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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Onychodromus

Onychodromus Stein, 1859 (ref. ID; 2014, 7336)

Class Polyhymenophora: Subclass Spirotricha: Order Hypotrichida: Suborder Sporadotrichina: Family Oxytrichidae (ref. ID; 2014)
Family Parakahliellidae Eigner, 1997 (ref. ID; 7423)

Synonym Himantophorus Ehrenberg, 1838 (ref. ID; 2014)

[ref. ID; 2014]
Large rigid, rectangularly oval, dorso-ventrally flattened body bearing a large AZM extending to the equator. 2 rows of marginal cirri joining posteriorly; transverse cirri present but caudal cirri are absent. Most of the front-ventral cirri arranged in 3 parallel rows with 3 large anterior frontal cirri. Contractile vacuole equatorial. Macronucleus in four parts each with an adjacent micronucleus. This genus is a voracious feeder found in saprobic environments. Single species genus.
Quote; Colin R. Curds, Michael A. Gates and David McL. Roberts "British and other freshwater ciliated protozoa Part II Ciliophora: Oligohymenophora and Polyhymenophora" Cambridge University Press, 1983 (ref. ID; 2014)

[ref. ID; 2129]
Note; Onychodromus is related very likely closely Onychodromopsis. This genus differs from Onychodromopsis by having ventral cirral rows and dorsal kineties also formed by multiple fragmentation. (ref. ID; 2129)
  1. Onychodromus acuminatus Fedriani et al., 1976 (ref. ID; 7336) reported author and year? (ref. ID; 191, 4130, 4756, 7552, 7752)
  2. Onychodromus grandis Stein, 1859 (ref. ID; 7336 redescribed paper, 7423) reported year? (ref. ID; 1308, 1335, 1618) reported author and year? (ref. ID; 191, 4471)
  3. Onychodromus indica amra & Sapra, 1993 (ref. ID; 7336) reported author and year? (ref. ID; 191)
  4. Onychodromus quadricornutus Foissner, Schlegel & Prescott, 1987 (ref. ID; 3675, 4265 original paper, 7423, 7440) reported author and year? (ref. ID; 191, 2129, 4471)

Onychodromus acuminatus Fedriani et al., 1976 (ref. ID; 7336) reported author and year? (ref. ID; 191, 4130, 4756, 7552, 7752)

Descriptions

The ultrastructural study of encystment and excystment. (ref. D; 7752)

Onychodromus grandis Stein, 1859 (ref. ID; 7336 redescribed paper, 7423) reported year? (ref. ID; 1308, 1335, 1618) reported author and year? (ref. ID; 191, 4471)

