Development of the cell covering in the dinoflagellate Scrippsiella hexapraecingula (Peridiniales, Dinophyceae)

The organization and development of cell coverings in two alternate phases of the life cycle in a marine dinoflagellate, Scrippsiella hexapraecingula Horiguchi et Chihara, were investigated by thin sectioning and freeze‐fracture electron microscopy. In one of these phases, the motile phase, cells have an outermost plasma membrane that is lined with flattened amphiesmal vesicles. Groups of microtubules lie beneath these vesicles. In mature motile cells, thecal plates are completely enclosed in individual amphiesmal vesicles. After settling, the cells enter the second, non‐motile phase. Here, ecdysis occurs, resulting in several steps including formation of the first pellicle layer (PI), fusion of the inner amphiesmal vesicle membranes to form the new plasma membrane, deposition of the second pellicle layer (PM) under PI, and the appearance and fusion of juvenile amphiesmal vesicles to form new territories, which eventually give rise to new thecal plates in the next motile phase. Thus, the pattern in which thecal plates are arranged in motile cells is determined at the time when the amphiesmal vesicles develop into non‐motile cells.     

Scrippsiella hexapraecingula sp. nov. (Dinophyceae), a tida pool dinoflagellate from the Northwest Pacific

Specimens of dinoflagellate collected in tide pools along the Pacific coast of central and southern Japan are described as a new species,Scrippsiella hexapraecingula Horiguchi et Chihara, of the Peridiniaceae (Class Dinophyceae). The plate formula is pp, x, 4′, 3a, 6″, 6c, 5‴, 2″" and, 5s, the same as that of other species ofScrippsiella, except in lacking one precingular plate. The genus must be emended, therefore, as having either six or seven precingular plates. This dinoflagellate migrates diurnally. In the morning motile cells are released from non-motile cells attached to the substrate and in the evening the motile cells swim down to settle on the bottom of the tide pool. Attached non-motile cells form either motile mono- or bispores. Sexual reproduction was not observed.     

Heterocapsa psammophila sp. nov. (Peridiniales, Dinophyceae), a new sand-dwelling marine dinoflagellate

A new armored dinoflagellate species, Heterocapsa psammophila Tamura, Iwataki et Horiguchi sp. nov. is described from Kenmin‐no‐hama beach, Hiroshima, Japan using light and electron microscopy. This dinoflagellate possesses the typical thecal plate arrangement of the genus Heterocapsa, Po, cp, 5′, 3a, 7′′, 6c, 5s, 5′′′, 2′′′′; and the 3‐D body scales of Heterocapsa on the plasma membrane. The cell shape is ovoidal. The spherical nucleus and the pyrenoid are situated in the hypotheca and the epitheca, respectively. The ultrastructure of H. psammophila is typical of dinoflagellates and the pyrenoid is invaginated by cytoplasmic tubules. H. psammophila is distinguished from all other hitherto‐described Heterocapsa species by the cell shape, the relative position of the nucleus and pyrenoid and the structure of the body scale. The habitat and behavior of this new species in culture suggest that the organism is truly a sand‐dwelling species.     

Dactylodinium pterobelotum gen. et sp. nov., a new marine woloszynskioid dinoflagellate positioned between the two families Borghiellaceae and Suessiaceae

A new marine woloszynskioid dinoflagellate Dactylodinium pterobelotum gen. et sp. nov., collected from a southern Vietnamese estuary, was described on the basis of LM, SEM, and TEM, and molecular phylogeny inferred from rDNA sequences. This species had the smallest number of amphiesmal vesicles (5 latitudinal series) in woloszynskioid dinoflagellates assigned to the Suessiaceae and Borghiellaceae. The eyespot was of type B, composed of osmiophilic globules and brick‐like material, located in‐ and outside of the chloroplast respectively. An apical structure comprised a pair of elongate anterior vesicles (PEV). A large peduncle was conspicuous, located in the sulcal extension in the epicone, and supported by a microtubular strand of ~140 microtubules. Ultrastructural features of trichocysts represent a novel type in the Dinophyceae, bearing lateral hairs besides anterior fibers. The molecular phylogeny based on partial LSU rDNA showed the species in a basal position in the family Suessiaceae; this indicates the eyespot type B and PEV of the Borghiellaceae are ancestral states of the eyespot comprising brick‐like material (type E) and an elongate apical vesicle of the Suessiaceae.     

