Morphology of Three Species of Amphileptus (Protozoa,Ciliophora, Pleurostomatida) from the South China Sea,with Note on Phylogeny of A. dragescoi sp. n.

Two new and one problematic species of pleurostomatids, Amphileptus dragescoi sp. n., A. wilberti sp. n., and A. marinus from coastal areas of the South China Sea, are described based on observations of live and protargol‐impregnated specimens. Amphileptus dragescoi is different from its congeners by the presence of an apical group of extrusomes and the possession of 12–15 right and five left somatic kineties, two macronuclear nodules, and a single terminally positioned contractile vacuole. Amphileptus wilberti is diagnosed by oval or pyriform body, 15–19 right and seven or eight left somatic kineties, extrusomes arranged only in anterior portion of oral slit, usually three ventrally located contractile vacuoles, and two macronuclear nodules. Amphileptus marinus (Kahl, 1931) Song et al., 2004 is redescribed and its diagnosis is improved. One isolate which was misidentified as A. marinus by Song et al. (2004) is believed to represent an unknown form, named here as Amphileptus songi sp. n. Phylogenetic analyses of the SSU rDNA sequences indicate that the genus Amphileptus is paraphyletic, but its monophyly is not rejected by statistical tree topology tests.     

Morphology and Phylogeny of Two Novel Pleurostomatids (Ciliophora,Litostomatea), Establishing a New Genus

Pleurostomatida Schewiakoff, 1896 is a cosmopolitan order of ciliates. In the present study, we investigated two new pleurostomatid species, Apolitonotus lynni gen. et sp. nov. and Protolitonotus clampi sp. nov., with state‐of‐the‐art methods. Apolitonotus lynni lacks its oral extrusomes and its right kineties form an anterior semi‐suture near the dorsal margin. Based on these two features, the new genus Apolitonotus was established within the Protolitonotidae Wu et al., 2017. Protolitonotus clampi differs from its congeners by its size of 80–130 × 15–30 μm, 4–6 left, and 9–11 right kineties, extrusomes arranged along the oral slit, and two macronuclear nodules. Because Litonotus antarcticus possesses an anterior semi‐suture and oral extrusomes, it was transferred to the genus Protolitonotus, becoming P. antarctius comb. nov. (basionym Litonotus antarcticus Song and Wilbert, 2002). Phylogenetic analyses based on SSU rDNA sequences suggest a sister group relationship of P. clampi and the family Kentrophyllidae, and A. lynni is adelphotaxon to Litonotus gracilis, both within the order Pleurostomatida. Based on the new findings, an improved diagnosis for Protolitonotus was also provided.     

Morphology and Phylogeny of Three Pseudovorticella Species (Ciliophora: Peritrichia) from Brackish Waters of China

The biodiversity of peritrich ciliates from brackish biotopes is rarely investigated, especially members of the genus Pseudovorticella. Here, the morphology of three species of Pseudovorticella, i.e. P. cf. vestita (Stokes, 1883) Jankowski, 1976, P. spathulata sp. n., and P. qinghaiensis sp. n. isolated from brackish waters were studied. Pseudovorticella cf. vestita is characterized by inverted bell‐shaped cell; a J‐shaped macronucleus; a single contractile vacuole ventrally located; P3 three‐rowed; pellicle striated with highly developed pellicular vesicles; 18–22 transverse silverlines between peristome and aboral trochal band, and 9–13 between aboral trochal band and scopula. Pseudovorticella spathulata sp. n. differs from its congeners by the following combination of characters: elongate‐elliptical cell; a single contractile vacuole near ventral wall of infundibulum; a J‐shaped macronucleus; P3 three‐rowed; 24–34 silverlines between oral area and aboral trochal band and 6–10 between aboral trochal band and scopula. Pseudovorticella qinghaiensis sp. n. is characterized by: cell with an oval outline; a single contractile vacuole near ventral wall of infundibulum; a C‐shaped macronucleus; P3 three‐rowed; 30–35 and 9–11 transverse silverlines above and below the trochal band, respectively. The SSU rDNA sequences of five Pseudovorticella species, namely P. annulata, P. monilata, P. parakenti, P. spathulata sp. n., and P. cf. vestita, plus that of Zoothamnium hartwigi, are reported for the first time and their evolutionary relationships are investigated. Five undefined Pseudovorticella forms are considered might be conspecific with P. monilata. Two congeners are conspecific with P. spathulata sp. n. Phylogenetic analyses based on SSU rDNA sequences reveal that Pseudovorticella is not monophyletic and Z. hartwigi clusters with its congeners as expected.     