Redescription

Body has broad, elliptical shape, narrowing towards the slightly tapering posterior end. Dorso-ventrally flattened 2-3:1. Adoral zone of membranelles (AZM) long, well developed, nearly one half of the body length. Three strong frontal cirri (FC) of a slate pencil shape on the frontal area, under which 9-10 less developed frontovental cirri (FVC) are situated in three short longitudinal rows, 3-4 of the cirri being directly adjacent to the paroral membrane (buccal cirri). Ventral rows of cirri begin in the middle of the cell at the level of the pharynx. The three central rows of cirri comprise 9 (2+5+2) ventral cirri (VC). Row VR2 formed by 5 VC extends to the level of transverse cirri. The first two cirri of VR2 usually parallel these two of VR1. The row of two VR3 cirri begins in the posterior third of the cell and extends to the fifth transverse cirrus. (We observed no variation in number of ventral cirri.) Ventral cirri are as well developed as frontoventral ones. The five transverse cirri (TC) are well developed, too, located in an oblique "J" shape (approx. 30-38 um), reaching beyond the posterior end of the cell. Rows of 18-20 um long marginal cirri (MC) do not meet posteriorly. LMC turns slightly towards AZM. There are 3 caudal cirri (CC) at the posterior end of the cell, 20-25 um long in vivo. Dorsal surface swelling in the middle, with two horns pointing towards the left side. Nine rows of dorsal kineties (DK). Some individuals have an additional short row of 5-8 kineties. The four oval or ball-shaped macronuclei form a longitudinal, slightly bent row. The four micronuclei are ball shaped or ellipsoid, usually near the macronuclei. A number of chromatin bodies in the nuclei. Contractile vacuole is located near the posterior left side of AZM with two ducts during diastole. There are no subpellicular granules in the pellicules, but the cytoplasm often contains crystal-like inclusions. The organism feeds voraciously on mainly bacteria and flagellates. The species has been isolated from the astatic, thin, alkaline-like water bodies of Pentezug (Hortobagy, National Park, Hungary). (ref. ID; 7336)
  • Morphogenesis of cell division: Morphogenesis begins with kinetosome proliferation between the transversal cirri (TC) and the left marginal row (LMR), on the left side of the transversal cirri. From the oral primordium (OP) developing in this was and by the subsequent proliferation of basal bodies, a large inverse drop-like and slightly bending primordial area is formed, situated postorally, extending to the level of the cytostome. Differentiation of the adoral membrane of the opisthe starts on the right anterior part of this area. In this stage of morphogenesis replication bands in the macronuclear segments are already recognizable. At the same time the development of the frontoventral-transversal (FVT) anlagen commences in both dividing parts. In the proter two FVT-anlagen are formed on the right of the buccal cirri, while three FVT-anlagen develop from the last member of the right row of cirri. The three FVT-anlagen of the opisthe develop on the right side of the anterior end of the lengthened oral primordium, which is elongating and migrating anteriorly. Thus initially, in both the proter and the opisthe, 5 FVC-anlagen are organized. In the meantime the reorganization of undulating membranes begins and is completed. The undulating membranes of the proter derive from a primordial area at the level of the first BC, in the anterior third part of the parental undulating membranes. On the right side of the primordial field of the opisthe the UM anlage appears as a row of kinetosomes from which the new undulating membranes of the opisthe will differentiate. (Our protargol preparations do not allow further description of the reorganization stages of the Um). Subsequently the developed 5-5 FVT-anlagen of the proter and opisthe become thick and then disintegrate. This marks the end of the segregation of new cirri. Differentiation of the adoral membranelles advances along the elongated oral primordial area, and then bending to the right acquires the shape characteristic of the AZM. This becomes the AZM of the opisthe, while the proter inherits the parental AZM. A new FVT-anlage joins the formerly developed ones, the origin of which has not been made clear on the basis of the protargol preparations. This anlage evolves spontaneously or-what is more possible-derives from the division of the paroral anlage as in the case of Paraurostyla weissei (Jerka-Dziadosz 1981; Wirnsberger 1985). The replication of macronuclear segments finishes, and macronuclear fragments and micronuclei being to fuse. Marginal primordia develop inside the parental marginal rows with the participation of marginal cirri. Later these early primordia move slightly to the right of the parental rows, where marginal rows of cirri are derived from them. After the bending of these marginal rows the cell divides step by step. At the same time, remaining parental marginal, frontal, transversal and caudal cirri gradually become resorbed. In the subsequent stage cirri begin to migrate. In the course of cirrus migration rows I-II, the first three and four members of rows III and VI, stay in place. The last member of row III and those of rows IV, V and VI migrate to the central and posterior-ventral areas of the proter and opisthe, respectively, forming transversal cirri. Frontal cirri (FC) develop from the first member of anlagen I, II, III, fronto-ventral cirri from the II, III, VI, ventral cirri from the IV, V, VI. Dorsal morphogenesis begins together with the differentiation of the oral primordial area. Short rows of dorsal kineties appear in the proter and opisthe, 3-3 on the two sides of the cell (dorso-marginally), and 3 (1+2) in the middle of the cell. Right side rows develop close to the anterior part of the right marginal anlage row, while left side ones along the left marginal row. Anlagen of the three caudal cirri (CC) differentiate at the posterior end of the left and middle dorsal anlagen. The dorsal cortical pattern is developed by the partial fragmention of dorsal primordium rows. We did not find, however, fragmentation in the dorsal primordium row attached on the second caudal cirrus anlagen. On the basis of our findings and observations, development of dorsal primordia in the species can be related to the fourth type according to the classification of Foissner and Adam (1984). Marginal cirri anlagen gradually become thicker in the marginal rows, while old marginal cirri are being resorbed. In the newly formed cells the nuclei and cirri acquire the configuration characteristic of the species. (ref. ID; 7336)