Heterocapsa busanensis sp. nov. (Peridiniales,Dinophyceae): A new marine thecate dinoflagellate from Korean coastal waters

During daily monitoring in Yongho Bay off Busan, Korea in 2019, an isolate of the dinoflagellate genus Heterocapsa was established in clonal culture. Light and electron microscopic examination revealed that the isolate was ellipsoid in shape, exhibiting a thecal plate arrangement (Po, cp, X, 5′, 3a, 7″, 6c, 5s, 5‴, 2ʹʹʹʹ) consistent with most other Heterocapsa species. A large, elongated nucleus was positioned on the left side of the cell, a single reticulate chloroplast was located peripherally, and a single, starch-sheathed, spherical pyrenoid was present in the episome or near the cingulum. Morphologically, the isolate most closely resembles H. circularisquama and H. illdefina. Transmission electron microscopic examination of whole mounts revealed that the isolate had two body scale types, one of which was a complex, three-dimensional, fine structure distinct from other Heterocapsa species, whereas the other simpler type was structurally similar to the scales of H. horiguchii. Molecular phylogeny based on rRNA sequences revealed that the isolate was distantly related to morphologically similar species, but formed a sister lineage to H. horiguchii, a species characterized by a similar body scale morphology. Based on morphological, ultrastructural, and molecular data, we proposed it as a new species, Heterocapsa busanensis sp. nov.     

Madanidinium loirii gen. et sp. nov. (Dinophyceae), a new marine benthic dinoflagellate from Martinique Island,Eastern Caribbean

A new benthic phototrophic dinoflagellate is described from sediments of a tropical marine cove at Martinique Island and its micromorphology is studied by means of light and electron microscopy. The cell contains small golden-brown chloroplasts and the oval nucleus is posterior. It is laterally compressed, almost circular in shape when viewed laterally. It consists of a small epitheca tilted toward the right lateral side and a larger hypotheca. In the left view, the cingulum is more anterior and the epitheca is reduced. The cingulum is displaced and left-handed. This organism is peculiar in having no apical pore and its thecal plate arrangement is 2′ 1a 7′′ 5c 3s 5′′′ 1′′′′. The plates are smooth with small groups of pores scattered on their surface. An area with 60–80 densely arranged pores is found near the centre of the 2′′′ plate, on the left lateral side. Morphologically, these features are different from all other laterally compressed benthic genera. In addition, molecular genetic sequences of SSU and partial LSU form a distinct and well-supported clade among dinoflagellates and support the erection of a new genus. However, molecular phylogenies inferred from ribosomal genes failed to confirm any clear relationship with other benthic taxa and affinity with other laterally compressed dinoflagellates has not been demonstrated. Hence, the taxonomic affinity of Madanidinium loirii with a defined order and family is unclear at the moment.     

Testudodinium gen. nov. (Dinophyceae), a new genus of sand‐dwelling dinoflagellates formerly classified in the genus Amphidinium

A new genus of sand‐dwelling photosynthetic dinoflagellate, Testudodinium Horiguchi, Tamura, Katsumata et A. Yamaguchi is proposed based on Testudodinium testudo (Herdman) Horiguchi, Tamura, Katsumata, et A. Yamaguchi comb. nov. (Basionym: Amphidinium testudo Herdman) and a new species in this new genus, Testudodinium maedaense Katsumata et Horiguchi sp. nov. is described. Amphidinium corrugatum is also transferred to this genus, making a new combination T. corrugatum (Larsen et Patterson) Horiguchi, Tamura et A. Yamaguchi. These three species are similar to the members of the genus Amphidinium in having an extremely small episome and a dorsoventrally flattened cell body. They are, however, distinguished from the genus Amphidinium seusu stricto by the possession of a distinct longitudinal furrow in the middle of ventral side of the episome. Phylogenetic trees based on small subunit (SSU) rDNA revealed that all three of these Testudodinium species formed a robust clade and, although statistical support is not high, the tree suggests Testudodinium clade is not closely related to Amphidinium seusu stricto clade. The morphological differences together with molecular data support the establishment of a new genus for A. testudo and its related species.     

