Rhipidodendron Stein, 1956 (ref. ID; 3748)

[ref. ID; 1618]
Similar to Cladomonas, but tubes are fused lengthwise; fresh water. (ref. ID; 1618)

[ref. ID; 3748]
The genus Rhipidodendron was erected by Stein (1956) for colonial colorless flagellates with 2 equal length flagella in which the cells were embedded in granular matrix material. Stein described only a single species of Rhipidodendron, R. splendidum, which had fan-shaped colonies composed of dichotomously branched tubes, each of the ultimate ones containing a flagellate at its anterior end. (ref. ID; 3748)

Rhipidodendron huxleyi Saville-Kent, 1882 (ref. ID; 3748)
Description; See R. splendidum.
Rhipidodendron splendidum Penard (ref. ID; 2683)
Description; This is a colonial flagellate, sometimes alternatively classified as a chrysomonad alga. The cells are located at the ends of long gelatinous tubes which are arranged in flattened contiguous bundles. (ref. ID; 2683)
Rhipidodendron splendidum Stein, 1956 (ref. ID; 3748) reported year? (ref. ID; 1618, 3343, 3517)
Description; Oval. (ref. ID; 1618)
[Light microscopy and Shadowcast Preparations]: Colonies of R. splendidum are fan-shaped, rust-brown in color and are composed of 1 to numerous granular, dichotomously branched tubes, each containing a single flagellate cell at its anterior end. The majority of observations have been made on free-floating material, but many young colonies were attached to the sides of the collecting jars. The 1-cell stage is ~ 35 um in length, increasing to ~ 60 um as the 4-cell stage is reached. Eight-celled colonies are ~ 85 x 30 um and have a single dichotomy. Colonies with up to 3 dichotomies are more or less symmetrical in shape and have 4 or 8 cells in each branch, but larger ones become more irregular, having 10-20 cells/branch arranged in 3 or 4 rows. The largest colony seen in the course of this investigation was 440 x 310 um at its broadest point. The cells of R. splendidum are ~ 20 x 4.5 um and each bears 2 flagella about as long as the cell which emerge through the open distal ends of the tubes. Where the cells emerge from the colony in preparations flattened by coverslip pressure, they become spherical and it can be seen that the 2 flagella appear to be inserted close together in a basal granule near which lies the single nucleus; the remainder of the cytoplasm is occupied by vesicles and dense granules. Mastigonemes have not been detected on either flagellum by shadowcast whole mounts nor in sectioned material. (ref. ID; 3748)
[Electron microscopy]: The cells are cylindrical with a more or less circular cross section, though the structure of the anterior end is relatively complex. The 2 flagella arise, with their basal bodies parallel, from a shallow flagella pocket on one side of the cell, termed here for convenience the ventral side. The rim of the flagellar pocket is asymmetrical appearing as a low ridge on the ventral side and rising to a tongue ~ 1 um high, in median position relative to the cell as a whole on the dorsal side. The dorsal side of the anterior end of the cell is extended for a distance of up to ~ 5 um beyond the level of the flagellar insertion. This anterior prolongation tapers at its distal end and, for the proximal 2/3 of its length, thinner curved arms of cytoplasm extend ventrally and toward the flagella in a pincer-like configuration. In transverse sections near the mouth of the colony tubes there is usually a profile, in addition to those of the flagella, of a narrow protoplasmic strand which probably represents a further extension of the anterior prolongation. Between the dorsal prolongation and the dorsal lip of the flagellar pocket there is a pit, which curves around the dorsal side of the flagella pocket and has a base that is approximately level with the proximal ends of the flagellar basal bodies. The pit is lined by a uniformly thick layer of what is probably mucoid material, and similar but more loosely organized material surrounds the most proximal 1 um of the flagella. The cells of R. splendidum are separated from each other by a single layer of closely and evenly spaced spherules all of which have a similar diameter of 0.9-1.1 um. These tubes of spherules are lined by a delicate layer of mucilage, retained even in older pats of the colony not occupied by cells. The contents of the spherules appear finely granular with interposed thin rods or sheets and large, very dense granules they have less dense centers which contain mostly fibrous-reticulate material. The posterior end of each cell tapers into a thin tail by means of which it is attached to the tube walls; numerous microtubules are visible in this tapered part of the cell. Young cell occurring singly have flattened posterior ends surrounded by a pad of mucilage by means of which they, and presumably the older colonies, may be attached. The nucleus varies in shape from roughly spherical to pyriform, though the extreme anterior end is always drawn out toward the dorsal flagellar basal body to which it is attached by a conspicuous cross-banded root (rhizoplast) visible even in low-power sections. The nucleus contains a single spherical nucleolus and irregularly-shaped chromatin bodies. A peculiar organelle is always found associated with the nucleus in R. splendidum. This takes the form of a flattened sac pressed closely against its posterior end and with a long tail extending deep into the cytoplasm. There is only a single bounding membrane and the contents are amorphous and homogenous so there is no reason for considering the sac to be a modified mitochondrion. Its origin and function therefore remain unknown, though it may have some phylogenetic significance. The single Golgi body occupies a position similar to that of the parabasal body in certain other zooflagellates, lying against the anterior