    Notes

    In many respects, the specimens of our population resemble those originally described by Stein (1859), especially in the dimensions of the cell, in the number of macro- and micronuclei and in the two dorsal horns. There is, however, a remarkable difference in the number and location of frontoventral and ventral cirri. Stein (1859) found 16 frontal and 18 ventral cirri, situated in 4 rows, but did not write about caudal cirri. The number of ventral cirri in the population observed by Kahl was less (13 VC), than that of Stein's (1859), so Kahl (1932) considered it to be the feeding variety of Stylonychia mytilus. The number of ventral cirri in our population always exceeds 5 (Min=9; Max=9; x=9; SO=0,0; Sx=0,0) which, together with the number of frontal ones (Min=12; Max=13; x=12,2: SO=0,42; Sx= 0,13) clearly separates it from Stylonychia grandis (Hemberger 1982). According to our observations, individuals of the populations studied had 13 cirri (FC:3 + FVC:9-10) as a maximum on the frontoventral area, 9 ventral and 5 transversal cirri. Rows of marginal cirri do not join at the posterior end of the cell. On the left dorsal surface there are two horns, clearly visible on Stein's (1859) figure. Unfortunately this important morphological character is missing from subsequent descriptions (Jones 1974; Smith 1914; Stiller 1974). O. quadricornutus has four horns (Foissner et al. 1987), while O. indica Kamra and Sapra (1993) has about 3 horn and a smaller swelling. It seems that the presence and number of these horns is one of the most important characters in the course of the identification of the species of genus Onychodromus. These are special traits of the dorsal side, and not homologous with the caudal appendices of Aspidisca and Psilotricha species (Foissner et al. 1987). The function of these horns has not yet been completely understood. It is possible that they serve as defensive spines against intraspecific predation (Wicklow 1988). In our opinion, despite some morphometrical differences on the basis of the remarkable similarity O. quadicornutus and O. indica are the same species. Dissimilarities can be the result of different geographical conditions or perhaps of another type of food organisms. Schlegel and Foissner (Foissner, pers. commun.) have found individuals with cirri situated in a manner similar to that of our population, in Tubingen, Germany. The species studied in this paper was mentioned earlier as O. grandis var. simplex by Szabo (1992), because of the lower number of ventral cirri. Morphogenesis of the species was only approximately known from Stein's description (1859). The development of oral primordia is apokinetal and not related to the transversal cirri (Foissner et al. 1987). In O. quadricornutes (Foissner et al. 1987) and O. indica (Kamra & Sapra 1993) the oral primordium develops close to TC2 and TC3 while in O. grandis it develops in the vicinity of TC1. Cirral anlagen of proter and opisthe are independent of one another. Six rows of cirri develop as in the majority of Oxytrichids. The posterior-most elements of the right and middle FVC row are included in the formation of the FVT anlagen in the proter, while second cirri of the two middle VC rows contribute in this way in the opisthe. First five anlagen develop, following the several fragmentations of primordia here, and later on similarly to that of Paraurostyla weissei (Jerka-Dziadosz 1981) a new (sixth) anlage joins to them after bipartition of the paroral one. Further, e.g. EM studies are required to elucidate this problem. In O. grandis marginal primordia are formed in the marginal rows with the participation of parental cirri. This is also true for Laurentiella acuminata, while in the case of O. quadricornutus and O. indica they develop independently of the parental cirri. Marginal cirri developed in this way shift slightly to the right then, as in most Oxytrichids. According to Kamra and Sapra (1993) only the first dorsal primordium from the left fragments, while Foissner et al. (1987) mention several fragmentations of primordia. On the basis of our observations, dorsal primordia of O. grandis except for the middle row divide only twice or three times, whereas in O. quadricornutus and O. indica rows of dorsal primordia fragment many times (Foissner et al. 1987; Kamra and Sapra 1993). Dorsal fragmenation of O. grandis displays similarity to that of Laurentiella acuminata (Martin et al. 1983), considering especially the development of the numerous small dorso-marginal kineties. Such kind of multiple fragmentation was found in Kerona polyporum (Hemberger et al. 1982), although this species belongs to another family. In summary, in the development of the ventral and dorsal pattern and in some morphogenetical events there are certain similarities among O. quadricornutus, O. indica, L. acuminata, Stylonychia vorax (Wirnsberger et al. 1985) and O. grandis. The remarkable differences in the interphase cells can be related simply to the significantly different numbers of cortical units (number of cirri), and also to the various cell sizes. In this species, development of dorsal kinety anlagen is of the 4th types, according to the type list set by Foissner and Adam (1984). (ref. ID; 7336)
    Both N1 anlagen-parts generate three cirral rows each for proter and opisthe, thus, cirri of row four are unused like those of row six. Several dorsal kineties are generated by only three within anlagen. (ref. ID; 7423)

