Oligotrich Ciliates Research Articles

Relating ciliary propulsion morphology and flow to particle acquisition in marine planktonic ciliates II: the oligotrich ciliate Strombidium capitatum

Abstract The marine oligotrich ciliate Strombidium capitatum is a cruise-feeder, relying on ciliary motion and propulsion flow to individually detect and capture particles. High-speed, high-magnification digital imaging revealed that the cell swims forward by sweeping its anterior adoral membranelles (AAMs) backward, achieving a mean path-averaged speed of U = 1.7 mm s−1 (31 cell-lengths per second). Particle detection occurs through either hydrodynamic signal perception or ciliary contact perception, with a mean reaction distance of R = 20.4 μm. While executing a ciliary reversal of AAMs to handle and capture a perceived particle, the cell coordinates the ciliary motion of ventral adoral membranelles (VAMs, the “lapel”) with the ciliary reversal of AAMs (the “collar”), causing a sudden halt of cell motion, thereby functioning as a motion “brake” that is crucial for effective particle capture. The encounter rate with small prey particles is calculated using πR2U (~8.0 μL h−1, equivalent to ~ 3.5 × 106 cell volumes per day). Based on hydrodynamic modeling results, it is hypothesized that spatial structures of the flow velocity vector and acceleration fields in front of the swimming cell are essential for pushing an embedded particle forward, creating a strong enough slip velocity and hydrodynamic signal for prey perception, even for a neutrally buoyant small particle.

On Papulifères, putative ciliate cysts of diverse morphologies, with new observations from the plankton of the Chukchi Sea (Arctic Ocean)

In the early 20th century, Alphonse Meunier described "Papulifères" as a group of enigmatic forms of unknown taxonomic affinity characterized by possessing a hyaline pimple, a "papula". In the early 1980's Papulifères were equated with cysts of tintinnid ciliates. The most conspicuous forms, the large Fusopsis, have been widely found, and are now known to resemble the cyst of a certain species of oligotrich ciliate (i.e., Cyrtostrombidium boreale). Thus today, Papulifère forms are often assumed to be cysts of oligotrich ciliates. Here we report on 26 Papulifère forms, of more or less distinct morphologies, found in the plankton of the Chukchi Sea. We found forms resembling some of those described by Meunier, and recorded here for the first time since Meunier's reports, and others that do not resemble any of Meunier's Papulifères. Here, we first review the literature on Papulifères, then we present the surprising variety of forms we found in Chukchi Sea, and for some, we provide for the first time data on morphological variability. With this report we have expanded the catalogue of observed Papulifère forms and documented variability in the dimensions of some morphotypes. However, we urge caution in assigning a ciliate species name to any given Papulifère form in the absence of corroborating data. There is a need for observational and/or sequence-based data to elucidate the identity of Papulifère forms.

Diversity Patterns of Protists Are Highly Affected by Methods Disentangling Biological Variants: A Case Study in Oligotrich (s.l.) Ciliates.

Protists are a dominant group in marine microplankton communities and play important roles in energy flux and nutrient cycling in marine ecosystems. Environmental sequences produced by high-throughput sequencing (HTS) methods are increasingly used for inferring the diversity and distribution patterns of protists. However, studies testing whether methods disentangling biological variants affect the diversity and distribution patterns of protists using field samples are insufficient. Oligotrich (s.l.) ciliates are one group of the abundant and dominant planktonic protists in coastal waters and open oceans. Using oligotrich (s.l.) ciliates in field samples as an example, the present study indicates that DADA2 performs better than SWARM, UNOISE, UPARSE, and UCLUST for inferring diversity patterns of oligotrich (s.l.) ciliates in the Pearl River Estuary and surrounding regions. UPARSE and UNOISE might underestimate species richness. SWARM might not be suitable for the resolution of alpha diversity owing to its rigorous clustering and sensitivity to sequence variations. UCLUST with 99% clustering threshold overestimates species richness, and the beta diversity pattern inferred by DADA2 is more reasonable than that of the other methods. Additionally, salinity is shown to be one of the key factors responsible for variations in the community distribution of ciliates, but infrequent marine–freshwater transitions occurred during evolutionary terms of this group.

Sex in the wild: repeated observations of planktonic ciliate conjugation from field samples.