Plagiodinium ballux sp. nov. (Dinophyceae), a deep (36 m) sand dwelling dinoflagellate from subtropical Japan

A new species of marine sand‐dwelling dinoflagellate, Plagiodinium ballux N. Yamada, Dawut, R. Terada & T. Horiguchi is described from a deep (36 m) seafloor off Takeshima Island, Kagoshima Prefecture, Japan in the subtropical region of the northwest Pacific. The species is thecate and superficially resembles species of Prorocentrum, but possesses an extremely small epitheca. The cell varies from ovoid to a rounded square, and is small (15.0–22.5 μm in length) and laterally compressed. The thecal plates are smooth and the thecal plate arrangement (Po, 1′, 0a, 5″, 5C, 2S, 5?, 0p, 1″″) is similar to that of Plagiodinium belizeanum, the type species of the genus. Molecular phylogenetic analyses based on SSU rDNA and partial LSU rDNA reveal that the dinoflagellate is closely related to P. belizeanum, but it can be clearly distinguished by its size and cell shape. This suite of morphological and molecular differences leads to the conclusion that this deep benthic dinoflagellate represents a new species of the genus Plagiodinium.     

Ultrastructure of Stylodinium Iittorale (Dinophyceae) with special reference to the stalk and apical stalk complex

The ultrastructure of Stylodinium littorale Horiguchi et Chihara, a marine, sand-dwelling coccoid dinoflagel-late, was investigated with special emphasis on its stalk and the apical stalk complex. The dinoflagellate alternates between non-motile and motile cells in its life cycle. The non-motile cell possesses a long and distinct stalk. The stalk, consisting of a main cylindrical part and a holdfast, is firmly attached to a thecal plate (the apical pore plate). A part of its proximal portion is hollow and V-shaped in section. The V-shaped hollow space is underlain by a projection from the apical pore plate. An apical stalk complex is present in the motile cells and consists of a large apical pore plate and mucilaginous material. The apical pore plate is depressed into the cell, but has a narrow central tubular projection. The mucilaginous stalk-building material is stored between this plate and the outer plate membrane. The tubular projection of the apical pore plate corresponds to the apical pore of other dinoflagellates and its lumen is filled with electron-dense material. The structure of the apical stalk complex is compared with the homologous structure in Bysmatrum arenicola, the only other example of an apical stalk complex that has been investigated. A general ultrastructural survey revealed that S. littorale possesses a typical dinoflagellate cellular structure.     

Discovery of a novel type of body scale in the marine dinoflagellate,Amphidinium cupulatisquama sp. nov. (Dinophyceae)

A new species of Amphidinium, A. cupulatisquama Tamura et Horiguchi, from sand samples from Ikei Island, Okinawa Prefecture in subtropical Japan, is described based on light, scanning and transmission electron microscopy and the partial sequencing of the large subunit rDNA gene. The species has a typical morphology for the genus, but is distinguished from previously described species by having a combination of the following characteristics: (i) a relatively large cell (over 30 µm in length); (ii) possessing an eyespot on the dorsal side of the cingulum; (iii) the longitudinal flagellum emerging from a point close to the cingulum; (iv) cell division taking place in the motile phase; and (v) possessing body scales. This is the third species of this genus to possess body scales. The body scales of A. cupulatisquama are uniform and cup‐shaped in side view and elliptical in face view. Their dimensions are 136.4 nm by 91.0 nm by 81.8 nm high. In side view, the scale is seen to have a thick lower half and a thin upper half. This scale type is very different from those of previously reported Amphidinium species (HG114 and HG115). The molecular tree indicated that A. cupulatisquama and the two other strains of body scale‐bearing Amphidinium are distantly related within the Amphidinium clade.     

Transferring the freshwater dinoflagellate Peridinium wisconsinense (Dinophyceae) to the family Thoracosphaeraceae,with the description of Fusiperidinium gen. nov.