    Ecology

    pH=7.1-7.8. Water temperature=9-19 degrees C. Dissolved O2 6.2-8.5 mg/liter. Conductivity 430-650 us. Salinity 200-500 mg/liter, 0.25-0.5 0/00. (ref. ID; 7336)

    Measurements

    100-300 um long. (ref. ID; 1618)

    Onychodromus quadricornutus Foissner, Schlegel & Prescott, 1987 (ref. ID; 3675, 4265 original paper, 7423, 7440) reported author and year? (ref. ID; 191, 2129, 4471)

    Diagnosis

    In vivo about 370x200x70 um (n=6). Body extraordinarily large, stiff; outline elliptical; four prominent horns on the dorsal side; 130 adoral membranelles; 20 macronuclear segments; 11 ventral cirral rows, 12 transverse cirri on the average. Numerous (>20) dorsal kineties. (ref. ID; 4265)

    Descriptions

    The ciliates are epibenthic and dorso-ventrally flattened. On the ventral surface are longitudinal rows of locomotory cirri and in the anterior left ventral quadrant, a buccal cavity. Dorsally there are longitudinal rows of single cilia and a unique set of four dorsal spines (middorsal, right and left posterodorsal, and caudal). Onychodromus quadricornutus is remarkable in its potential for voluminous size (up to 900 um in length), its possession of a unique set of four dorsal spines or horns, and its capability to express two kinds of developmental polymorphism induced by intraspecific predation. (ref. ID; 3675)