Three New Loxophyllum Species (Ciliophora: Pleurostomatida) from China with a Brief Review of the Marine and Brackish Loxophyllum Species

Recent studies indicate that there is a high diversity of pleurostomatid ciliates in the coastal waters of China. Here, three new congeners of Loxophyllum, L. caudatum sp. n., L. rugosum sp. n., and L. chinense sp. n., are described following observations of live cells and protargol‐impregnated specimens. All three species usually have two macronuclear nodules and prominent warts along the dorsal margin formed by clustered extrusomes. In addition, L. caudatum sp. n. is characterized by its long conspicuous tail, dot‐like cortical granules, 4 or 5 left and 9 or 10 right kineties, and a single subterminal contractile vacuole. Loxophyllum rugosum sp. n. is distinguished by possessing three prominent ridges on the left side, 7–11 right and 5–7 left kineties. Loxophyllum chinense sp. n. is characterized by having several contractile vacuoles distributed along the ventral margin, 13–18 right and 6–8 left kineties. The small subunit ribosomal DNA (SSU rDNA) sequence similarities among six congeners range from 96.46% to 99.94%. Phylogenetic trees based on the SSU rDNA sequences indicate that all Loxophyllum spp. form a well‐supported monophyletic group. A brief review of the marine and brackish Loxophyllum species is supplied and one new combination, Litonotus multiplicatus (Kahl 1931) comb. n. (basionym Loxophyllum multiplicatum Kahl 1931), and one new name, Litonotus dragescoi nom. n. (basionym L. fasciolatus Dragesco 1966), are suggested.     

A Contribution to the Morphology and Phylogeny of Chlamydodon,with Three New Species from China (Ciliophora,Cyrtophoria)

Three new cyrtophorian ciliates isolated from coastal areas of China were described based on morphological and genetic data. The Chlamydodon mnemosyne‐like species Chlamydodon similis sp. n. differs from its congeners mainly by its number of somatic kineties. Chlamydodon oligochaetus sp. n. is distinguished from its congeners mainly by having fewer somatic kineties, and/or an elongated body shape. Chlamydodon crassidens sp. n. is characterized mainly by an inverted triangular body shape, a posteriorly interrupted cross‐striated band (5–6 μm wide), and a large cytostome. Moreover, we provided small‐subunit (SSU) rDNA sequences of C. similis sp. n. and C. oligochaetus sp. n. Maximum likelihood (ML) and Bayesian inference (BI) consistently placed C. similis sp. n. as a sister to C. paramnemosyne, but showed different branching position of C. oligochaetus sp. n., which may be due to a low taxon sampling in the Chlamydodontidae and/or an insufficient resolution of the marker gene at species level.     

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.     

Morphology and Molecular Phylogeny of a New Hypotrich Ciliate,Pseudourostyla guizhouensis sp. nov. from Southern China,with Notes on a Chinese Population of Hemicycliostyla franzi (Foissner, 1987) Paiva et al., 2012 (Ciliophora,Hypotricha)

The morphology and molecular phylogeny of a soil hypotrich ciliate, Pseudourostyla guizhouensis sp. nov., collected from southern China, were investigated. Pseudourostyla guizhouensis sp. nov. has an elongate elliptical body measuring 180–310 × 65–85 μm in vivo; invariably two right and three or four left marginal rows; six or seven dorsal kineties; adoral zone consisting of 57–70 membranelles; 12–16 frontal cirri, one buccal cirrus, 13–20 midventral pairs, two frontoterminal cirri, two pretransverse cirri, and five to seven transverse cirri. Morphogenesis during physiological regeneration indicates that the marginal rows of each side originate from a common anlage that differentiates into several rows. Molecular phylogenetic analysis based on SSU rDNA sequence data reveals that P. guizhouensis sp. nov. clusters with the type species P. cristata (Jerka‐Dziadosz, 1964) Borror, 1972 and that the genus Pseudourostyla is monophyletic. The morphological characters of another soil hypotrich ciliate, Hemicycliostyla franzi (Foissner, 1987) Paiva et al., 2012, are also described based on a Chinese (Guizhou) population.     

