Rhynchopus
Rhynchopus Skuja, 1948 (ref. ID; 4950)
[ref. ID; 4898]
This highly metabolic genus with a sub-apical pocket containing flagellar 'stubs' was originally described as an unusual euglenid by Skuja (1948). Electron-microscopical studies have shown instead that this genus is extremely similar to Diplonema (Schnepf 1994 (= ref. ID; 4839); Simpson & Patterson, unpubl.). (ref. ID; 4898)
- Rhynchopus amitus (ref. ID; 4839, 4950)
- Rhynchopus coscinodiscivorous Schnepf, 1994 (ref. ID; 4839 original paper)
Diagnosis
Cell pear-shaped but highly variable in form, 10-25 um long, 4-12 um thick (depending on the presence or absence of food vacuoles), with an apical papilla and subapical cytostome of the plicate mouth type. Two short flagella, lacking a regular axoneme largely included in a lateral, subapical pocket and generally not visible in the light microscope. Mitochondria with very few invaginations of the inner envelope membrane. No trichocysts, muciferous bodies, paramylum grains, no plicate euglenoid pellicle. Feeds on the marine diatom Coscinodiscus concinnus. (ref. ID; 4839)
Descriptions
Rhynchopus coscinodiscivorous is colorless and highly variable in shape and size. Small cells lacking food vacuoles are ovoid or oblong, about 10 um long and 4-6 um thick and not so variable in shape as the larger trophonts. The size and shape of the latter depend on the number of food vacuoles. The trophonts are more or less pear-shaped, 20-25 um long and 10-12 um thick and change their shape permanently by euglenoid ("metabolic") contortions, which are often nearly ameboid-like. Nevertheless, the cells maintain a strict polarity. Old trophonts, however, are an exception and may round up and even encyst. Feeding trophonts have a distinct cell apex in the form of a cone-like, refractive structure which can become nearly invisible when the cell bulges out in the same location. It is generally also seen in the smaller cells, with the exception of liberated migratory cells. Subapically, near the apical cone, there is a slight indentation. Due to the changes of the cell shape this indentation. Due to the changes of the cell shape this indentation can frequently remain undetected. Under favourable conditions a cytostome can be recognized here which extends into a cytopharynx. The oral apparatus seems to be less well developed in the non-feeding small cells. Some other structures can also be observed in this region but it is generally not possible to identify them with the flagellar groove and the nucleus which both are situated here. Flagella are generally not visible. The apical part of the cell is devoid of food vacuoles but may contain a few small refractive reserve granules. These are scattered mainly over the basal part of the cell which in active trophonts is filled with food vacuoles. There are 10-20 spherical food vacuoles that may bulge out from the cell surface. Their size is not uniform and their contents vary in color and structure, being always inhomogeneous, obviously depending on the composition of the food particles which have been ingested and on the degree of digestion. During food uptake R. coscinodiscivorus uses its cytostome initially to suck the surface of the retracted diatom protoplast and, later, when the protoplast is decaying, the cell debris. During this process, the cell apex is suddenly and repeatedly drawn back. By the resultant jerking contractions, small food particles are torn out, then ingested, transported through the cytopharynx, and added to a food vacuole. Non-feeding R. coscinodiscivorus cells, including both trophonts and smaller cells, migrate slowly with a smooth gliding motion, preferentially along the diatom shell or the bottom of the Petri dish. With a similar gliding motion, the cells can even cross empty regions of the Coscinodiscus cell without any need for attachment to a substrate. This gliding motion as well as the contraction of the cell apex and the euglenoid contortions are all reversibly inhibited by cytochalasin D (2 ug/ml). Such a treatment does not induce the cells to round up. Frequently, small cells were observed to be attached with their cytostome to a rimoportula (a short, thin, inner tub-like process of the diatom valva). There were even small R. coscinodiscivorus cells in contact with each other at their cytostomes for considerable length of time. R. coscinodiscivorus divides longitudinally. Cytokinesis begins at the cell apex; the two sister cells can be relatively unequal. (ref. ID; 4839)
Remarks