    [ref. ID; 4265]
  • Cells cultured in Pringsheim's medium with Chlorogonium elongatum as food organism are considerably larger than those from the spring-water cultures. The reason is unknown, but suboptimal culture conditions may probably be excluded because ciliates divided readily and reached high abundance in the spring-water cultures too. The differences, however, should be considered in the interpretation of the biometrical data, which surely do not reflect the whole potential variability of this species. A comparison with a natural (non-cultured) population would be of great interest. If not otherwise stated, all data in this study refer to individuals culture in spring water. Body shape in vivo and growing cultures rather variable, outline elliptical to lanceolate, sometimes distinctly truncated posteriorly. Many undersized individuals of very variable shape develop in old cultures. Body stiff, a little flexible under the cover glass, conspicuously (2-3:1) flattened dorso-ventrally, especially the rims. Constantly four prominent horns on the dorsal side, one horn in the upper half of the body near its center, two horns subterminal right and left of the median, and one smaller caudal horn. Horns cone-shaped, immobile, tips sometimes bifurcated, function unknown. The number and shape of the horns did not change with culture conditions and culture age. Macronuclear segments elliptical, connected by a fine strand, arranged like a question mark left of the median. Micronuclei spherical, most of them lying near the macronucleus, some scattered in the cytoplasm. About four to six contractile vacuoles along the left and anterior body margin. Pellicle without subpellicular granules, very brittle; shape-edged fragments can be broken out by touching the cell with a hair or fine needle. Cytoplasm colorless, filled with numerous fatty shining globules, 1-5 um in diameter. No cytoplasmic crystals. Movement strikingly slow and jerky. Feeds on algae (Chlorogonium elongatum), zooflagellates, ciliates (Chilodonella uncinata, Cyclidium glaucoma, Tetrahymena thermophila), and wheat starch. Cannibalistic when starved. Adoral zone of membranelles nearly 1/2 of body length, its posterior part covered by the pellicle. Bases of the largest membranelles in vivo about 25 um wide, their fine details as in other hypotrichs. Buccal area large, considerably deepened, a small bulge at its right side. Two straight undulating membranes, cilia about 12 um long. Cirral pattern constant. One left and right row of marginal cirri, about 20 um long; those of the right row are sometimes rather irregularly distributed. Transverse cirri conspicuously enlarged and during movement of the ciliate, strikingly motionless, about 25 um long; only the lowermost ones project slightly above the posterior body margin. Two slightly enlarged ventral cirri near the transverse cirri. Caudal cirri fine, the left cirrus being constantly inserted near the tip of the caudal horn. One row of buccal cirri, no clustered (oxytrichid) postoral cirri. Ventral cirri arranged in 8-13 straight rows at the right of the undulating membranes. The rows elongate from right to left, with exception of the rightmost row, which is nearly as long as the body. Thus, a suture ("systeme secant") is produced at the right part of the ventral cirral field. The first 2-4 cirri of the leftmost rows are conspicuously enlarged, about 25 um long, and form 2-3 arched "rows" along the anterior body margin. Dorsal cilia in vivo 4 um long, arranged in many (>20) rows, most of them being more or less shortened. This shortening and the small distances between the rows were the reason why it was impossible to do exact counting. All dorsal kineties appear composed of densely packed pairs of basal bodies, only the anterior being ciliated. Each pair is surrounded by fibrils forming a rhomboid figure. (ref. ID; 4265)
  • Cortical development during cell division: The morphogenesis of O. quadricornutus is most similar to that of Laurentiella acuminata. To make a comparison easy, the style of description follows closely the paper of Martin et al. (1983). A few basal bodies develop at the left border of the first one to three transverse cirri, which appear intact. Basal bodies increase in number forming a narrow patch that extends anteriorly and reaches the proximity of the peristomial vertex. The resulting oral primordium (OP) is club-shaped, rounded anteriorly, and pointed posteriorly. About 50% of the dividing cells show one or two round clusters of basal bodies at the posterior end of the anarchic field. From the anterior end of the OP, three streaks separate; the left one becomes the paroral primordium, the middle one the buccal primordium, ant the right one develops to a ventral row. The buccal row of the proter is formed by an anlage that develops near the parental row, whose cirri are resorbed at the end of the division process. Some parental cirri of the ventral rows disintegrate and rearrange their basal bodies into the ciliary streaks appearing next, which average 13 for each tomite. Those of the proter evolve a little bit later than those of the opisthe. On the average these are two frontal-ventral-transverse (FVT) anlagen more in the filial products than ventral rows in the interphase individuals, suggesting that some reduction occurs in later division phases. This reduction probably begins after the new transverse cirri have been separated, because the number of transverse cirri of the non-dividers is on the average the same as the number of FVT anlagen. No FVT anlagen develop within the two to three rightmost ventral rows, which remain unchanged during the whole division process. These rows are produced by splitting of some of the rightmost FVT anlagen, in both the proter and the opisthe. Development of cirral primordia is accomplished by growth, in both length and width, of thin streaks and by posterior fragmentation in cirri, which progresses as an anteroposterior wave, transverse cirri being the last and largest, except for the frontal cirri that derive from the approximately eight leftmost cirral streaks. While this happens, the rest of the OP differentiates into membranelles. Alignment of membranelles starts at the anterior right edge of the OP. The paroral primordium of the opisthe develops as a long anarchic field of basal bodies parallel to the right border of the OP and remains connected with streaks II and III until the start of organization of the endoral membrane and the first (leftmost) frontal cirrus. Thus, the FVT system of the opisthe derives from usually 13 cirral primordia: three of these come from the OP and 10 originate by proliferation and rearrangement of basal bodies from old ventral cirri. The paroral and endoral membrane, the buccal area, and some fibrillar structures of the proter undergo a partial disorganization. The basal bodies lose temporarily their linear arrangement and later rearrange and form again the paroral and endoral membrane and a new first frontal cirrus. The buccal area appears smaller, less deep, and the fibrils in the peristomial vertex disappear. The parental adoral zone of membranelles is inherited without any appreciable changes. Thus, the FVT system of the proter evolves from one paroral primordium and, on the average, 12 primordia which originate by proliferation and rearrangement of basal bodies from parental ventral cirri. All cirral streaks of the proter develop without participation of the primordia of the opisthe. The evidence is the high number of unchanged parental cirri and the large gap between the two anlagen field in the early and middle stages of the division process. New marginal cirri appear a little bit later than the FVT anlagen and are built from streaks always formed closely at the right of the old rows at two sites on each side, one anterior and the other posterior to the future fission plane. No parental marginal cirri are involved in this process, as indicated by the unchanged parental right marginal row in the early and late division stages. Anterior and posterior elongation of each primordium and subsequent fragmentation form the new marginal cirral rows. The dorsal ciliature derives from many kineties in three sets. One set of three kineties develops by proliferation of some basal bodies of three parental dorsal ciliary rows. At the posterior end of these, kineties organize the new caudal cirri. A second set of two kineties develops near the anterior limit of each newly developing right marginal row and then extends to the anterior half of the cell. A third set of many short kineties develops by multiple fragmentation of the anterior ends of the three dorsal ciliary rows which produce the caudal cirri. All ciliary organelles -ventral, marginal, and dorsal- that do not participate in morphogenesis disappear after cytokinesis. The dorsal horns develop at the end of the fission process. The macronuclear segments fuse to a rod-like mass during the middle stages of the division. Later, this mass segregates into the typical nodulated macronucleus of the interphase individuals. (ref. ID; 4265)

