PHYLOGENETIC POSITION OF TOXARIUM,A PENNATE‐LIKE LINEAGE WITHIN CENTRIC DIATOMS (BACILLARIOPHYCEAE)1

The diatom genus Toxarium Bailey has been treated as a pennate because of its elongate shape and benthic lifestyle (it grows attached to solid substrata in the marine sublittoral). Yet its valve face lacks all structures that would ally it with the pennates, such as apical labiate processes, a midrib (sternum) subtending secondary ribs and rows of pores extending perpendicularly out from the midrib, or a raphe system. Instead, pores are scattered irregularly over the valve face and only form two distinct rows along the perimeter of the valve face. In our nuclear small subunit rDNA phylogenies, Toxarium groups with bi‐ and multipolar centrics, as sister to Lampriscus A. Schmidt. Thus, the genus acquired a pennate‐like shape and lifestyle independently from that of the true pennates. The two species known, T. hennedyanum Grunow and T. undulatum Bailey, differ only in a single feature: the valve perimeter of the former shows only a central expansion, whereas that of the latter possesses in addition a regular undulation. Yet both forms were observed in our monoclonal cultures, indicating that the two taxa represent extremes in a plasticity range. Toxarium resembles another elongate and supposedly araphid diatom, Ardissonea De Notaris, in being motile. Cells can move at speeds of up to 4 μm·s ^{? 1} 1 Received 7 June 2002. Accepted 4 October 2002. through secretion of mucilage from the cell poles or they remain stationary for longer periods, when they form short polysaccharide stalks. Division during longer periods of quiescence leads to the formation of small colonies of linked or radiating cells.     

THE EVOLUTION OF ELONGATE SHAPE IN DIATOMS1

Diatoms have been classified historically as either centric or pennate based on a number of features, cell outline foremost among them. The consensus among nearly every estimate of the diatom phylogeny is that the traditional pennate diatoms (Pennales) constitute a well‐supported clade, whereas centric diatoms do not. The problem with the centric–pennate classification was highlighted by some recent analyses concerning the phylogenetic position of Toxarium, whereby it was concluded that this “centric” diatom independently evolved several pennate‐like characters including an elongate, pennate‐like cell outline. We performed several phylogenetic analyses to test the hypothesis that Toxarium evolved its elongate shape independently from Pennales. First, we reanalyzed the original data set used to infer the phylogenetic position of Toxarium and found that a more thorough heuristic search was necessary to find the optimal tree. Second, we aligned 181 diatom and eight outgroup SSU rDNA sequences to maximize the juxtapositioning of similar primary and secondary structure of the 18S rRNA molecule over a much broader sampling of diatoms. We then performed a number of phylogenetic analyses purposely based on disparate sets of assumptions and found that none of these analyses supported the conclusion that Toxarium acquired its pennate‐like outline independently from Pennales. Our results suggest that elongate outline is congruent with SSU rDNA data and may be synapomorphic for a larger, more inclusive clade than the traditional Pennales.     

Morphology of four plagiogrammacean diatoms; Dimeregramma minor var. nana,Neofragilaria nicobarica,Plagiogramma atomus and Psammogramma vigoensis gen. et sp. nov., and their phylogenetic relationship inferred from partial large subunit rDNA

A marine araphid pennate diatom obtained from sand grains sampled at two sites at Vigo, Spain is described as Psammogramma vigoensis S. Sato et Medlin, a member of the family Plagiogrammaceae, based on observations of frustule fine structure. The species possesses elongated valves with apical pore fields and parallel rows of striae oriented perpendicular to the apical axis. Valve poroids are occluded by perforated rotae. There is a weak sternum along the apical axis and rimoportulae are absent. Detailed observations of Dimeregramma minor var. nana (Gregory) Ralfs, Neofragilaria nicobarica Desikachary, Prasad et Prema and Plagiogramma atomus Greville, were also undertaken. Based on morphological features, the transfer of Neofragilaria nicobarica from the Fragilariaceae to the Plagiogrammaceae is proposed. Partial large subunit rDNA phylogeny also supported inclusion of both Psammogramma and Neofragilaria in Plagiogrammaceae.     

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.     

