Morphology, Morphogenesis and Phylogenetic Position of the Soil Hypotrichous Ciliate, Parabistichella dieckmanni (Foissner, 1998) Foissner, 2016 (Ciliophora, Hypotrichia), with Notes on the Phylogeny of Parabistichella.

The morphology, morphogenesis and molecular phylogeny of Parabistichella dieckmanni (Foissner, 1998) Foissner, 2016, isolated from north China, were investigated. The Chinese population was characterized as having five to seven frontal cirri in corona, four to seven buccal cirri arranged in a row, two to four short frontal rows and two long frontoventral rows, three to seven transverse cirri, four macronuclear nodules, three dorsal kineties following a Gonostomum-pattern, and caudal cirri absent. Morphogenetic research reveals that the main characteristics during binary fission are as follows: (1) the long left frontoventral row is formed by two or three anlagen; (2) the posterior part of the parental adoral zone of membranelles is renewed, and the oral primordium of the opisthe is formed intrakinetally; (3) FVT-anlagen I to VI (or V, VII) produce each a frontal cirrus to form the frontal corona; (4) development of dorsal kineties follows the Gonostomum-pattern. Phylogenetic analyses showed that P. dieckmanni does not group with other Parabistichella species. Therefore, the genus Parabistichella is polyphyletic. Additionally, Parabistichella variabilis n. comb. (basionym: Bistichella variabilis He & Xu, 2011) and Parabistichella cheni n. nom. (basionym: Parabistichella variabilis Jiang et al., 2013) were suggested.

Morphology and ontogenesis of two new Hemiholosticha species (Ciliophora, Hypotrichia, Hemiholostichidae nov. fam.).

The morphology and ontogenesis of two new hypotrich ciliates, Hemiholosticha solitaria and Hemiholosticha germanica, were studied using live observation, protargol impregnation, and scanning electron microscopy. Both species share a medium-sized, almost globular body with a short anterior projection; two macronuclear nodules with a single micronucleus in between; a central contractile vacuole; three or four ventral, one postoral, one right and one left marginal cirral row; and three dorsal kineties extending along ribs. However, H. germanica is distinguished from congeners by a higher number of cirri in ventral rows R1 and R2 (3-6 vs. 2 cirri in each row). Hemiholosticha solitaria differs from congeners by having four (vs. three) ventral cirral rows and by the lack (vs. presence) of intracellular green algae. The ontogenesis of H. solitaria follows the H. pantanalensis mode in that (i) the oral primordium develops in a deep pouch and generates the first two cirral streaks in addition to adoral membranelles and undulating membranes, (ii) the undulating membrane anlage does not produce any cirri, and (iii) the longitudinal ventral cirral row R3 originates from two anlagen. The ontogenetic peculiarities along with the 18S rRNA gene phylogenies suggest classification of Hemiholosticha, Psilotrichides, and Urospinula into a new family, Hemiholostichidae.

An Integrative Investigation of Parabistichella variabilis (Protista, Ciliophora, Hypotrichia) Including Its General Morphology, Ultrastructure, Ontogenesis, and Molecular Phylogeny.

Hypotrichs are a highly differentiated and very diverse group of ciliated protists. Their systematics and taxonomy are challenging and call for detailed investigations on their general morphology, ultrastructure, ontogenesis, and molecular phylogeny. Here, a comprehensive study is conducted on a brackish water population of Parabistichella variabilis using light and electron microscopy and phylogenetic analyses based on small subunit ribosomal DNA sequence data. Its morphology, including the infraciliature, pellicle, nuclei, buccal seal, and extrusomes, is documented. The present findings indicate that in P. variabilis: (i) the cortical granules are extrusomes, which differ from those of other hypotrichs; (ii) the buccal seal is bounded by the plasma membrane and contains a single layer of longitudinal microtubules; (iii) two contractile vacuoles might be present rather than one; and (iv) the pharyngeal disks are bounded by a single membrane. Early-to-middle stages of ontogenesis are described for the first time, enabling the complete characterization of this process. Phylogenetic analyses indicate that Parabistichella variabilis is closely related to several species from different genera, such as Orthoamphisiella breviseries, Uroleptoides magnigranulosus, and Tachysoma pellionellum. However, ultrastructural and gene sequence data for more taxa are needed in order to resolve the systematics of Parabistichella.