    Nomenclatural rectification

    Lin & Prescott (1985) introduced the name "Styx sp." for our species but gave no description that would satisfy the rules of the International Code of Zoological Nomenclature. Thus, this name is a nomen nudum and needs not be considered further. In addition, the "Styx sp." of these authors is a junior homonym of Styx Staudinger, 1875, a butterfly, and Styx Oppenheim, 1895, a mollusc. (ref. ID; 4265)

    Comparison with other species

    Onychodromus quadricornutus is unique among all described hypotrichs by having four dorsal horns. Its probably the most voluminous hypotrich ciliate known (ca. 2x10E6 um3 for individuals from spring-water cultures and ca. 5x10E6 um3 for those cultured in Pringsheim's medium, reaching peak values up to 20x10E6 um3 for the largest cells). To our knowledge, there is only one other species with horns: O. grandis Stein, 1859. This species has two bulges near the left margin, clearly depicted by Stein. Unfortunately, this important character is lacking in later descriptions of this species and even in Kahl (1932), indicating misidentification of species or superficial observation. The caudal projections of some other hypotrichs, e.g. Psilotricha (Foissner 1983), Aspidisca (Kahl 1932), and various terrestrial species (Foissner 1987), are very probably not homologous to the dorsal horns O. quadricornutus because they are simple elongations at the end of the cell, whereas the horns of O. quadricornutus and O. grandis are a special differentiation of the dorsal side. Concerning the general appearance, the nodulated macronucleus, and the infraciliary pattern, O. quadricornutus resembles species of the genus Laurentiella and Coniculostomum. The bewildering nomenclatural and taxonomic problems within this and related genera have been discussed by others (Jankowski 1979 and H. Hemberger, Dissertation, University of Bonn, 1982) and are beyond the scope of this paper. With respect to the size, the shape, the moniliform macronucleus, and the extensive ventral and dorsal ciliature, Coniculostomum monilata is probably the most similar species. This species lacks, however, dorsal horns, and the left half of its ventral ciliature is much more irregularly arranged than in O. quadricornutus. (ref. ID; 4265)