Ciliate conjugation is considered a rare event to encounter in the field and it is mostly reported from cultures. In this work, we describe a synchronized conjugation event of planktonic ciliates that was discovered twice; in September 2019, at two different locations in the Cretan Sea, Eastern Mediterranean, and in October 2020. In 2019, first, at 2m depth of the coastal station POSEIDON-HCB, in samples fixed with acid Lugol and formaldehyde, we found 340 and 200 mating pairs L-1of different ciliate species, respectively; and second, at the Heraklion port, we found 220 mating pairs L-1 of Strombidinopsis sp. and 1960 mating pairs L-1 of Strombidium sp. At the Heraklion port visited again in 2020, we found 800 mating pairs L-1 of Strombidinopsis sp. and 200 mating pairs L-1 of Strombidium sp. Since detailed descriptions of conjugation in pelagic oligotrich ciliates are missing, our observations indicate that ciliate conjugation could be a frequent and periodic phenomenon, under specific conditions.

Morphological, Molecular, and Growth Characteristics of a Cryptic Species, Strombidium parasulcatum n. sp. (Alveolata: Ciliophora: Oligotrichida).

A new marine planktonic ciliate from Taiwan, Strombidium parasulcatum sp. n., is described based on live observations, protargol staining, and molecular data. Its morphological characters are similar to those of Strombidium sulcatum Claparède and Lachmann, 1859 sensu Song et al., 2000 but differ from S. sulcatum sensu Fauré-Fremiet, 1912 and sensu Fauré-Fremiet and Ganier, 1970 by several morphological characters. The 18S rRNA gene sequences of the two forms display 76 base pair differences (about 5%), indicating that they should be considered separate species. The highest observed specific growth rates of S. parasulcatum in culture were 1.79 day–1 over 1 day and 1.52 day–1 over 2 days, both starting at day 5. Bacteria appear to be an important food resource for the cultivation of this medium-sized oligotrich ciliate. This and other recent studies suggest that cryptic species may be common in the genus Strombidium, and an integrative approach including morphological, ecological, and molecular data should be used to address this question.

Spatial distribution of the protist community in the southern part of the Okhotsk Sea off Hokkaido during summer

The southern part of the Okhotsk Sea off Hokkaido has rich fishery resources supported by high primary production. The phytoplankton community has been described by several studies based on size-fractionated chlorophyll a concentration and primary production; however, the details of the species composition of nano- and micro-protist communities have not been well investigated. We evaluated the spatial distribution and relationship with hydrography of nano- and micro-protist communities in the southern region of the Okhotsk Sea off Hokkaido during the summer of 2019. Forty-two species of diatoms from 19 genera, dinoflagellates from 7 genera, tintinnid ciliates, oligotrich ciliates, and 1 genus of silicoflagellates were identified. Their spatial distribution showed distinct differences among the taxa. Cluster analysis based on the protist cell density identified six groups with geographically well-distinguished distributions. Diatom-dominant communities were distributed in the Cold Water Belt and Soya warm current along the coastal area of Hokkaido. Nanoflagellate-dominated communities were distributed in the offshore area. Some harmful species of diatoms (Pseudonitzschia pseudodelicatissima) and dinoflagellates (Alexandrium spp.) were dominant in the Cold Water Belt off Wakkanai. We suggest that the upwelling of cold water could increase the abundance of harmful species, with immanent negative impacts on marine ecosystems or fishery resources. In summary, the species composition and abundance of the summer protist community was strongly related to different water masses in the southern part of the Okhotsk Sea off Hokkaido.

A biodiversity survey on the planktonic protistan grazers of the Gulf of Naples (Italy)

Plankton communities include both unicellular and multicellular organisms. An important unicellular component is represented by those protists (i.e., unicellular eukaryotes) that are non-strictly autotrophic organisms and consume bacteria and other protists. These organisms are an important link between primary producers and metazoans and are usually known as microzooplankton, protozooplankton, or mixoplankton, as many of them couple phagotrophic and photoautotrophic behaviours. Herein we report on the diversity of these organisms sampled at two sampling sites (coastal and offshore stations), at two depths (0 and 10 m), in the Gulf of Naples during the early autumn of 2020. Despite efforts to list plankton biodiversity of primary producers and metazoan grazers made in this area so far, protistan grazers are still poorly investigated and previous information date back to decades ago. Our survey identified dinoflagellates and oligotrich ciliates as the most abundant groups, while tintinnids were less quantitatively relevant. The taxonomic composition in samples investigated herein remarked that reported by previous studies, with the sole exception of the tintinnid Ascampbeliella armilla, which was never reported before. A coastal-offshore gradient in the taxonomical composition of protistan grazers was also observed, with some species more abundant within coastal waters and other better thriving in offshore ones. Surface and sub-surface communities also differed in terms of species composition, with the deeper communities in the two sites being more similar reciprocally than with communities at the surface. These differences were associated with distinct environmental conditions, such as light availability, as well with the standing feeding environment, arising potential implications in the functioning of the planktonic food web at the local scale.

