Reticulamoeba

[ref. ID; 5694]
Amoeboid protists. (ref. ID; 5694)


Reticulamoeba gemmipara Grell, 1994 (ref. ID; 4838 original paper, 4847)
Description; The cell body of the amoeboid stage is more or less flattened. Frequently, a central nucleus with nucleolus is recognizable. The surrounding cytoplasm contains a single layer of large digestive vacuoles, interspersed with small light-reflecting concrements. Though being completely naked, the cell body shows no changes of shape or locomotion. As an expression of its stationary mode of life, the motility of the amoeboid stage is predominantly restricted to the alterations of the reticulopodia. In contrast to "real" amoebae, the amoeboid stages of Reticulamoeba gemmipara grow up from little zoospores which settle on the substrate, resorb their flagella and protrude reticulopodia in various directions. At the beginning of their growth period, the cell bodies are more or less roundish, later on they become quite irregular. Where a reticulpodium arises, the outline is often drawn out to a tip. The recticulpodia form extended networks which can best be seen when the surface of the slide is smooth and clean. Usually, their threads are rather thin, but sometimes distinctly thickened, especially when they serve for transporting diatoms to the cluster or food material to the cell body. The so-called granules ("Kornchen"), whose back and forth motion indicates bidirectional streaming, are characteristic for the thin treads. On the lateral slides, whose surface is covered with scattered diatoms only, it can readily be seen, that the networks of amoeboid stages, derived from different zoospores, fuse with each other. One or more of the specimens feed on diatoms in such cases, whereas the others participate in the food engulfed by them. Hence, the amoeboid stages of Reticulamoeba gemmipara can be establish a "feeding community" which is, however, only temporary, since the amoeboid stages causing clusters remain separate. The main function of the reticulopodia is feeding. One gets the impression that they penetrate the diatoms, probably at the slit where both halves of the shell are separated. Phagocytosis may take place within the shell. Though the diatoms can be membranced by reticulopodia, Reticulamoeba gemmipara was not seen to engulf whole specimens as ist the case in Reticulosphaera socilais (Grell 1989) and Chlorarachnion reptans (Grell 1990). That digestion begins already in the reticulpodia ("reticulopodial digestion") is indicated by the fact that the contents of the digestive vacuoles within the cell bodies are thoroughly homogeneous from the beginning. Later on, the digestive vacuoles disappear, whereas the light-reflecting concrements remain in the buds and become transmitted to the zoospores. (ref. ID; 4838)
[Zoosporogenesis] While the amoeboid stage of other amoeboflagellates (e.g. Naegleria gruberi) transforms as a whole into the flagellate stage, a process which is interconvertible, Reticulamoeba gemmipara develops its flagellate swamers or zoospores in a rather different way. Details of this process, which is difficult to analyse by light microscopy alone, are not sufficiently understood. Evidently, two periods of development can be distinguished. The first period consists of a series of unequal fissions by which one to four, sometimes more (5-8) buds are formed. As slightly compressed specimens show, the buds contain no food vacuoles but always light-reflecting concrements. A specimen with three, four and five buds photographed at different times, demonstrates that the buds may have tapering projections at their distal ends. The second period starts with the lengthening of one of the projections. The differentiation of the zoospore occurs at its end. In the beginning, a stretching takes place, combined with a vivacious pseudopodial activity. Before this activity ceases, the flagella become recognizable. While the zoospore gets its definite shape, it leaves the visual field by fast swimming. The zoospore has short anterior flagellum which moves by gyration and a long posterior flagellum trailing behind. The latter is most active when the zoospore creeps on the substrate. In the posterior part of the zoospore are the light-reflecting concrements, probably a storage material. (ref. ID; 4838)
Comments; Reticulamoeba gemmipara and Leucodictyon marinum share distinctive features. Apart from the fact that both species are heterotrophic, have reticulopodia and feed on diatoms, they practice the same manner of reticulopodial digestion. In contrast to Reticulosphaera socialis and Chlorarachnion reptans, which phagocytose while diatoms, the reticulpodia of Leucodictyon marinum and Reticulamoeba gemmipara penetrate the diatom shells and engulf parts of the cells. The cell bodies of the both species contain large digestive vacuoles with homogeneous contents. Most striking is the similarity of zoosporogenesis in both species. In Leucodictyon marinum, it starts with a single unequal division of the cell body that is enclosed by a lorica, in Reticulamoeba gemmipara with one of several successive unequal fissions of a naked cell. The final step, the differentiation of the zoospore, takes place in the same manner. The zoospores with their short anterior and long posterior flagellum are nearly identical. In both cases, the posterior end is endowed with light-reflecting concrements. The main peculiarity of Leucodictyon marinum is its meroplasmodial organization, i.e. the permanent