R. coscinodiscivorus in its body shape, in its euglenoid "metaboic" contortions, in its gliding motion and in the occurrence of largely concealed flagella most closely resembles the colorless englenozoon Rhynchopus amitus, described by Skuja (1948). In some details of its fine structure Rhynchopus coscinodiscivorus resembles phagotrophic euglenids like Isonema nigricans (Schuster et al. 1968), Isonema papillatum (Porter 1973) and Diplonema ambulator (Triemer & Farmer 1991). The name Isonema was found to be a junior synonym of Diplonema (Larsen & Patterson 1990; Triemer & Ott 1990), a genus described by Griessmann (1913). The two Isonema species have therefore been transferred to the genus Diponema (Triemer & Ott 1990). Two other Diplonema (= Isonema)- like flagellates (Bodammer & Swayer 1981; Kent et al. 1987) are also similar. Diplonema has two flagella which reach about one third of the cell length. Rhynchopus amitus has flagella that hardly protrude beyond the opening of the flagellar pocket (Skuja 1948). The present Coscinodiscus parasite is, therefore, placed in the genus Rhynchopus as Rhynchopus coscinodiscivorus (the name characterizes its nutrition). It has many similarities, especially in ultrastructure, to Diplonema. R. conscinodiscivorus differs from R. amitus in being a marine organism; not living in fresh water it therefore lacks a contractile vacuole. The reported presence of paramylum as reserve substance is perhaps an error; Skuja (1948) has obviously mistaken other reserve granules as paramylum grains. Rhynchopus coscinodiscivorus resembles Diplonema in the fine structure of the "plicate mouth" feeding apparatus (Porter 1973; Larsen & Patterson 1990) and the adjacent flagellar pocket, the abnormal structure of the mitochondria, the absence of paramylum granules, eyespot and pellicle and a pronounced euglenid-like movement. There is no doubt that Rhynchopus coscinodiscivorus belongs other euglenozoa with plicate mouths (Larsen & Patterson 1990; Triemer & Farmer 1991). Triemer & Farmer (1991) group the feeding apparatuses of euglenids into four types. The cytostome of R. coscinodiscivorus resembles most closely the Type II feeding apparatus which is found e.g. in Diplonema. Other euglenids with a Type II feeding apparatus are, however, bacteriotrophs, whereas those (and only those) with a Type III apparatus are said to capable of engulfing whole eukaryotic prey organisms (Triemer & Farmer 1991). They include, e.g., Peranema trichophorum, Diplonema sulcatum, and Urceolus cyclostomum. R. coscinodiscivorus consumes eukaryotic cytoplasm and obviously does not belong to the Type III group. The supporting rods of these euglenids are composed primarily of highly ordered bundles of microtubules. Their mitochondria have, more over, a "normal" appearance. Phagocytosis and the presence of food vacuoles are also observed in D. ambulator (Triemer & Ott 1990) but not reported for D. papillatum (Porter 1973) and D. nigricans (Schuster et al. 1968) though these species also have a very similar feeding apparatus. R. coscinodiscivorus differs from Diplonema in that the flagella are not only reduced in length but lack also an ordered axoneme, in contrast to D. nigricans (Schuster et al. 1968) and D. papillatum (Porter 1973). One of the characteristic features of R. coscinodiscivorus and Diplonema is the absence of normal looking mitochondria. In R. coscinodiscivorus as well as in D. nigricans (Schuster et al. 1968) and D. papillata (Porter 1973) only very few but well preserved invaginations of the inner envelope membrane are found. Furthermore, in D. ambulator, where mitochondrial invaginations are abundant (Triemer & Ott 1990), they do not have the characteristic disc-like appearance of mitochondrial cristae in other euglenid flagellates (Leedale 1967). In some respect these mitochondria resemble proplastids. That they are euglenid leukoplasts can, however, be excluded because they have an envelope of two and not of three membranes. In this respect a certain similarity exists with hydrogenosomes (Benchimol & De Souza 1983; Muller 1985). It remains to be determined whether the Diplonema and Rhynchopus mitochondria are biochemically normal, how the scarcely developed inner membranes system can provide enough ATP for the cell, and whether they are primitive organelles or reduced ones (Cavalier-Smith 1987). This problem is also important or the question of whether Diplonema (and Rhynchopus) are "primitive" forms of euglenids (Larsen & Patterson 1990). Another unresolved problem is the movement of Rhynchopus and of the euglenid flagellates (Leedale 1967; Hausmann 1985). The lack of a pellicle demonstrates that this special form of cytoskeleton is not necessary for euglenoid contortion. In R. coscinodiscivorus it could be driven by interactions between the peripheral microtubules and the contractile and elastic structures which surround the cytopharynx. They may also generate of the force for the jerking contractions during feeding. The flagella, being too short and lacking a regular axoneme, can hardly cause the sliding motion of R. coscinodiscivorus. It is rather probable that they bear chemoreceptors and serve to recognize the food, a possible function also common in Diplonema species. The absence of trichocysts and muciferous bodies indicates also that these structures are not involved in the sliding motion. Skuja (1948) suggested that Rhynchopus (note the name!) glides by the vibration of the apical papilla. The inhibitory effects of cytochalasin demonstrate that actin is involved in the euglenoid contortion, the jerking contractions and the sliding motion. (ref. ID; 4839)
Type locality
North Sea, near List/Sylt. (ref. ID; 4839)