Distinctive spindle‐shaped thecae first described by Samuel Eddy in 1930 and assigned to the genus Peridinium Ehrenberg are commonly reported from freshwater environments in eastern North America. We demonstrate that thecae incubated from cysts of Peridinium wisconsinense Eddy have six cingular plates and a protuberant apical pore complex characteristic of the family Thoracosphaeraceae Schiller 1930 emend. Tangen in Tangen et al . 1982. Small subunit ribosomal DNA (SSU rDNA) and internal transcribed spacer (ITS) sequences confirm the close genetic similarity with Chimonodinium lomnickii (Wo?oszyńska) Craveiro, Calado, Daugbjerg, Gert Hansen & Moestrup and with species recently reassigned to the genus Apocalathium Craveiro, Daugbjerg, Moestrup & Calado that was inferred from previously published LSU rDNA analysis of cysts of P. wisconsinense . Despite sharing identical tabulation with the thoracosphaeracean genera Chimonodinium Craveiro, Calado, Daugbjerg, Gert Hansen & Moestrup and Apocalathium , substantial morphological differences in the morphology of both the thecate and cyst stages of P. wisconsinense led us to reassign this species to the genus Fusiperidinium gen. nov. The phylogenetic position of Fusiperidinium wisconsinense comb. nov., inferred from concatenated data of SSU and LSU sequences, suggests that it evolved from the brackish Scrippsiella lineage, independently of the transition that produced the family Peridiniaceae. Cysts described as Geiselodinium tyonekensis Engelhardt from nonmarine strata from Alaska are apparently identical to the resistant cysts produced by F. wisconsinense . The palynologically‐constrained late Middle Miocene age for the Tyonek Formation provides a minimum age of 11.6 Ma for the evolution of this lineage, coinciding with a rapid glacioeustatic decline in sea level. Our findings also call into question the inclusion of the family Thoracosphaeraceae within the order Peridiniales Haeckel.     

Morphology,ultrastructure and molecular phylogeny of cyst-producing Caladoa arcachonensis gen. et sp. nov. (Peridiniales,Dinophyceae) from France and Indonesia

The dinoflagellate order Peridiniales encompasses several well circumscribed families. However, the family level of some genera, such as Bysmatrum and Vulcanodinium, has remained elusive for many years. Four Peridinium-like strains were established from the Atlantic coast of France and North Sulawesi, Indonesia through cyst germination or isolation of single cells. The cyst-theca relationship was established on specimens from the French Atlantic. Their morphologies were examined using light, scanning and transmission electron microscopy. The cells were characterized by a much larger epitheca relative to the hypotheca, a large anterior sulcal (Sa) plate deeply intruding the epitheca and a small first anterior intercalary plate. The plate formula was identified as Po, cp, X, 4′, 3a, 7′′, 6C, 5S, 5′′′, 2′′′′, shared by Apocalathium, Chimonodinium, Fusiperidinium and Scrippsiella of the family Thoracosphaeraceae but the configuration of Sa plate and anterior intercalary plates is different. Transmission electron microscopy showed that the eyespot was located within a chloroplast comprising two rows of lipid globules and thus belongs to type A. All four strains were classified within a new genus Caladoa as C. arcachonensis gen. et sp. nov. Small subunit ribosomal DNA (SSU rDNA), partial large subunit ribosomal DNA (LSU rDNA) and internal transcribed spacer ribosomal DNA (ITS rDNA) sequences were obtained from all strains. Genetic distance based on ITS rDNA sequences between French and Indonesian strains reached 0.17, suggesting cryptic speciation in C. arcachonensis. The maximum likelihood and Bayesian inference analysis based on concatenated data from SSU and LSU rDNA sequences revealed that Caladoa is monophyletic and closest to Bysmatrum. Our results supported that Caladoa and Bysmatrum are members of the order Peridiniales but their family level remains to be determined. Our results also support that Vulcanodinium is closest to the family Peridiniaceae.     

Laciniporus arabicus gen. et sp. nov. (Dinophyceae,Peridiniales), a new thecate,marine, sand‐dwelling dinoflagellate from the northern Indian Ocean (Arabian Sea)1

A new thecate, photosynthetic, sand‐dwelling marine dinoflagellate, Laciniporus arabicus gen. et sp. nov., is described from the subtidal sediments of the Omani coast in the Arabian Sea, northern Indian Ocean, based on detailed morphological and molecular data. Cells of L. arabicus are small (16.2–30.1 μm long and 13.1–23.2 μm wide), dorsoventrally compressed, with a small apical flap‐shaped projection pointing to the left. The thecal plate pattern is distinguished by minute first precingular plate and sulcus, which extends into the epitheca, with large anterior and right sulcal plates. The Kofoidian thecal tabulation is Po, X, 4′, 2a, 7′′, 6c, 6s, 5′′′, 2′′′′. Morphologically, the revealed plate pattern has an affinity to the Peridiniales, and LSU rDNA based phylogenetic analyses placed L. arabicus within the Thoracosphaeraceae, close to calcareous‐cyst producing scrippsielloids, predatory pfiesteriaceans, and photosynthetic freshwater peridinioids Chimonodinium lomnickii and Apocalathium spp. However, the thecal plate arrangement of L. arabicus differs noticeably from any currently described dinoflagellates, and the species stands out from closely related taxa by extensive differences in physiology and ecology.     