A Study of Two Morphological Variants of the Diatom Fragilaria crotonensis Kitton using Electron Microscopy

Two forms of Fragilaria crotonensis Kitton, one with flaredends to the cells, the other with ends that taper to a narrowrod-like tip, were previously distinguished by light microscopyand by their contrasting relationships with the parasitic chytridsRhizophydium fragilariae Canter and one referred to as species3. Examples of the above diatom types, taken from a number ofgeographically widely separated lakes, have been examined byelectron microscopy. Additional morphological differences aredescribed. The ocellus in flared-type cells consists of 4–5rows of pores while in rod-type cells fewer (2–3) rowsof pores are present. Valve thickenings on the mantle at theapex of rod-type cells are rarely visible whereas in flared-typecells they are usually very prominent. A rimoportula was presentin both cell types. In flared cells its external aperture liesimmediately behind the apex usually in line with the striaeand the internal aperture is often orientated with its longaxis oblique to that of the diatom cell. The rimoportula inrod-type cells opens at the centre of the valve face and theinternal aperture is orientated at 90° to the apical axisof the valve. Comparison made with type material suggests thatthe rod-form should be recognized as F. crotonensis var. crotonensisand the flared-form as var. prolongata Grunow ex van Heurck. Fragilaria crotonensis, diatom, systematics, chytrid infection, rimoportula     

MATING SYSTEM,SEXUAL REPRODUCTION,AND AUXOSPORULATION IN THE ANOMALOUS RAPHID DIATOM EUNOTIA (BACILLARIOPHYTA)1

The diatom genus Eunotia is unusual among raphid diatoms in having a raphe system consisting of two short slits that are not integrated into the primary pattern center. This and other characteristics, particularly the presence of rimoportulae, are consistent with the hypothesis that Eunotia is a basal lineage within the raphid group. We studied auxosporulation in E. bilunaris (Ehrenberg) Mills and E. tropica Hustedt for comparison with other raphid pennate diatoms and with araphid pennates; E. bilunaris was studied in parental and F1 generations. Like araphid pennates, E. bilunaris and E. tropica are heterothallic. Clones of the same mating type did not interact sexually, and intraclonal sexual reproduction was absent or very rare. Clones retained the same sex throughout the life cycle, as shown by experiments using abrupt size reduction to produce clones of similar age but different size and using subclones derived from a single initial cell within six mitotic generations. Unlike in araphid pennate diatoms, in the Eunotia species the gametes are not visibly or behaviorally differentiated. Gametogenesis is merogenous, because the gametangium formed a supernumerary cell as well as a single gametic cell, both undergoing meiosis II to form a surviving functional nucleus and a nucleus that quickly degenerated. Plasmogamy is via papillae that grew out toward each other from the ends of the gametangia to create a copulation canal. After plasmogamy, the gametes moves bodily into the copulation canal, producing an elongate zygote, which expands to form a curved sausage‐like auxospore.     

OOGAMOUS REPRODUCTION,WITH TWO‐STEP AUXOSPORULATION,IN THE CENTRIC DIATOM THALASSIOSIRA PUNCTIGERA (BACILLARIOPHYTA)1

Thalassiosira species are common components of marine planktonic communities worldwide and are used intensively as model experimental organisms. However, data on life cycles and sexuality within the genus are fragmentary. A clone of the cosmopolitan marine diatom Thalassiosira punctigera Cleve emend. Hasle was isolated from the North Sea and oogamous sexual reproduction was observed in culture. Cells approximately 45 μm and smaller became sexualized. Oogonia were produced preferentially and spermatogenesis was infrequent. Unfertilized oogonia always aborted and their development was apparently arrested at prophase of meiosis I. Further progression through meiosis and auxospore formation occurred only after a sperm had penetrated into the oocyte. Many cells of the new large‐celled generation (approximately 90–120 μm in size) immediately became sexualized again but only oogonia were produced. A few of the large oogonia became auxospores and produced initial cells 132–153 μm in diameter. The second step of auxosporulation probably involved fertilization of large‐celled oocytes by the sperm of the small‐celled spermatogonangia that were still present in the culture. An F₁ clone obtained after selfing within the small‐celled auxosporulation size range was investigated. Like the parent clone, the F₁ clone was homothallic but no auxosporulation was observed: spermatogonangia were unable to produce viable sperm, apparently because of inbreeding depression. Aggregation and interaction of oogonia were documented, and may be relevant for understanding the mechanisms of signaling and recognition between sexualized cells and the evolution of sexuality in pennate diatoms.     