Morphogenesis and Molecular Phylogeny of the Soil Ciliate Holostichides chardezi (Ciliophora, Hypotrichia, Bakuellidae), with Redefinition of Holostichides Foissner, 1987 and Establishment of a New Genus Anteholostichides.

Morphogenesis of the soil hypotrich ciliate, Holostichides chardezi Foissner, 1987, collected from southeastern China, was investigated using the protargol staining method. The main morphogenetic events follow a similar process with that of its congeners. Phylogenetic analyses based on the SSU rDNA sequence data indicate that Holostichides is nonmonophyletic; H. chardezi, the type species of Holostichides, clusters with H. heterotypicus, while H. terrae is distinctly separate from these species. H. terrae can be distinguished from H. chardezi (type species of Holostichides) and H. heterotypicus by undulating membranes relatively long and distinctly curved (vs. relatively short and straight), pharynx with (vs. without) rod-shaped structure, and two (vs. more than two) frontoterminal cirri. Therefore, a new genus, Anteholostichides nov. gen., has been proposed for H. terrae. Further, the diagnosis of the genus Holostichides is improved.

Morphology and morphogenesis of a new soil urostylid ciliate, with the establishment of a new genus Caudiurostyla gen. nov. (Ciliophora, Hypotrichia).

The living morphology, infraciliature and morphogenesis of a new urostylid ciliate, Caudiurostyla sinensis gen. nov., spec. nov., isolated from a soil habitat in China, was investigated. The new, monotypic genus is defined by having frontal cirri arranged in a bicorona, distinct transverse cirri, midventral pairs only, buccal cirrus, frontoterminal cirri, multiple marginal cirral rows that replicate independently via within row anlagen formation, more than three dorsal kineties, and caudal cirri. The main morphogenetic features of the new species are: (1) the posterior part of the parental adoral zone is renewed in situ; (2) the marginal cirral rows and dorsal kineties develop intrakinetally; (3) a caudal cirrus is formed at the posterior end of each dorsal kinety; and (4) macronuclear nodules fuse into a single mass during the middle stage. Based on the morphology and morphogenesis, the new genus differs from Pseudourostyla and Hemicycliostyla by presence of caudal cirri.

Morphology and morphogenesis of a new soil urostylid ciliate, Australothrix xianiensis nov. spec. (Ciliophora, Hypotrichia).

The living morphology, infraciliature and morphogenetic events of a new soil urostylid ciliate, Australothrix xianiensis nov. spec., collected from Chanba National Wetland Park in Xi'an, China, were studied in vivo and after protargol preparation. Australothrix xianiensis nov. spec. is characterized as follows: about 190-240 × 40-60 µm in life; body pisciform and dark; cortical granules arranged in longitudinal rows on both sides, colourless, rod-shaped and about 2 × 1 µm in size; single contractile vacuole slightly ahead of mid-body with two long collecting canals; macronuclear nodules scattered throughout cytoplasm; adoral zone occupies about 20% of body length, composed of about 32 membranelles; three frontal cirri and one buccal cirrus; two to four midventral pairs and four or five midventral rows, one left and two right marginal rows; four or five dorsal kineties; four or five caudal cirri. The main features of divisional morphogenesis are: (1) each posterior streak generates a midventral row together with the midventral pair; (2) the old adoral zone of membranelles is retained with the exception of the posterior part, which is renewed in situ; the undulating membranes are completely renewed; (3) parental cirri do not contribute to the construction of the oral primordium in the opisthe in very early dividers; (4) two sets of frontoventral cirral anlagen are formed.

Morphology, Morphogenesis, and Molecular Phylogeny of a New Soil Ciliate, Sterkiella multicirrata sp. nov. (Ciliophora, Hypotrichia) from China.

The morphology and morphogenesis of a new soil hypotrich ciliate, Sterkiella multicirrata sp. nov., was investigated using live observation and protargol staining. The new species is characterised by: body elliptical, 110-180 × 45-75 µm in vivo, an average of 35 adoral membranelles; usually 19 frontoventral-transverse cirri, consisting of three frontal, five frontoventral, one buccal, four postoral ventral, two pretransverse, and four transverse cirri; four macronuclear nodules, and 2-5 micronuclei. S. multicirrata sp. nov. differs from its congeners mainly in the number of frontoventral-transverse cirri and macronuclear nodules. Morphogenesis of the new species is similar to its congeners; the primary difference exists in the segmentation of the frontoventral-transverse cirral anlagen, which is usually generated in a 1:2:3:3:5:5 pattern. Based on the small subunit ribosomal DNA sequence, the phylogenetic position of the new species is discussed.