Morphology and Molecular Phylogeny of Two New Brackish Water Ciliates of Bakuella (Ciliophora: Urostylida: Bakuellidae) from South Korea

Two new species of Bakuella, B. (B.) incheonensis n. sp. and B. (Pseudobakuella) litoralis n. sp., were discovered in brackish water near Aamdo Shore Park, Incheon, South Korea. We conducted a morphological study based on live observations and protargol‐impregnated specimens, and a molecular analysis using nuclear SSU rRNA gene sequences. Bakuella (B.) incheonensis is diagnosed by: body size 70–105 × 20–40 μm in vivo, 21–25 adoral membranelles, three or four frontoterminal cirri, midventral complex composed of 7–10 midventral pairs with one or two rows and terminating at about 62% of body length, 20–28 left and 25–32 right marginal cirri, 58–87 macronuclear nodules, and yellowish cortical granules. Bakuella (Pseudobakuella) litoralis is diagnosed by: body size 90–125 × 30–40 μm in vivo, 25–33 adoral membranelles, 3–5 buccal cirri, midventral complex composed 10–15 midventral pairs with one or two rows and terminated at 70% of body length, one or two pretransverse cirri, 3–6 transverse cirri, 26–39 left and 29–47 right marginal cirri, 49–84 macronuclear nodules, and two types of cortical granules. Molecular phylogeny using SSU rRNA gene sequences shows a nonmonophyletic relationship among Bakuella species and emphasizes the need for further morphogenetic studies of this genus and other related hypotrichs.     

Novistrombidium sinicum n. sp. and Novistrombidium orientale n. sp. (Protozoa: Ciliophora): Two New Oligotrich Ciliates from a Mangrove Wetland, South China

ABSTRACT. The morphology and infraciliature of two new marine oligotrich ciliates, Novistrombidium sinicum n. sp. and Novistrombidium orientale n. sp., isolated from a mangrove wetland near Guangzhou, southern China, were studied from live and protargol-stained specimens. Novistrombidium sinicum is different from its congeners by the combination of the following characters: three posteriorly directed thigmotactic membranelles, one ellipsoidal macronucleus, the extrusomes equidistantly arranged, and the ventral kinety commencing below the right end of the girdle kinety. The small-sized N. orientale can be separated from its congeners by two posteriorly directed thigmotactic membranelles, one ellipsoidal macronucleus, the extrusomes equidistantly arranged, and the ventral kinety commencing below the right end of the girdle kinety.     

Phylogeny of the Poorly Known Ciliates,Microthoracida, a Systematically Confused Taxon (Ciliophora), with Morphological Reports of Three Species

Three species of Microthoracids, Lopezoterenia paratorpens n. sp., Trochiliopsis australis Foissner et al., 1988 and Discotricha papillifera Tuffrau, 1954, collected from Chinese coastal waters, were investigated using live observation and protargol staining methods. Lopezoterenia paratorpens n. sp. was characterized by its squarely shaped cortical papillae and by dorsal kineties which contained loosely distributed basal bodies. Trochiliopsis australis was revealed to have two oral membranelles, which was not recorded in the original report. Phylogenetic analyses were carried out based on SSU rRNA gene sequence data from each of the three species, and on other available data for microthoracids. The results showed that the order Microthoracida is not monophyletic because the family Discotrichidae, which contains L. paratorpens and D. papillifera, forms a clade separated from the “core” Microthoracids clade. The topologies of the maximum likelihood and Bayesian inference trees, along with the distinct morphological characteristics found previously, suggest that the family Discotrichidae should not be assigned to the order Microthoracida. We propose to designate a new order, Discotrichida n. ord. which diagnosed as: flattened ciliates with conspicuous cortical papillae on both dorsal and ventral faces, rod‐shaped mucocysts, and an asymmetric cytopharyngeal basket. Also, the fact that Leptopharynx clusters with the assemblage including T. australis, and Pseudomicrothorax is located distantly from Leptopharynx indicates that the classification of Pseudomicrothoracidae and Microthoracidae by Foissner (1985) is justified.     