Ultrastructure and molecular phylogenetic position of a new marine sand-dwelling dinoflagellate from British Columbia,Canada: Pseudadenoides polypyrenoides sp. nov. (Dinophyceae)

Two monospecific genera of marine benthic dinoflagellates, Adenoides and Pseudadenoides, have unusual thecal tabulation patterns (lack of cingular plates in the former; and no precingular plates and a complete posterior intercalary plate series in the latter) and are thus difficult to place within a phylogenetic framework. Although both genera share morphological similarities, they have not formed sister taxa in previous molecular phylogenetic analyses. We discovered and characterized a new species of Pseudadenoides, P. polypyrenoides sp. nov., at both the ultrastructural and molecular phylogenetic levels. Molecular phylogenetic analyses of SSU and LSU rDNA sequences demonstrated a close relationship between P. polypyrenoides sp. nov. and Pseudadenoides kofoidii, and Adenoides and Pseudadenoides formed sister taxa in phylogenetic trees inferred from LSU rDNA sequences. Comparisons of morphological traits, such as the apical pore complex (APC), demonstrated similarities between Adenoides, Pseudadenoides and several planktonic genera (e.g. Heterocapsa, Azadinium and Amphidoma). Molecular phylogenetic analyses of SSU and LSU rDNA sequences also demonstrated an undescribed species within Adenoides.     

Pyramidodinium atrofuscum gen. et sp. nov. (Dinophyceae), a new marine sand-dwelling coccoid dinoflagellate from tropical waters

A new marine sand‐dwelling coccoid dinoflagellate Pyramidodinium atrofuscum Horiguchi et Sukigara gen. et sp. nov. is described from Jellyfish Lake, Republic of Palau. The dinoflagellate alternates a non‐motile vegetative stage with a motile gymnodinioid stage within its life cycle. The non‐motile stage is dominant in the life cycle and the dinoflagellate reproduces itself by means of the production of two motile cells. The released motile cell swims only for a short period and is directly transformed into the non‐motile cell. The non‐motile cell is sessile, pyramidal in shape, with a single longitudinal ridge and a double transverse ridge. The surface of the cell wall is covered with many processes. The motile cell has a Gymnodinium‐like morphology, but no apical groove is present. An ultrastructural study revealed that the dinoflagellate possesses typical dinoflagellate organelles. Based on the unique morphology of the vegetative non‐motile stage, we propose a new genus Pyramidodinium for this dinoflagellate, with the type species Pyramidodinium atrofuscum Horiguchi et Sukigara, gen. et sp. nov.     

Serial replacement of a diatom endosymbiont in the marine dinoflagellate Peridinium quinquecorne (Peridiniales, Dinophyceae)

To infer the phylogeny of both the host and the endosymbiont of Peridinium quinquecorne Abé, the small subunit (SSU) ribosomal DNA (rDNA) from the host and two genes of endosymbiont origin (plastid‐encoded rbcL and nuclear‐encoded SSU rDNA) were determined. The phylogenetic analysis of the host revealed that the marine dinoflagellate P. quinquecorne formed a clade with other diatom‐harbouring dinoflagellates, including Kryptoperidinium foliaceum (Stein) Lindeman, Durinskia baltica (Levander) Carty et Cox and Galeidinium rugatum Tamura et Horiguchi, indicating a single endosymbiotic event for this lineage. Phylogenetic analyses of the endosymbiont in these organisms revealed that the endosymbiont of P. quinquecorne formed a clade with a centric diatom (SSU data indicated it to be closely related to Chaetoceros), whereas the endosymbionts of other three dinoflagellates formed a clade with a pennate diatom. The discrepancy between the host and the endosymbiont phylogenies suggests a secondary replacement of the endosymbiont from a pennate to a centric diatom in P. quinquecorne.     