Morphological and Molecular Redefinition of Euplotes platystoma Dragesco & Dragesco-Kernéis, 1986 and Aspidisca lynceus (Müller,1773) Ehrenberg, 1859, with Reconsideration of a "Well-known" Euplotes Ciliate, Euplotes harpa Stein, 1859 (Ciliophora, Euplotida).

We documented the morphology, infraciliature, silverline system, and molecular data of two euplotid species isolated from China, including two populations of the poorly known Euplotes platystoma Dragesco & Dragesco-Kernéis, and the previously well described Aspidisca lynceus (Müller, ) Ehrenberg, 1830. Based on the information available, an improved diagnosis of Euplotes platystoma is given, including: a narrow adoral zone with 44-68 membranelles, 10 frontoventral, 5 transverse, 2 left marginal and 2 caudal cirri, 11-13 dorsal kineties with 17-25 dikinetids in the mid-dorsal row, and dorsal silverline system of the double-eurystomus type. The Chinese population of Aspidisca lynceus closely resembles previously described populations. Phylogenetic analyses inferred from SSU rDNA sequences show that E. platystoma is closely related with E. neapolitanus, and the internal position of A. lynceus within this genus is still not robust. A reconsideration of the "well-known" Euplotes harpa and a comparison of all SSU rDNA sequences of E. harpa in GenBank are provided. We speculate that the sequences available from GenBank under the name of E. harpa are very likely from misidentified materials, that is, the identity of the species currently associated with the SSU rDNA of this "well-known" form in molecular databases requires further confirmation.

Interaction Between a Bacterivorous Ciliate Aspidisca cicada and a Rotifer Lecane inermis: Doozers and Fraggles in Aquatic Flocs.

Activated sludge is a semi-natural habitat composed of macroaggregates made by flocculating bacteria and inhabited by numerous protozoans and metazoans, creating a complicated interactome. The activated sludge resembles the biological formation of naturally occurring floc habitats, such as "marine snow." So far, these two types of habitat have been analyzed separately, despite their similarities. We examined the effect of a bacterivorous ciliate, Aspidisca cicada, on the quality of the macroaggregate ecosystem by estimating (i) the floc characteristics, (ii) the proliferation of other bacterivores (rotifers), and (iii) the chemical processes. We found that A. cicada (i) positively affected floc quality by creating flocs of larger size; (ii) promoted the population growth of the rotifer Lecane inermis, an important biological agent in activated sludge systems; and (iii) increased the efficiency of ammonia removal while at the same time improving the oxygen conditions. The effect of A. cicada was detectable long after its disappearance from the system. We therefore claim that A. cicada is a very specialized scavenger of flocs with a key role in floc ecosystem functioning. These results may be relevant to the ecology of any natural and engineered aggregates.

Morphogenesis of a saline soil ciliate Urosoma salmastra (Dragesco and Dragesco-Kernéis, 1986) Berger, 1999 with notes on the phylogeny of Urosoma (Ciliophora, Hypotrichia).

Morphogenesis in a population of Urosoma salmastra collected from saline soil in Weinan, China, was investigated using protargol staining. The main morphogenetic event that characterizes U. salmastra is the inclusion of cirrus III/2 in the formation of the frontoventra-transverse cirral anlagen. We also provide small subunit ribosomal DNA gene sequences for a population of U. salmastra and for two populations of U. emarginata. The molecular phylogeny indicates that Urosoma is not monophyletic, viz, U. emarginata branches separately from its congeners and the closest relative of U. salmastra is Oxytricha granulifera.

Morphology, morphogenesis and molecular phylogeny of a new soil ciliate Paragonostomoides xianicum n. sp. (Ciliophora, Hypotrichia, Gonostomatidae).