AUXOSPORE FORMATION AND THE MORPHOLOGY OF THE INITIAL CELL OF THE MARINE ARAPHID DIATOM GEPHYRIA MEDIA (BACILLARIOPHYCEAE)1

Recent studies have led to a rapid increase in knowledge of auxospore formation in diatoms. However, these studies have been limited to centric and raphid pennate diatoms, and there is still very little information for the araphid pennate diatoms. Using LM and SEM, we studied the development of the auxospore and the initial cell of the marine epiphytic diatom Gephyria media Arnott. Auxospores were bipolar and curved in side view, as in many other pennate diatoms. SEM revealed many transverse perizonial bands, all of which were incomplete rings. There was an elongate, sprawling, silicified structure beneath the ventral suture of the transverse perizonial bands. This structure is presumably equivalent to the longitudinal perizonial band in other pennate diatoms, although we could not determine the homologous relationship between the two features. Scales were found both in the inner wall of the perizonium and around the primary perizonial bands. The presence or absence of scales may be of phylogenetic significance in diatoms, only during the final stages of auxospore formation because scales are found in early spherical stages. The distinctive finger‐like structures observed throughout all stage of G. media have not been observed before in the other diatom taxa.     

Molecular Phylogeny and Taxonomy of the Genus Minidiscus (Bacillariophyceae), with Description of Mediolabrus gen. nov

The genus Minidiscus comprises a group of ecologically-important and globally-distributed planktonic diatoms that are characterized by their small cell size, high mantle and processes more or less concentrated in the valve center. Monoclonal strains were established from collections along the Chinese coast. In the phylogenetic analyses inferred from a LSU and SSU dataset, six Minidiscus species clustered into two well-supported clades. The first clade was located within a larger clade formed mainly by Thalassiosira taxa, and the second clade appeared as sister to a clade comprising the genus Skeletonema. Hence, presently known Minidiscus do not form a monophyletic clade, but rather make up a phenotypic grouping. Based on the morphology of the type species, M. trioculatus, as well as morphological characters of all taxa in the clade with M. trioculatus, Minidiscus is characterized by having fultoportula(e) in the valve center or sub-centered close to a single rimoportula, and the cells are usually cylindrical. Mediolabrus gen. nov. is proposed to accommodate species in the second clade. The main difference between Minidiscus and Mediolabrus is the type of process found in the valve (sub-)center, with fultoportula(e) close to a rimoportula in the former, and only a single rimoportula in the latter. According to the above criteria, previously described Minidiscus taxa were re-examined, and either retained in Minidiscus or transferred to Mediolabrus.     

Auxosporulation of Licmophora communis (Bacillariophyta) and a review of mating systems and sexual reproduction in araphid pennate diatoms

The auxosporulation of Licmophora communis is allogamous and dioecious. Pairing between sessile, shortstalked cells of compatible clones is followed by meiosis and gametogenesis, to form two gametes in each gametangium. The behavior of the gametes differs between the gametangia. In the male gametangium, the gametes detach from the frustule, round up, and migrate out of the gametangium after its dehiscence at the broader, unattached pole. In the female gametangium, both gametes remain attached to the adjacent theca over almost their whole length and do not move. Plasmogamy therefore occurs within the female gametangium and this is where the zygotes are formed and remain. After fertilization, the zygotes detach from the thecae of the female gametangia, contract, and become ellipsoidal, before expanding parallel to the apical axis of the gametangium. We review the types of auxosporulation in other pennate diatoms and the systems used for classifying these. Dioecy and cis‐type anisogamy (in which one gametangium produces active gametes and the other produces passive gametes), as in L. communis, are probably primitive within the pennate group (although there is no information on the AsterionellopsisRhaphoneis clade). However, size can also be restored in various araphid pennates by allogamous sexual reproduction involving the formation of only one gamete per gametangium, or in rare cases by automixis or (apparently) vegetative enlargement.     