Two New Species of Zoothamnium (Ciliophora,Peritrichia) from Korea,with New Observations of Z. parahentscheli Sun et al., 2009

Three peritrichous ciliates, Zoothamnium arcuatum n. sp., Z. grossi n. sp., and Z. parahentscheli Sun et al., 2009, were collected from an estuary of the Taehwagang River, Korea. All these species were investigated based on live observations and silver staining, and their small subunit (SSU) rRNA gene was also sequenced. Zoothamnium arcuatum can be identified by a goblet‐shaped colony, double‐layered peristomial lip, and abstomally shortened row 3 of infundibular polykinety 3 (P3). Zoothamnium grossi is morphologically characterized by an alternately branched stalk with the lowest secondary stalk diverging from the main part of colony, asymmetrically bell‐shaped zooids, and three short, parallel ciliary rows in P3. Phylogenetic analysis revealed that the three Zoothamnium species described in this paper clustered with other members of the family Zoothamniidae, as expected.     

Phylogeny of three choreotrich genera (Protozoa,Ciliophora, Spirotrichea), with morphological,morphogenetic and molecular investigations on three strobilidiid species

Liu, W., Yi, Z., Lin, X., Warren, A. & Song, W. (2012). Phylogeny of three choreotrich genera (Protozoa, Ciliophora, Spirotrichea), with morphological, morphogenetic and molecular investigations on three strobilidiid species. —Zoologica Scripta, 41, 417–434. The phylogenetic relationships among three strobilidiid genera, namely Strobilidium, Rimostrombidium and Pelagostrobilidium, are investigated using a combination of morphological, morphogenetic and molecular data. The results indicate that all three genera belong to the same lineage, in which Rimostrombidium evolved first and Strobilidium and Pelagostrobilidium derived later. Improved genus diagnoses for Rimostrombidium and Pelagostrobilidium are supplied. The curved kinety 2 and the caudal spiralling of some somatic kineties are confirmed as generic characters for Pelagostrobilidium and Strobilidium, respectively. In addition, the morphology and morphogenesis of three species, namely R. veniliae ( Montagnes & Taylor, 1994 ) Petz et al., 1995 , P. paraepacrum sp. n. and P. minutum sp. n., isolated from the South China Sea are described. Pelagostrobilidium paraepacrum sp. n. is characterized by the presence of six somatic kineties, 30–32 external and two internal membranelles. Pelagostrobilidium minutum sp. n. is characterized by its extremely small body size, four somatic kineties, and in having one internal and 19–21 external membranelles. Rimostrombidium conicum Kahl, 1932 is transferred to the genus Pelagostrobilidium as P. conicum (Kahl, 1932) comb. nov.     

Pelagostrobilidium liui n. sp. (Ciliophora,Choreotrichida) from the Coastal Waters of Northeastern Taiwan and an Improved Description of Pelagostrobilidium minutum Liu et al., 2012

The number of somatic kineties in Pelagostrobilidium ranges from 4 to 6 according to the present state of knowledge. This study investigates Pelagostrobilidium liui n. sp. using live observation, protargol stain, and small subunit rDNA data sequencing. Pelagostrobilidium liui n. sp. is characterized by having a spherical‐shaped body, four somatic kineties, with kinety 2 spiraled around the left side of body, about six elongated external membranelles, and invariably no buccal membranelle. It differs from its most similar congener, Pelagostrobilidium minutum Liu et al., 2012 , in (i) cell shape; (ii) macronucleus width; (iii) oral apparatus; (iv) anterior orientation of kinety 2; (v) location where kinety 2 commences; (vi) arrangement of kinety 1; (vii) distance between the anterior cell end and the locations where kineties commence; and (viii) the presence of 12 different bases (including two deletions) in the small subunit rDNA sequences. The diagnosis of P. minutum Liu et al., 2012 is also improved to include the following new characteristics: invariably four somatic kineties; kineties 2 and 4 alone commence at the same level; kinety 2 originates from right anterior cell half on ventral side, extends sinistrally posteriorly, over kinety 1, around left posterior region, terminates near posterior cell end on dorsal side; kinety 1 commences below anterior third of kinety 2.     