A hypotrichous ciliate, Paragonostomoides xianicum n. sp., was discovered in soil on the surface of the Xi'an Circumvallation, Xi'an, China. Its morphology and some major ontogenetic stages were studied and the phylogenetic position was estimated using standard methods Paragonostomoides xianicum n. sp. is characterized as follows: body size about 75-90×15-25µm in vivo, elliptical and flexible; colourless mitochondrion-like cortical granules scattered throughout cell surface, about 0.8×0.6µm in size; contractile vacuole positioned at mid-body; frontoventral row IV extends beyond buccal vertex and frontoventral row VI extends to about body end; frontoventral cirri not in pairs; transverse cirri absent; three caudal cirri; two macronuclear nodules and one to four micronuclei. The ontogenetic process in P. xianicum n. sp. is basically similar to that in Metagonostomum gonostomoidum and Gonostomum-species and featured by: frontoventral-transverse cirri develop in five-anlage pattern, the rearmost cirri generated in the two rightmost FV anlagen are not set off posteriorly (like an ordinary TC), as well as FVT-anlagen n and n-1 form in secondary mode in P. xianicum. Phylogenetic analyses based on small subunit ribosomal DNA sequence data reveal a close relationship of P. xianicum n. sp. with Cotterillia bromelicola and two Gonostomum-species.

Morphologic and phylogenetic studies of two hypotrichous ciliates, with notes on morphogenesis in Gastrostyla steinii Engelmann, 1862 (Ciliophora, Hypotrichia).

Two oxytrichid ciliates, Stylonychia (Metastylonychia) nodulinucleata Shi and Li, 1993 and Gastrostyla steinii Engelmann, 1862; collected from Huguangyan Maar Lake and Shenzhen Mangrove Nature Protection Area, southern China, respectively, were investigated using standard methods. The uncommon species Stylonychia (Metastylonychia) nodulinucleata can be recognized by its large body size, the conspicuous caudal cirri in vivo, and a moniliform macronucleus. We provide the first record of the small subunit ribosomal RNA (SSU rRNA) gene sequence for the species. The division of Stylonychia into two subgenera based on morphological and morphogenetic data is supported by the phylogenetic analyses, in which Stylonychia (Metastylonychia) nodulinucleata is placed as a sister branch to the clade including all the sequences of Stylonychia (Stylonychia) mytilus complex. The first brackish water population of Gastrostyla steinii is described in detail, with emphasis on its morphogenesis, which corresponds well with previous populations. In the phylogenetic trees, all the available Gastrostyla sequences, except for Gastrostyla sp. Y2 (KT780432) (probably a misidentification), nest together in the big group of the subfamily Stylonychinae with moderate to high support (ML/BI, 91%/0.95), very likely revealing the monophyly of the genus Gastrostyla.

Morphology and Morphogenesis of a Novel Saline Soil Hypotrichous Ciliate, Gonostomum sinicum nov. spec. (Ciliophora, Hypotrichia, Gonostomatidae), Including a Report on the Small Subunit rDNA Sequence.

Morphology, cirral pattern, and morphogenesis of the new saline soil hypotrich, Gonostomum sinicum nov. spec. collected from Longfeng Wetland in Daqing, north China, were studied, using detailed live observations and protargol-stained specimens. The new species is characterized as follows: (i) a size in vivo of 100-125 × 30-40 µm, (ii) colorless cortical granules, 0.5 µm across, arranged in short rows, (iii) an adoral zone composed of 28-33 membranelles, (iv) three or four frontoventral rows, one of which extends onto the postoral area, (v) left and right marginal rows composed of 18-27 and 21-35, cirri, respectively, and (vi) usually two transverse cirri. Morphogenesis is as usual for the genus Gonostomum, i.e. the cirral primordia II-VI are primary primordia which split into two sets for proter and opisthe in division middle stages, except for anlage I which develops independently. However, the number of frontoventral transverse anlagen is either five or six not only in different individuals but even in proter and opisthe of the same divider. The phylogenetic analyses based on SSU rDNA sequences showed that the genus Gonostomum is nonmonophyletic, indicating that the patterns of cirri and dorsal kineties are homoplasious characters. The new species G. sinicum nov. spec. is perhaps closely related to Cotterillia bromelicola and two congeners.

Morphology, Ontogeny, and Molecular Phylogeny of Two Freshwater Species of Deviata (Ciliophora, Hypotrichia) from Southern China.

The morphology and partial morphogenesis of two freshwater hypotrichous ciliates, Deviata brasiliensis Siqueira-Castro et al., 2009 and Deviata rositae Küppers et al., 2007, isolated from southern China, were investigated using live observation and protargol staining. Our populations resemble the original ones in terms of their live characters and ciliary patterns. The main determinable morphogenetic features of D. brasiliensis basically correspond with those of the type population. However, the origin of anlage V for either proter or opisthe is ambiguous: whether anlage V for the proter originates from parental frontoventral row 2 (the same as in the original population) or parental frontoventral row 3 (the same as in Deviata abbrevescens) or even de novo is not clear; the anlage V for the opisthe is possibly derived from frontoventral row 3 and further migrates to frontoventral row 2, like that in D. abbrevescens. In addition, the SSU rRNA gene was first sequenced for both species. Molecular phylogenetic analyses suggest that the genus Deviata is non-monophyletic and has a close relationship with Perisincirra paucicirrata.