end of the nucleus and partially surrounding the rhizoplast. It is 1.5 um in diameter and usually consists of 8-10 relatively closely adpressed cisternae with swollen edges and amorphous contents. Blebs on the outer membrane of the nuclear envelope appear to form vesicles with coalesce with the most proximal Golgi cisterna. Profiles of mitochondria with narrow-stalked saccate cristae are irregularly distributed throughout the cytoplasm, though one consistently appears to lie close to the flagellar basal bodies. The remainder of the cell is packed with various types of vesicle. The most numerous of these each contains a dense spherule, identical in structure to those forming the tubes of the colony. It is proposed that these are not food vacuoles containing ingested spherules but that they are spherule-producing vesicles and that the colony is built up by release of their content to the outside. Although no evidence for this release has been seen in sections, this interpretation is supported by the facts that ~ 15% of these vesicles contain 1, or in some cases 2 bacteria, which partially occupy a concavity in the contents of the vesicle, and that a similar proportion of the colony spherules have a crescentic segment missing, this space presumably having been occupied by a bacterium during spherule formation. The fate of the bacteria after the spherules have been released has not been determined, though vesicles containing only bacteria have been seen occasionally. The vesicles containing the nature spherules appear to develop from larger vesicles with less dense fibrous-reticulate contents which become denser and granular as they mature. A notable feature of these large vesicles is the occurrence on their membranes of fields of regular spaced tubules or chains of small vesicles, and it is possible that the transfer of material from the cytoplasm to the vesicle occurs by a process of membrane invagination and budding, though movement of the vesicles in the opposite direction cannot be ruled out. The largest vesicles in the cell, commonly up to 2 um in diameter, have a dense central sphere, measuring ~ 1/2 their diameter in section, and much less dense amorphous material peripherally. Their function is unknown though they may represent a form of reserve material as is the suggested function of similar vesicles in the amebo-flagellate Pseudospora. Much smaller vesicles of moderate electron density and with wound fibrous contents are found directly beneath the plasmalemma, mostly at the anterior end of the cell. This superficial position indicates that they may secrete the mucilage lining the colony tubes or that around the flagella or lining the pit. Phagotrophy was not been seen in living cells of R. splendidum and food vacuoles could not be clearly identified by the nature of their contents in sectioned material. It is possible, however, that some of the putative spherule-producing vesicles are in fact food vacuoles, since what might be part of a silica scale of a species of the chrysomonad genus Synura has been seen in a single case. No sings of digestion of the endogenous bacteria have been seen and these are almost certainly symbiotic. Profiles of contractile vacuoles could also not be identified, though it is unlikely that these are missing completely; they have been reported previously in both Rhipidodendron and Spongomonas. (ref. ID; 3748)
Comments; On the basis of the structural similarity of the cells of R. splendidum and Spongomonas spp. when free from the colony matrix, Hartmann & Chagas (1910) transferred the former species to be genus Spongomonas and combined the 3 species of Spongomonas described by Stein (1878) under the name S. uvella. They retained "Spongomonas splendida" as a separate species mainly because the granules in the colony matrix were always smaller than those in S. uvella. Although the present observations on the basic similarity of cell structure in R. splendidum and S. uvella support and considerably extend those of Hartmann & Chagas, it has been decided to retain the genus Rhipidodendron for species forming fan-shaped colonies built of tubes and to refer to the genus Spongomonas those forming less well defined colonies not built of tubes. There appears to be no overlap between these 2 basic types and the distinction between them is well marked at a very early stage of growth since even solitary cells of Rhipidodendron are cylindrical in shape and completely surrounded by a tube of spherules. It is possible that characters such as the presence or absence of a nuclear sac and detained differences in the structure of the flagellar apparatus may also become useful taxonomically when more species have been investigated. With regard to the taxonomy of the genus Rhipidodendron, the species described by Saville-Kent (1882) as Rhipidodendron huxleyi differs from R. splendidum in possessing only 4 and not an indefinite number of tubules in each branch of the colony and in having more symmetrically branched colonies. The species investigated here may therefore be clearly identified as Stein's R. splendidum because of the rather irregular shape of the larger colonies which have up to 20 tubes in each branch. Since, however, younger colonies are more regular in form and have mostly 4 tubes per branch, it is possible that the 2 taxa may only be growth forms of one species, though it should also be pointed out that very large richly branched colonies of R. huxleyi of the sort figured by Saville-Kent (1882) have not been seen. Work on material in culture would therefore seem to be necessary for a definite solution to this problem. (ref. ID; 3748)
Measurements; Body about 13 um long; flagella about two to three times body length; fully grown colony 350 um high. (ref. ID; 1618)
Length of cells 10-12 um. (ref. ID; 3343)