MORPHOLOGY OF THE DIATOM GENUS CAMPYLONEIS (BACILLARIOPHYCEAE,BACILLARIOPHYTA), WITH A DESCRIPTION OF CAMPYLONEIS JULIAE SP. NOV. AND AN EVALUATION OF THE FUNCTION OF THE VALVOCOPULAE1

A revision of the monoraphid pennate diatom genus Campyloneis Grunow was carried out based on LM and EM observations. The material examined originated from various herbarium collections and from extant epiphytic diatom communities on leaves of Posidonia spp. We also examined the generitype C. grevillei (Smith) Grunow and the fossil material of C. gheyselinchi Reinhold from which the author extracted the type. Our results clarified the fine structure of C. grevillei and C. gheyselinchi. Of the various varieties of C. grevillei, only the variety argus (Grunow) Cleve was retained. This differs from the nominate variety in the arrangement and shape of the areolae adjacent to the sternum of the araphid valve. The newly described taxon Campyloneis juliae De Stefano differs from all Campyloneis species in areolae ultrastructure and morphology of the valvocopulae. As for the fossil species C. gheyselinchi, the sternum valve areolae are similar to those of C. grevillei, but scarcity of frustules in the type material prohibited evaluation of its variability. For this reason we provisionally maintained its rank of species. The elaborate linking systems among the valvocopulae and valves in Campyloneis species appear to provide structural reinforcement against pressure from neighboring epiphytic diatoms and scouring of seagrass leaves.     

Morphology and molecular phylogeny of Thalassiosira sinica sp. nov. (Bacillariophyta) with delicate areolae and fultoportulae pattern

Thalassiosira Cleve is one of the most species-rich marine diatom genera. Previous studies have mainly focused on polar and temperate areas, but recent studies on material from Asian waters suggested that a high and undescribed species diversity of Thalassiosira occurs in Asia. On the basis of plankton samples collected from the South China Sea, a new species, T. sinica sp. nov. Y. Li & Y. Q. Guo is described. The morphology of the cells was examined by light and scanning electron microscopy. The hypervariable region of the nuclear large-subunit ribosomal DNA and the relatively conserved region of the nuclear small-subunit ribosomal DNA were sequenced for information on phylogenetic relationships. The living cells are usually solitary and drum-shaped. The areolae on the valve are delicate, small and arranged in fascicles. In addition to a regular ring of marginal fultoportulae on the valve edge, T. sinica possesses one central fultoportula and a number of fultoportulae arranged into 2–3 irregular rings on the valve face. A rimoportula located inside the ring of marginal fultoportulae possesses a long and strong external tube. The valvocopula and the copulae have rows of pores, but the pores on the valvocopula are larger than those on the copulae. Thalassiosira sinica appears to be included in subgroup C sensu Gedde because of a rimoportula with a distinct external tube located on the valve face. The molecular phylogeny, inferred from both SSU and LSU sequences, does, however, not support the validity of subgroup C, as the closest allies of T. sinica here turned out to be T. diporocyclus and T. lundiana, species in which the rimoportulae are located on the valve margin.     

Valve morphogenesis in an araphid diatom Rhaphoneis amphiceros (Rhaphoneidaceae,Bacillariophyta)

The pattern centre in valve morphogenesis is an annulus in centric diatoms or a sternum in pennate diatoms. The genus Rhaphoneis is currently placed within a lineage that diverges at the root of the pennate diatom clade in most molecular phylogenies, and its valves have a unique pattern to their striae, i.e. radiating from both apices, giving the impression that a pattern centre exists at both ends of the valve and virgae (ribs) formation proceeds centripetally. The present study, however, shows that the pattern centre is actually a linear sternum and the formation of virgae proceeds centrifugally, a pattern centre that is commonly found in most araphid diatoms. Thus, the hypothesis that valve morphogenesis based on a linear sternum and perpendicular virgae is a synapomorphy of pennate diatoms is supported. Our study also demonstrates that the pattern of valve formation can be observed by light microscopy with a direct mounting method when the specimen is relatively large, i.e. exceeding approximately 50 µm in valve length. An important advantage of the use of the direct mounting method is that it requires no repeated centrifugation steps for dehydration, steps necessary for observation by a scanning electron microscope, causing the loss and/or collapse of the specimen, particularly with fragile valves in the early stages of development.     