Resting cysts of Parentocirrus hortualis Voß, 1997 (Ciliophora, Hypotrichia), with preliminary notes on encystation and various types of excystation.

Resting cysts of Parentocirrus hortualis were investigated, using live observation, SEM and TEM. Processes during encystation and excystation were observed in vivo under the light microscope. During encystation, the trophic body becomes globular, the ciliature is resorbed in an anterior direction, the macronuclear nodules fuse into an elongated mass, and finally a cyst wall develops. As typical for oxytrichids, the resting cysts of P. hortualis are of the kinetosome-resorbing type and their wall is made of four layers: ectocyst, mesocyst, endocyst, and metacyst. The beginning of excystation is indicated by the formation of an excystation vacuole that helps the regenerating specimen to break the cyst wall. The excysting specimen leaves the resting cyst in a thin membrane that is gradually resorbed in the outer environment. Also two other excystation modes were observed. During the rare mode, the excystation vacuole breaks the thin membrane instead of the cyst wall that ruptures under the pressure of the body of the regenerating specimen. During the reproduction mode, the regenerating specimen divides within the resting cyst, producing two to four tomites. This is the first report of division in resting cysts of oxytrichids, but reproduction in division cysts was already described in keronopsids.

Morphology and molecular phylogeny of Apoterritricha lutea n. g., n. sp. (Ciliophora, Spirotrichea, Hypotrichia): a putative missing link connecting Cyrtohymena and Afrokeronopsis.

A new hypotrichous ciliate, Apoterritricha lutea n. g., n. sp., was discovered in a sample of a terrestrial liverwort from Korea. Its morphology was studied using detailed in vivo observation and protargol impregnation. Its phylogenetic relationships were revealed by analyses of the 18S rRNA gene. This new taxon is characterized by a combination of the following traits: (i) ellipsoidal to narrowly ellipsoidal body with an average size of 230 × 85 µm; (ii) two macronuclear nodules and two to five micronuclei; (iii) golden yellow cortical granules, forming small groups along the microtubular appendages of cirri, adoral membranelles, and dorsal kineties; (iv) typically three frontal cirri, one buccal cirrus, four frontoventral cirri, seven midventral cirri, two pretransverse cirri, seven transverse cirri, ca. 38 left, and ca. 36 right marginal cirri; and (v) on average six dorsal kineties, three dorsomarginal kineties, and three caudal cirri. In molecular phylogenies, A. lutea clusters with strong support within a clade containing Afrokeronopsis aurea and several "typical" oxytrichids having golden yellow to brown cortical granules. In this light we propose a hypothesis that is not unambiguously rejected by the present phylogenetic analyses, which shows how the Afrokeronopsis-like pattern could have evolved from a Rubrioxytricha-like ancestor via an Apoterritricha-like stage by cirri-multiplication.

Molecular phylogeny and taxonomy of two novel brackish water hypotrich ciliates, with the establishment of a new genus, Antiokeronopsis gen. n. (Ciliophora, Hypotrichia).

Two novel brackish water urostyloid ciliates, Anteholosticha paramanca sp. n. and Antiokeronopsis flava gen. n., sp. n., isolated from the Shenzhen Mangrove Nature Protection Area on the coast of the South China Sea, were investigated using live observation and protargol impregnation techniques. Anteholosticha paramanca sp. n. is characterized by its spherical yellowish cortical granules arranged in lines, shortened midventral complex and three transverse cirri. Morphogenesis is similar to that in Anteholosticha manca. The new genus Antiokeronopsis is diagnosed by having a continuous adoral zone of membranelles, frontal cirri arranged in a bicorona, midventral complex composed of midventral pairs only, one marginal cirral row on each side, the presence of frontoterminal and transverse cirri, and the lack of buccal and caudal cirri. The type species A. flava sp. n. is characterized by its elongated body shape, brown to yellowish body color and two types of cortical granules. Small subunit ribosomal RNA gene sequence data justify the classification of both species. Phylogenetic analyses indicate that A. paramanca clusters with Bakuella subtropica within a clade that includes two other Anteholosticha species, while Antiokeronopsis groups within the core urostylids and is most closely related to the well-known genera Pseudokeronopsis and Uroleptopsis.