    Morphogenetic comparison and systematic position of O. quadricornutus

    Onychodromus quadricornutus and Laurentiella acuminata shape some important characters (cirral pattern, number of dorsal kineties and FVT anlagen, fragmentation of dorsal primordia) with the oxytrichids sensu lato (e.g. Paraurostyla) in the phylogenetic system of the oxytrichids (Wirnsberger, Foissner and Adam 1986). Parakahliella and Pleurotricha clearly belong to other lineages as indicated by their increased number of self-replicating marginal rows and -in Pleurotricha- by the clustered frontal cirri and postoral ventral cirri. There is, however, no clear generic separation possible with non-morphogenetic characters between species like Onychodromus quadricornutus, O. grandis, Laurentiella acuminata, Parakahliella macrostoma, and Paraurostyla weissei. The differences are rather vague or simply quantitative. A main difference between O. quadricornutus and L. acuminata concerns the development of the cirral primordia of the proter and the opisthe. They evolve independently in O. quadricornutus and in some continuity in L. acuminata. This character has been used to separate some oxytrichid genera (Foissner 1983) and is perhaps an argument for the validity of the genus Laurentiella, which has been synonymized with Onychodromus (H. Hemberger, Dissertation, University of Bonn, 1982). Unfortunately, nothing has been published about the morphogenesis of the type species, O. grandis. Thus, it is impossible to make any defensible conclusion about the generic status of Laurentiella acuminata and Onychodromus quadricornutus; however, the dorsal horns occurring in both, O. grandis and O. quadricornutus, are a strong apomorphic character suggesting congenerity of these species. Likewise, the occurrence of multiple fragmentation of the dorsal primordia in L. acuminata support the suggested inclusion of this species into the genus Onychodromus (Jareno & Tuffrau, 1979). This is strengthened by a study on the morphogenesis of O. grandis which is in progress (Foissner, unpubl.). The main results are 1) the oral primordium originates apokinetally and has no contact with the transverse cirri; 2) six cirral streaks are formed as in the oxytrichids s. str.; 3) multiple fragmentation of the dorsal primordia occurs and it is very similar to that described in L. acuminata especially in developing numerous small dorsalmarginal kineties close to the new right marginal cirri. Unfortunately, the phylogenetic significance of the dorsal infraciliature is not well understood, but some authors propose that it is important (Foissner & Adam 1983; Martin, Fedriani & Perez-Silva 1983). Different modes of origin, however, occur in species with a nearly identical ventral morphogenetic pattern and which are thus at present in the same genus (Foissner & Adam 1983). Multiple fragmentation is not very common but occurs also in Kerona pediculus, which belongs to a different family from Onychodromus and Laurentiella with regard to the development of the frontal cirri (Hemberger & Wilbert 1982). Nevertheless, O. quadricornutus and K. pediculus have many morphological and morphogenetic characters in common, indicating a close relationship of these families or highly convergent evolution. Another striking similarity exists between O. quadricornutus and Paraurostyla weissei. The rightmost ventral row of P. weissei evolves from two cirral streaks and appears very similar to the rightmost ventral rows of O. quadricornutus during the late divisional stages. Another species, Urostyla hologama (very probably a Paraurostyla species) even shows the same pattern as O. quadricornutus during the interphase. It is, however, very likely an analogy because these rows are self-replicating in P. weissei, whereas they evolve by division of the cirral streaks of the neighboring ventral rows in O. quadricornutus. (ref. ID; 4265)