Roscoffia minor sp. nov. (Peridiniales, Dinophyceae): A new, sand-dwelling, armored dinoflagellate from Hokkaido, Japan

A new, sand-dwelling, armored dinoflagellate, Roscoffia minor sp. nov., is described from Ishikari beach, Hokkaido, Japan. The dinoflagellate has been collected from sand samples taken both near the water's edge and further upshore (25 m from the water's edge at a depth of 1 m), indicating that it is a true sand-dwelling species. Roscoffia minor is heterotrophic and lacks both a chloroplast and an eye-spot. The cell consists of a flattened cap-shaped epitheca and a large hemispheroidal hypotheca, and it is quite different from cells of the typical armored dinoflagellates. The thecal plate formula is: Po, 3′, la, 5″, 3c, 3s, 5″, 1″″. Its distinct cell shape and the thecal plate arrangement indicate affinity to the monotypic genus Roscoffia. Roscoffia minor is distinguished from Roscoffia capitata, the type species, by its smaller size and the possession of a finger-like apical projection. The thecal arrangement of the epitheca is similar to those of the members of the family Podolampaceae, while the hypothecal arrangement is the same as that of members of the subfamily Diplopsalioideae (family Congruentidiaceae). The organism seems to be positioned somewhere intermediate between these two families, but the family to which this dinoflagellate should be affiliated could not be determined.     

Cyst-theca relationship and phylogenetic positions of Scrippsiella plana sp. nov. and S. spinifera (Peridiniales,Dinophyceae)

Species belonging to the dinophyte genus Scrippsiella are frequently reported in marine waters, but information on their distribution in brackish environments is limited. Here we describe a new species, S. plana, through incubation of non-calcified cysts from sediments collected in the South China Sea and Caspian Sea. The vegetative cells consist of a conical epitheca and a rounded hypotheca with the plate formula of Po, X, 4′, 3a, 7′′, 5C+t, 5S, 5′′′, 2′′′′. It differs from other Scrippsiella species by its flattened body in dorsoventral view and a small first anterior intercalary (1a) plate (half the size of plate 3a). Scrippsiella plana strains from the South China Sea and Caspian Sea share identical internal transcribed spacer (ITS) sequences, and show phenotypic plasticity and local adaptation in growth rate at various salinities, consistent with the environments in which they originated. In addition, two strains of S. spinifera were obtained by incubating ellipsoid cysts with calcareous spines from sediments collected along the Turkish and Hawaiian coast. They also share identical ITS sequences and differ from Duboscquodinium collinii (a parasite of tintinnids) only at two base pair positions (in the ITS2 region). Molecular phylogeny based on ITS and large subunit ribosomal DNA (LSU rDNA) sequences revealed that S. plana was nested within the Calciodinellum (CAL) clade and S. spinifera within the S. trochoidea (STR) clade. The phylogenetic position of ‘Peridinium’ wisconsinense is reported for the first time, which supports multiple transitions of the Peridiniales to freshwater.     

Bispinodinium angelaceum gen. et sp. nov. (Dinophyceae), a new sand‐dwelling dinoflagellate from the seafloor off Mageshima Island,Japan1

A new athecate dinoflagellate, Bispinodinium angelaceum N. Yamada et Horiguchi gen. et sp. nov., is described from a sand sample collected on the seafloor at a depth of 36 m off Mageshima Island, subtropical Japan. The dinoflagellate is dorsiventrally compressed and axi‐symmetric along the sulcus. The morphology resembles that of the genus Amphidinium sensu lato by having a small epicone that is less than one third of the total cell length. However, it has a new type of apical groove, the path of which traces the outline of a magnifying glass. The circular component of this path forms a complete circle in the center of the epicone and the straight “handle” runs from the sulcus to the circular component. Inside the cell, a pair of elongated fibrous structure termed here the “spinoid apparatus” extends from just beneath the circular apical groove to a point near the nucleus. Each of two paired structures consists of at least 10 hyaline fibers and this is a novel structure found in dinoflagellates. Phylogenetic analyses based on the SSU and LSU RNA genes did not show any high bootstrap affinities with currently known athecate dinoflagellates. On the basis of its novel morphological features and molecular signal, we conclude that this dinoflagellate should be described as a new species belonging to a new genus.