Morphology and molecular phylogeny of two new species of Spirostrombidium (Ciliophora,Oligotrichia), with a key to the species of Spirostrombidium

Microscopic methods were used to investigate the morphological characterization of two novel oligotrich ciliates, Spirostrombidium paraurceolare sp. nov. and Spirostrombidium faurefremieti sp. nov., isolated from a mangrove wetland in Zhanjiang and an intertidal sandy beach in Qingdao, respectively. Spirostrombidium paraurceolare sp. nov. is characterized by three thigmotactic and 8–10 buccal membranelles, the girdle kinety spiralling around cell with one and a half whorls, and located at right anterior third of dorsal side anteriorly. Spirostrombidium faurefremieti sp. nov. can be recognized by a prominently deep and broad buccal cavity, two thigmotactic and 15–19 buccal membranelles, and the girdle kinety spiralling around cell with two whorls. The small subunit ribosomal RNA genes of these two species were sequenced and compared with those of their congeners to reveal nucleotide differences. Phylogenetic analyses revealed that the genus Spirostrombidium is non-monophyletic. Spirostrombidium faurefremieti sp. nov. falls into a clade comprising most congeners, but Spirostrombidium paraurceolare sp. nov. branches off and groups with Varistrombidium kielum with moderate support. A key to the identification of Spirostrombidium species is also provided.www.zoobank.org/urn:lsid:zoobank.org:pub:AB96BEE6-BE3A-4B95-B75A-3469B1C53ABB2     

Rubrioxytricha guamensis nov. spec. (Ciliophora,Spirotricha), a Novel Hypotrich Ciliate from Guam (United States), Micronesia

The oxytrichid ciliate Rubrioxytricha guamensis nov. spec. isolated from water samples collected from a small freshwater pond near the Hagåtña River in Hagåtña, Guam (United States territory), Micronesia, was investigated, using live observation and protargol impregnation. The morphology, ontogenesis, and molecular phylogeny inferred from the small‐subunit rRNA gene sequences were studied. The new species is mainly characterized by a cell size of about 100 × 35 μm in vivo, two elongate ellipsoidal macronuclear nodules and two micronuclei, a single contractile vacuole, a colorless cytoplasm, yellowish cortical granules, arranged in short rows and in small groups, an adoral zone occupying about 34% of body length and comprising 27 membranelles on average, about 27 cirri each in the right and left marginal rows, 18 frontoventral transverse cirri, four dorsal kineties including one dorsomarginal row, and one or two caudal cirri at the posterior end of dorsal kinety 3. The ontogenesis of the new species is similar to that of Rubrioxytricha indica Naqvi et al., 2006. Phylogenetic analyses based on SSU rRNA gene sequences consistently place the new species within the family Oxytrichidae Ehrenberg, 1838, where it clustered with other Rubrioxytricha species, viz., R. tsinlingensis, R. ferruginea, and R. haematoplasma.     

Morphological and Molecular Description of Three New Species of the Cyrtophorid Genus Chlamydodon (Ciliophora,Cyrtophoria)

This study investigates the morphology and molecular characteristics of three new cyrtophorid ciliates isolated from China seas: Chlamydodon salinus n. sp., Chlamydodon caudatus n. sp., and Chlamydodon paramnemosyne n. sp. Of these, C. salinus n. sp. differs from its congeners through a combination of body size, a cross‐striated band that is not continuous, the presence of 30–34 somatic kineties, 11–15 nematodesmal rods, and 13 contractile vacuoles. Chlamydodon caudatus n. sp., meanwhile, is characterized by having a conspicuous tail, a continuous cross‐striated band, 34–40 somatic kineties, about 15 contractile vacuoles, and 20–24 nematodesmal rods. Compared with other Chlamydodon species, the third new one, C. paramnemosyne n. sp., could be identified by its continuous cross‐striated band, 16–18 somatic kineties, 5 contractile vacuoles, and 9–12 nematodesmal rods. Based on the sequence of the small subunit (SSU) rRNA gene, the phylogeny of these three new species was analyzed, indicating that they all clustered with other congeners to form a monophyletic assemblage. Based on previous studies and the present work, a brief revision of the genus Chlamydodon is supplied, and a key to aid the identification of Chlamydodon species is given.     