Morphology and morphogenesis of Apoholosticha sinica n. g., n. sp. (Ciliophora, Hypotrichia), with consideration of its systematic position among urostylids.

This paper investigates the morphology, morphogenesis and SSU rRNA gene-based phylogeny of Apoholosticha sinica n. g., n. sp., isolated from mangrove wetland in Shenzhen, southern China. The new genus Apoholosticha is characterized by its bipartite adoral zone, clearly differentiated frontal cirri arranged in a bicorona, midventral complex composed of midventral pairs only, one marginal cirral row on each side, presence of frontoterminal and transverse cirri, and the lack of a buccal cirrus and caudal cirri. The type species, Apoholosticha sinica n. sp. is diagnosed by the elongated body shape and two kinds of cortical granules. Its main morphogenetic features are similar to that of Pseudokeronopsis except for (1) no buccal cirrus is formed and (2) its macronuclear nodules fuse into a single mass during cell division. Phylogenetic analyses for the new taxon indicate that Apoholosticha n. g. is most closely related to Nothoholosticha and Heterokeronopsis, and falls into the family Pseudokeronopsidae within the core Urostylida clade. In addition, a species that had been misidentified in previous literature is here recognized and assigned to the new genus as Apoholosticha sepetibensis (Wanick and Silva-Neto, 2004) n. comb. (basionym: Pseudokeronopsis sepetibensis Wanick and Silva-Neto, 2004).

Establishment of a new hypotrichous genus, Heterotachysoma n. gen. and notes on the morphogenesis of Hemigastrostyla enigmatica (Ciliophora, Hypotrichia).

A marine hypotrich ciliate, Heterotachysoma multinucleatum (Gong and Choi, 2007) n. comb., found in coastal waters near Qingdao, China, was investigated. Heterotachysoma multinucleatum is characterized by its dorsal ciliature arranged in Gonostomum-pattern. Additionally, a new genus, Heterotachysoma n. gen., is established which is mainly characterized by: 18-cirri pattern; flexible body; three dorsal kineties with no dorsomarginal kineties nor kinety fragmentation; one right and one left row of marginal cirri; caudal cirri absent. The genus Tachysoma is redefined, and three new combinations, T. multinucleatum, T. ovatum and T. dragescoi, are proposed. The morphogenesis of Hemigastrostyla enigmatica (Dragesco and Dragesco-Kernéis, 1986) Song and Wilbert, 1997, is also described. Compared with that of its congeners, the differences are mainly in the dorsal ciliature: (1) the dorsal kinety anlagen are formed de novo in H. enigmatica (vs. intrakinetally in H. paraenigmatica and H. elongata); (2) the dorsal kineties anlagen develop in secondary mode in H. enigmatica (vs. primary mode in H. paraenigmatica); (3) the kinetal fragment anterior to the right marginal row in both filial product is absent in both H. enigmatica and H. elongata (vs. present in H. paraenigmatica). These findings suggest that morphogenesis is not uniform among members of the genus Hemigastrostyla.

Morphology and morphogenesis of a new marine ciliate, Apokeronopsis bergeri nov. spec. (Ciliophora, Hypotrichida), from the Yellow Sea, China.

The morphology and morphogenesis of a new marine hypotrich ciliate, Apokeronopsis bergeri nov. spec., collected from mussel-farming waters near Qingdao, China, are described from living and protargol-impregnated specimens. This ciliate has characteristics that place it in the family Pseudokeronopsidae, namely, two long rows of frontal cirri (bicorona), which are continuous with the long midventral rows, and a single row of marginal cirri on each side of the body. It shares with its only congener, Apokeronopsis crassa, the long rows of buccal and transverse cirri and the wide spacing between the midventral rows of cirri. These characters separate the genus Apokeronopsis from Pseudokeronopsis, which has a single buccal cirrus, fewer transverse cirri and midventral rows of cirri arranged in a typical zig-zag pattern. A. bergeri differs from A. crassa in its shape, colour and in the numbers of membranelles and transverse cirri. Although morphogenesis in A. bergeri is similar to that of A. crassa in most respects, the mode of formation of the buccal cirri is slightly different. The close relationship of A. bergeri with A. crassa, and the more distant relationship with three Pseudokeronopsis species, is supported by a comparison of the sequences of their ITS1-5.8S-ITS2 rDNA regions.