    Phenotypic plasticity

    Onychodromus quadricornutus is remarkable in its extreme phenotypic plasticity. Multiple locomotory cirral rows and the buccal cavity are located ventrally; on the dorsal surface are longitudinal rows of single cilia and a unique set of four dorsal spines: middorsal, right and left posteriodorsal, and caudal. Macronuclei are moniliform. This ciliate is capable of expressing developmental polyphenisms that include behavioral, morphological, and physiological responses. Morphological plasticities involve changes in cell size and dorsal spine development. Intraclonal cell size ranges from <100 um up to 900 um (Wicklow 1988) depending on the culture conditions; this ciliate is considered the most voluminous hypotrich known (Foissner et al. 1987). Furthermore, I reported both intraspecific and interspecific predator-released signals stimulate dorsal spine growth in this species (Wicklow 1988). In response to changing environmental conditions individuals within an Onychodromus clone may express one of three general phenotypes: basic, laceolate, or giant cells. Basic cells are herbivores and are expressed when an algal food source is abundant. When algae are depleted, however, basic cells undergo a rescaling process, transforming into smaller lanceolate cells, or if consumption of a conspecific occurs, into predacious giant cells (Wicklow 1988). In addition of differences in cell dimensions, O. quadricornutus giants have increased numbers of cirral rows, membranelles, and macronuclei. A correlation between cell size and macronuclear size is also observed in the various phenotypes of Onychodromus indicus (Kamra and Sapra 1994). Studies show that an increase in macronuclear volume is accompanied by an increase in DNA content. Onychodromus quadricornutus phenotypes also express ecological differences. Basic cells are primary consumers, filter-feeders, algivorous and although capable of swimming are primarily benthic. Lanceolate cells are primary consumers, filter-feeders, bacterivorous and spend a significant amount of time swimming in the water column. Giant cells are secondary consumers, raptorial-feeders, and due to their voluminous size are obligatory benthic. (ref. ID; 7440)
  • Induction of morphological defenses: The predacious giant phenotype of O. quadricornutus releases a morphogenetically active substance, Onychodromus-factor (O-factor), that induces defensive spine growth in conspecifics both intraclonally and interclonally. All four dorsal spines become hypertrophied presumably providing protection from potential predators approaching in anterior, posterior, and lateral directions. Postero-dorsal spines may grow to >40 um in lanceolate cells and up to 90 um in basic cell exposed to O-factor. O-factor also serves as an interspecific inducing signal that triggers defensive morphological changes in Sterkiella cells. In addition of the intraspecific inducing signal, O-factor, a weaker but significant defensive spine growth in O. quadricornutus is triggered by the interspecific inducing signal. Evidence from transmission electron microscopy shows that defensive spines are supported primarily by microtubules arranged subcortically along the long axis of the spine. These supporting bundles of microtubules are also apparent in protargol-stained cells and are likely responsible for the rapid growth of the spines. (ref. ID; 7440)

    Remarks

    Morphogenesis: The only parakahliellid species in which obviously both anlagen-parts of N1 develop in the third ventral rows from right (row 11). The anterior N1 anlagen-part develops de novo and generates proter's rows 11-13, the posterior N1 anlage-part develops from cirri of the old row 11 and generates opisthe's new rows 11-13. The two to three rightmost ventral rows are reported to generate no anlagen or streaks. Proter and opisthe's right marginal row 14 (RM) and opisthe's left marginal row L1 develop by typical neokinetal 5 (N5) anlagen development, i.e. all or nearly all cirri of these three old rows are intact (unused) while the new cirri develop. Therefore, in late stages of morphogenesis these old and new rows can be observed lying side by side; surprisingly, no old cirri in interphase are reported for this species. The many dorsal kineties develop by only three within anlagen for each proter and opisthe. Like in most evolved parakahliellids, two dorsomarginal kineties develop. (ref. ID; 7423)

    Type location

    Unknown; the population investigated was isolated from a freshwater aquarium in Boulder, Colorado. (ref. ID; 4265)

    Type specimens

    One slide of holotype specimens and one slide of paratype specimens have been deposited in the collection of microscope slides of the Upper Austrian Museum in Linz. (ref. ID; 4265)