually took on the limax shape. Another flattened form was found most commonly on agar surfaces, where many of the amoebae might be divided into several branches. A rare form with more than one pseudopod (though only one advancing) on a glass substratum is figure; this was only a transitory form. The only active locomotive form, except during changes of direction or cessation or resumption of locomotion, was the monopodial, limax shape. Floating forms were irregularly rounded up, but in a few minutes assumed the locomotive form. In diameter they measured 22-43 um. As with some other marine amoebae, the relative paucity of cytoplasmic vacuoles and absence of distinguishable crystals gave the cytoplasm of R. polyura rather more monotonous appearance than that of fresh-water amoebae, though compressed cells showed more detail. Among the cytoplasmic inclusions were food vacuoles containing a single bacterium each; optically empty vesicles mostly about 0.5 um but occasionally up to 2.0 um in diameter; granules with a diameter of approximately 0.2-0.3 um; rods approximately 1.0-1.5 um long or slightly longer and about 0.2-0.3 um broad; and spherules with a diameter of 2 um or less, possibly compacted food remains. There was a greater accumulation of granules in the more slowly moving lateral endoplasm than in the central stream, which consequently had a lighter appearance. On periodic acid-Schiff preparations, the granular cytoplasm alone showed a diffuse coloration which could not be associated with any discrete inclusions; the appearance was the same on controls treated with saliva for 20 min at 37 degrees C. The nucleus, though containing a single central nucleolus, differed in other respects from that common among amoebae. Apposed to one side of the nucleolus was a cluster of granular material resembling in appearance the chromosomal material occurring in other amoebae with a central nucleolus as a layer between nucleolus and nuclear membrane. However, no part of the interphase nucleus gave a positive Feulgen reaction, making definite identification of the DNA-containing material impossible by this method. The side of the nucleolus against their presumptive chromatin was often depressed into a cup-like shape. On the inner surface of the nuclear membrane was a layer of more finely granular material resembling that found in a similar position in some other amoebae, e.g., Hartmannella limax. A second noticeable difference in the nuclei of R. polyura was their tendency to be drawn out into elongate shapes, spindle-shaped if both ends were tapered, comet-shaped if one end was greatly drawn out. Such comet-shaped nuclei in which the main part of the nucleus had a diameter of 5 or 6 um might be drawn out to 14 um long in the axis of elongation of the cell as a whole. The nucleolus had a diameter of approximately 2.5-3.5 um. It was difficult to distinguish the nucleus in living amoebae other than flattened ones. In binucleate or multinucleate cells, the nuclei were usually close together, though flattened amoebae with two lobes joined by a cytoplasmic isthmus sometimes had two unequal groups of nuclei. Outside these tallies, as many as 16 nuclei were found in a cell. Bi- and multinucleate amoebae were larger than most uninucleate ones. The mitotic figures were not prominent, no matter which of the fixing or staining procedures were used; and it was necessary to examine a fair number of dividing cells to find distinct ones. One photograph of a prophase nucleus is included. In the examples seen, the nuclear membrane had disappeared by metaphase, and by that stage the cell had assumed a lens-like shape. Centrioles were not found in either interphase or dividing cells. In cytokinesis the two daughter cells elongated and drew apart until the connecting cytoplasmic thread snapped. Multiple fission was not seen. Partly because of the adhesive nature of the cell membrane, alcian blue preparations were made, but these failed to demonstrate any positive material on the cell surface. (ref. ID; 2618)
Comments; Although several other species show similarities to R. polyura, it is clearly distinguishable from all of them. Schaffer's (1926) description of his Trichamoeba sphaerarum indicates a similar locomotive behavior and uroid, but T. sphaerarum is much smaller and has a different nuclear structure. T. pallida Schaeffer, 1926 seems much like R. polyura in locomotive and stationary forms and in the peripheral attachment of the stationary form to the substratum, but the holdfast structures apparently are less fine and the uroidal filaments fewer than in R. polyura, and the nuclear structure differs. T. caerulea appears similar in holdfast and the tendency of clear cytoplasm to flow back along the side. However, it is a fresh-water organism, and it is not clear how nuclear number and structure compare with those of R. polyura; Schaeffer saw only one example, which had 16 nuclei. T. schaefferi Radir, 1927 is a marine amoeba more than twice as large as R. polyura. Its uroid seems to have a different form in locomotive cells, but the appearance of the peripheral holdfast in stationary cells resembles that of R. polyura. The configuration of nucleolus and presumptive chromatin is like that of R. polyura, but Radir's account of mitosis is uncertain and unhelpful. (ref. ID; 2618)
Marine amoeba. (ref. ID; 3847)
Measurements; Length actively locomotive amoebae approximately 25-135 um, mean 76 um; mean length: breadth ratio approximately 4.3:1. Diameter of rounded nucleus approximately 5-7 um. (ref. ID; 2618)
Rhizamoeba schnepfii Kuhn, 1996/97 (ref. ID; 4890 original paper)
Diagnosis; Size approximately 15-45 x 10-20 um; nucleus rounded, 5.5-7 um diameter; central nucleolus, 1-1.5 um; mostly uninucleate; feeds only on the protoplast of diatoms; marine. (ref. ID; 4890)
Description; [Non-feeding stages]:
Pseudopodia: Broad, lobate, cylindrical, smooth, granular, with clear caps (if visible), lack of ridges, rounded at tips, semi-eruptive. (ref. ID; 4890) Locomotive form: Irregular, usual number of pseudopodia 2-5, oriented in several directions, occasionally eruptive activity during locomotion. (ref. ID; 4890) Single pseudopodium: 4-15 um long, 4-9 um wide. (ref. ID; 4890)
Afloat: Several pseudopodia projecting in several directions. (ref. ID; 4890)
After feeding: Structure very granular, colour of granules greenish to reddish; rounded, at rest 15-20 um diameter; monopodial stages 25-45 um long and 10-20 um in width. (ref. ID; 4890)
When starving: Less colourful, structure finely granular; monopodial stages 20-35 um long and 8-10 um wide; rounded, at rest 10-15 um in diameter. (ref. ID; 4890)
Uroid: Temporary: present in motile individuals, morulate to spiny, uroidal filaments more or less adhesive. (ref. ID; 4890)
Ectoplasm: Thin, hyaline, sometimes distinct at pseudopodial tips. (ref. ID; 4890)
Endoplasm: Fine to granular. (ref. ID; 4890)
Nucleus: One, singular; spherical, 5.5-7 um in diameter; central nucleolus, 1-1.5 um (hardly visible in living amoebae). (ref. ID; 4890)
Food vesicles: Size according to ingested food particles (corresponding to 2 to 6 um). (ref. ID; 4890)
Division: Binary fission. (ref. ID; 4890)
Cyst: 9-11 um in diameter, thin spines, 5-9 um long, on the periphery. (ref. ID; 4890)
Feeding: Algivorous. (ref. ID; 4890)
Habitat: Marine. (ref. ID; 4890)
Remarks; Rhizamoeba are distinguished from Saccamoeba by the presence of pronounced and more or less adhesive uroidal filaments (Bovee & Sawyer 1979). The feeding mode of Rhizamoeba schnepfii differs from other herbivorous Rhizamoeba species reported hitheto: R. schnepfii engulfs selectively only one part of the food organism, namely the intact protoplast of several marine diatom species whereas other amoebae incorporate whole algae. (ref. ID; 4890)
Type locality; North Sea, Wadden Sea off List/Sylt. (ref. ID; 4890)