Description of a New Freshwater Ciliate Epistylis wuhanensis n. sp. (Ciliophora,Peritrichia) from China,with a Focus on Phylogenetic Relationships within Family Epistylididae

Two populations of Epistylis wuhanensis n. sp., a new freshwater peritrich ciliate, were isolated from different freshwater ponds located in Hubei, China. Their morphological characteristics were investigated using live observation, protargol impregnation, and scanning electron microscopy (SEM). Specimens from the two populations showed identical arrangement of the infraciliature and identical small subunit ribosomal RNA (SSU rRNA) gene and ITS1‐5.8S‐ITS2 sequences. The zooids present bell‐shaped and 90–175 × 27–54 μm in vivo. Macronucleus is variable in shape and located in the middle of cell. Pellicle is usually smooth with 139–154 and 97–105 striations above and below the trochal band, respectively. SSU rRNA gene and ITS1‐5.8S‐ITS2 sequences of E. wuhanensis n. sp. did not match any available sequences in GenBank. Phylogenetically, E. wuhanensis n. sp. clusters with the other Epistylis within the family Epistylididae, but is distinct from the major clades of Epistylis. Above all, the morphological characteristics and molecular analyses support that the present Epistylis is a new species. Expanded phylogenetic analyses of sessilids based on both SSU rRNA gene sequences and ITS1‐5.8S‐ITS2 sequences reveal that the genus Epistylis consists of Epistylis morphospecies and taxonomic revision of the genus is needed.     

Morphology,Ontogeny, and Phylogeny of Two Brackish Urostylid Ciliates (Protist,Ciliophora, Hypotricha)

The diversity of hypotrichous ciliates has encouraged numerous researchers to use a combination of morphological, morphogenetic, and phylogenetic data to provide a better understanding of the evolutionary relationships within this complex group. In this study, we investigate the morphology and morphogenesis of Pseudourostyla subtropica sp. nov., isolated from mangrove wetland. The new species can be distinguished from its congeners by the huge body size, many more adoral membranelles and marginal cirral rows, and numerous macronuclear nodules. In addition, we provide a morphological characterization of a population of Pseudourostyla nova Wiackowski 1988 from an estuarine habitat. The main events during binary fission of P. subtropica sp. nov. and the Chinese population of P. nova are also revealed to be conservative. The morphological, ontogenetic, and phylogenetic analyses based on the SSU rDNA sequences corroborate the monophyly of Pseudourostyla Borror, 1972, which corresponds well with previous research. The phylogenetic analyses also show that Pseudourostyla and Hemicycliostyla Stokes, 1886, both of which are assigned to the family Pseudourostylidae based on morphological and morphogenetic data, in fact fall into separated clades. The approximately unbiased tests, however, do not reject the possibility that the family Pseudourostylidae is a monophyletic lineage.     

A New Heterotrophic Dinoflagellate from the North‐eastern Pacific,Protoperidinium fukuyoi: Cyst–Theca Relationship,Phylogeny, Distribution and Ecology

The cyst–theca relationship of Protoperidinium fukuyoi n. sp. (Dinoflagellata, Protoperidiniaceae) is established by incubating resting cysts from estuarine sediments off southern Vancouver Island, British Columbia, Canada, and San Pedro Harbor, California, USA. The cysts have a brown‐coloured wall, and are characterized by a saphopylic archeopyle comprising three apical plates, the apical pore plate and canal plate; and acuminate processes typically arranged in linear clusters. We elucidate the phylogenetic relationship of P. fukuyoi through large and small subunit (LSU and SSU) rDNA sequences, and also report the SSU of the cyst‐defined species Islandinium minutum (Harland & Reid) Head et al. 2001. Molecular phylogenetic analysis by SSU rDNA shows that both species are closely related to Protoperidinium americanum (Gran & Braarud 1935) Balech 1974. Large subunit rDNA phylogeny also supports a close relationship between P. fukuyoi and P. americanum. Three subgroups in total are further characterized within the Monovela group. The cyst of P. fukuyoi shows a wide geographical range along the coastal tropical to temperate areas of the North‐east Pacific, its distribution reflecting optimal summer sea‐surface temperatures of ~14–18 °C and salinities of 22–34 psu.