GALEIDINIIUM RUGATUM GEN. ET SP. NOV. (DINOPHYCEAE), A NEW COCCOID DINOFLAGELLATE WITH A DIATOM ENDOSYMBIONT1

A new sand‐dwelling dinoflagellate from Palau, Galeidinium rugatum Tamura et Horiguchi gen. et sp. nov., is described. The life cycle of this new alga consists of a dominant nonmotile phase and a brief motile phase. The motile cell transforms itself directly into the nonmotile cell after swimming for a short period, and cell division takes place in the nonmotile phase. The nonmotile cell possesses a dome‐like cell covering, which is wrinkled and equipped with a transverse groove on the surface. The cell has 10–20 chloroplasts and a distinct eyespot. The motile cell is Gymnodinium‐like in shape. The dinoflagellate possesses an endosymbiotic alga to which the chloroplasts belong and which is separated from the host (dinoflagellate) cytoplasm by a unit membrane. The endosymbiont cytoplasm also possesses its own eukaryotic nucleus and mitochondria. The eyespot is surrounded by triple membranes and is located in the host cytoplasm. Photosynthetic pigment analysis, using HPLC, revealed that G. rugatum possesses fucoxanthin as the principal accessory pigment instead of peridinin. The rbcL tree showed that G. rugatum is monophyletic with Durinskia baltica (Levander) Carty et Cox and Kryptoperidinium foliaceum (Stein) Lindemann and that this clade is closely related to the pennate diatom, Cylindrotheca sp. The endosymbiont of G. rugatum is therefore shown to be a diatom. Phylogenetic analysis based on small subunit rDNA sequences demonstrated that G. rugatum, D. baltica, and K. foliaceum, all of which are known to harbor an endosymbiont of diatom origin, are closely related.     

MORPHOGENESIS OF THE LABIATE PROCESS IN THE ARAPHID PENNATE DIATOM DIATOM A VULGARE1

Each valve of the araphid pennate diatom Diatoma has a labiate process (LP) at one end; in a frustule, the LPs are at diagonally opposite ends. After mitosis is over, an elongated dense body detaches from the spindle pole and migrates to one end of the daughter cell, always diagonally opposite the LP of the parental valve. This dense body trails a cone-shaped array of microtubules (MTs). Meanwhile, the new valve has begun to form within the Silica Deposition Vesicle (SDV). Having reached the end of the cell, this dense body moves back slightly and then settles onto the SDV, developing a layered substructure as it does so. Immediately beneath it, the LP of the new daughter valve differentiates. This dense object is clearly the homologue of the fibrous Labiate Process Apparatus (LPA) involved in the differentiation of the LP in several centric diatoms. In a few cases, these LPAs also hair been shown to originate from some component of the spindle pole. Thus, the homologue of the LPA of centric diatoms has now been found in an araphid pennate diatom; in each case, the LPA apparently comes from the pole of the spindle and presumably uses a cytoskeleton of MTs to locate the LP in its correct position. These observations support the possibility that the raphe evolved from the LP.     

Description of Conticribra tricircularis,a new genus and species of Thalassiosirales,with a discussion on its relationship to other continuous cribra species of Thalassiosira Cleve (Bacillariophyta) and its freshwater origin

A new diatom genus Conticribra is erected to accommodate C. tricircularis, described from a freshwater Pliocene deposit in Trout Creek, Oregon (USA). The genus accommodates species possessing: (i) loculate areolae with (semi-) continuous cribra; (ii) non-plicated valve face; (iii) rimoportula located on the valve mantle, replacing a fultoportula. Conticribra tricircularis has no valve face fultoportulae and can easily be distinguished by its marginal fultoportulae with four satellite pores arranged in three rings. Three species are transferred to the new genus from Thalassiosira sensu lato. Using evidence from the fossil record and recent molecular data, a hypothesis concerning the freshwater origin of Conticribra is discussed.     

MORPHOLOGY AND MOLECULAR PHYLOGENY OF PROROCENTRUM CONSUTUM SP. NOV. (DINOPHYCEAE), A NEW BENTHIC DINOFLAGELLATE FROM SOUTH BRITTANY (NORTHWESTERN FRANCE)1

A new marine benthic Prorocentrum species from sandy habitats of South Brittany (northwestern France), P. consutum sp. nov., is described using LM and SEM and molecular phylogenetic analyses. Cells have a subcircular to broadly ovoid shape and are plainly flattened. They are 57–61 μm long and 52–55 μm wide. A central pyrenoid is present, and the kidney‐shaped nucleus is positioned in the posterior region. In right valve view, the periflagellar area is deeply excavated, and the left valve forms a prominent apical ridge. The periflagellar area consists of nine platelets, and a small narrow collar is present around the flagellar pore. The ornamentation of this new species is very peculiar and is characterized by a ring of round areolae located at the periphery of the valves, each areola containing three or four pores. Apart from this ring of areolae, the cell surface is smooth and with scattered pores. Pores are not present in the center of the right or left valve. The intercalary band is generally narrow and faintly striated horizontally. The molecular phylogenetic position of P. consutum sp. nov. was inferred using SSU and LSU rDNA. In both analyses, this species branched with high support in the clade comprising species with a symmetric shape and appeared to be a sister group to that formed by P. lima and other tropical benthic species, such as P. arenarium, P. belizeanum, P. hoffmannianum, and P. maculosum.     