connection of the cell bodies by a common reticulopodial network. The meroplasmodium enlarges by equal binary fissions of its cell bodies. In Reticulamoeba gemmipara, where no equal divisions of the cell bodies have been observed, the situation is different. If enough food is available, as is the case on a diatom lawn, clusters are formed, which presuppose that the amoeboid stages remain separate. If the diatoms are sparsely distributed, however, the networks of adjacent amoeboid stages fuse with each other, forming temporary "feeding communities". From the Phylogenetic point of view, the assumed relationship of Reticulamoeba gemmipara and Leucodictyon marinum deserves some remarks. Although Reticulamoeba gemmipara may not be a direct ancestor of Leucodictyon marinum, one can assume that it belongs to a stem group from which heterotrophic meroplasmodia such as Leucodictyon marinum derive. There is no reason to believe that Reticulamoeba gemmipara derives from a plastid-containing ancestry. It depends on an ultrastructural investigation, whether this assumption can be maintained. Less probable seems a close relationship between Reticulamoeba gemmipara and Gymnophrydium marinum Dangeard (Kumar 1980), an amoeboid organism with "filopodia" (reticulopodia?). Feeding on bacteria, it reproduces by binary fission and exogenous budding. Like Reticulamoeba gemmipara, several amoebae can join by means of their "filopodia" to form a cellular association ("colonial state") comparable to a feeding community. In contrast to Reticulamoeba gemmipara, no flagellate stages have been observed, however. (ref. ID; 4838)
Type locality; Reticulamoeba gemmipara derives from little stones of volcanic origin which were collected in the littoral zone of the Greek island Thera (Santorin). (ref. ID; 4838)
Reticulamoeba minor Grell, 1995 (ref. ID; 4847 original paper)
Description; The amoeboid stages of Reticulamoeba minor have a diameter of only 3-8 um. Details like nucleus, digestive vacuoles or light-reflecting concrements, so clearly visible in Reticulamoeba gemmipara, are rarely recognizable. Specimens which settled on the surface on the slide as shown by the scanning electron micrographs are probably derived directly from zoospores. They are loaf-shaped and have short reticulopodia radiating in all directions. During their growth period, the amoeboid stages attain a roundish shape and the reticulopodia develop into the typical network. In contrast to Reticulamoeba gemmipara, the amoeboid stages of Reticulamoeba minor multiply by equal binary fission, so that two, four or more amoebae are formed, sometimes, though temporarily, connected by reticulopodia. Under as yet known conditions, an unequal division occurs which leads to the formation of a swarmer or zoospore. The beginning of this process indicated by the appearance of a rounded bud, a stage which is rather frequent and seems to last a certain time. The differentiation of the zoospore takes place close to the mother cell, not at the end of a cellular projection as in Reticulamoeba gemmipara. The zoospore has a short anterior and a long posterior flagellum. The short flagellum propels the cell by gyrations, whereas the long flagellum trails behind. Swimming takes place in a trembling manner. Since the zoospores are much smaller than those of Leucodictyon marinum and Reticulamoeba gemmipara, it was impossible to make satisfactory photographs. The scanning electron micrograph, shows a zoospore which sticks to the surface of the slide. Its flagella are disarranged. There are two ways how the reticulopodia contact the diatoms: they either adhere to them or they feed on them. Both possibilities are often difficult to distinguish. The reticulopodia may be single strands or branched networks contacting several diatoms. As described for Reticulamoeba gemmipara, the reticulopodia do not phagocytose whole diatoms but penetrate them, evidently through the narrow slit separating both valves of the shells. What happens within the shells goes without saying: phagocytosis of the intracellular material and predigestion within phagosomes. After these processes, some of the diatoms tear open. Most shells, however, simply shrink. After ascending into the overlaying assembly of diatoms, the amoeboid stages always appear roundish. By means of their reticulopodial networks they interweave the diatom "lawn" into a firmly connected mass, which includes not only dead but also living diatoms and even dividing specimens. The reticulopodia of Reticulamoeba minor can fulfill different functions. The extend and contrast when the amoebae move about or gather prey. They keep the diatoms together, either to feed on them or to have source of food available. Most impressive, however, is their role in the acquisition of food. After the processes inside the diatoms that are invisible but can be guessed the phagosomes must be passed to the cell body probably by microtubule-mediated cytoplasmic transport. Since the reticulopodia of Reticulamoeba minor are rather thin, this process is difficult to study with the light microscope. After Parducz' fixation which is generally thought to approach a good reflection of the living state, the reticulopodia acquire in most cases a "string of pearls" - appearance which as such is certainly an artifact of fixation. (ref. ID; 4847)
Type locality; From the Mediterranean Sea, more precisely, from little stones of a tide pool, east of the harbor of Alanya (Turkish Riviera). (ref. ID; 4847)