Nanoscale uniformity of pore architecture in diatomaceous silica: a combined small and wide angle x-ray scattering study

Combined small and wide angle X‐ray scattering (SAXS and WAXS) analysis was applied to purified biogenic silica of cultured diatom frustules and of natural populations sampled on marine tidal flats. The overall WAXS patterns did not reveal crystalline phases (WAXS domain between 0.07 to 0.5 nm) in this biogenic silica, which is in line with previous reports on the amorphous character of the SiO₂ matrix of diatom frustules. One exception was the silica of the pennate species Cylindrotheca fusiformis Reimann et Lewin, which revealed wide peaks in the WAXS spectra. These peaks either indicate the presence of a yet unknown crystalline phase with a repetitive distance (d‐value ≈0.06 nm) or are caused by the ordering of the fibrous silica fragments; numerous girdle bands. The SAXS spectra revealed the size range of pores (diameter d between 3.0 and 65 nm), the presence of distinct pores (slope transitions), and structure factors (oscillation of the spectra). All slopes varied in the range of ?4.0 to ?2.5, with two clear common regions among species: d < 10 nm (slopes –4, denoted as region I and also called the Porod region), and 10.0 < d < 40.0 nm (slopes ?2.9 to ?3.8, denoted as region II). The existence of these common regions suggests the presence of comparable form (region I) and structure (region II) factors, respectively the shape of the primary building units of the silica and the geometry of the pores. Contrast variation experiments using dibromomethane to fill pores in the SiO₂ matrix showed that scattering was caused by pores rather than silica particles. Electron microscopic analysis confirmed the presence of circular, elliptical, and rectangular pores ranging in size from 3 to 65 nm, determining the structure factor. The fine architecture (length/width ratio of pore diameters) and distribution of the pores, however, seemed to be influenced by environmental factors, such as the salinity of and additions of AlCl₃ to the growth medium. The results indicate that diatoms deposit silica with pores <50 nm in size and are highly homologous with respect to geometry. Consequently, it is suggested that in diatoms, whether pennate or centric, the formation of silica at a nanoscale level is a uniform process.     

Morphology and molecular phylogeny of epizoic araphid diatoms on marine zooplankton,including Pseudofalcula hyalina gen. & comb. nov. (Fragilariophyceae,Bacillariophyta)

Some diatoms are able to colonize as epibionts on their potential zooplankton predators. Here, we report Pseudohimantidium pacificum living on the copepod Corycaeus giesbrechti and as a new finding on Oithona nana, Protoraphis atlantica living on the copepod Pontellopsis brevis, Protoraphis hustedtiana on the cypris larvae of barnacles, and Falcula hyalina on the copepod Acartia lilljeborgii. The epizoic diatoms were able to grow as free‐living forms under culture conditions. Pseudohimantidium pacificum and P. atlantica appeared as the most derived species from their benthic diatom ancestors. The mucilage pad or stalk of the strains of these species showed important morphological distinction when compared with their epizoic forms. Barnacle larvae explore benthic habitats before settlement, and epibiosis on them is an example where P. hustedtiana profits from the host behavior for dispersal of its benthic populations. Molecular phylogenies based on the SSU rRNA and RuBisCO large subunit (rbcL) gene sequences revealed F. hyalina as an independent lineage within the Fragilariales (Tabularia, Catacombas, and others), consistent with its morphological distinction in the low number of rows (≤6) in the ocellulimbus, among other features. We propose the transfer of F. hyalina to the genus Pseudofalcula gen. nov. Molecular phylogeny suggests a single order for the members of the Cyclophorales and the Protoraphidales, and that the epibioses of araphid diatoms on marine zooplankton have been independently acquired several times. These clades are constituted of both epizoic and epiphytic/epilithic forms that evidence a recent acquisition of the epizoic modus vivendi.