Neglect of Presence of Bacteria Leads to Inaccurate Growth Parameters of the Oligotrich Ciliate Strombidium sp. During Grazing Experiments on Nanoflagellates

Oligotrich ciliates play a key role in linking microbial food webs to the traditional grazing food chain. Hence, the numerical (growth) and functional (grazing) responses of oligotrich ciliates are very important issues in studies of marine ecosystems. Most oligotrich ciliates feed mainly on nanoflagellates, while some of them also have the ability to consume bacteria. Up until now, studies of ciliates grazing on algae have not specifically excluded the effects of bacteria. In the present study, we found that the presence of bacteria in the algal culture medium affected the growth and grazing rates of ciliates grazing on nanoflagellates, resulting in an overestimate of gross growth efficiency at low relative algal concentrations. Strombidium sp. is prey selective mainly grazing on bacteria at low relative algal concentrations, but on algae at high relative algal concentrations. Carbon obtained from ciliate grazing on bacteria should be taken into account in the coastal zone surveys and especially in culture experiments to avoid unreasonable results of carbon flow.

Taxonomy and Phylogeny of a New Marine Planktonic Ciliate, Strombidium pseudorapulum sp. n. (Protozoa, Ciliophora, Oligotrichia)

As one of the main groups of marine planktonic protozoa, oligotrich ciliates have shown a high biodiversity. Recently, a new aloricate oligotrich ciliate, Strombidium pseudorapulum sp. n. was isolated from coastal waters of southeast China. The living morphology and infraciliature of this species were studied using live cell observation and protargol impregnation. Strombidium pseudorapulum sp. n. differs from its congeners by having a radish-like body with a bumped anterior end and an elongated tail-like posterior end, possessing a red ‘eyespot’ in apical protrusion of cell, adoral zone of membranelles composed of 12–16 collar and 9–12 buccal membranelles, one spindle-shaped macronucleus, girdle kinety in anterior 2/5 of cell with 45–60 densely spaced dikinetids, and ventral kinety occupying about the posterior 1/3 of the cell with 16–19 dikinetids. Strombidium pseudorapulum sp. n. clusters together with those species having ‘eyespot’ in apical protrusion in the phylogenetic tree based on the small subunit rRNA gene sequence. It is a sister species of S. cuneiforme with 31 unmatched nucleotides between their SSU rRNA gene sequences.

Impacts of meltwater discharge from marine-terminating glaciers on the protist community in Inglefield Bredning, northwestern Greenland

To evaluate the effects of meltwater discharge from marine-terminating glaciers on a fjord protist community in northwestern Greenland during summer, we investigated the distribution, abundance and biomass of the protist community and their relationships with hydrographic parameters. In the standing stock of protists, dinoflagellates (46.4%) and oligotrich ciliates (39.5%) were dominant throughout the study region. With respect to vertical distribution, oligotrich ciliates were abundant in the surface layer, mainly due to suitable food conditions (abundance of diatom and nanoflagellates). Near glaciers, relatively high chlorophylla(chla) concentrations were found in the subsurface layers associated with the low-temperature, high-turbidity and slightly high nutrient levels, indicating that the nutrient inputs from the upwelling glacial meltwater plume increased primary production. Large-sizedProtoperidiumspp. were found only at stations near glaciers where nutrients were abundant, and heterotrophic dinoflagellates showed strong relationships with nanoflagellates. These findings suggest that the upwelling associated with subglacial meltwater discharge can stimulate nanoflagellate production, resulting in increases in ciliate and heterotrophic dinoflagellate production.

Morphology and Molecular Phylogeny of Two Freshwater Oligotrich Ciliates (Protozoa, Ciliophora, Oligotrichia), Pelagostrombidium fallax (Zacharias, 1895) Krainer, 1991 and Limnostrombidium viride (Stein, 1867) Krainer, 1995, with Brief Notes on Stomatogenesis.

The morphology of two oligotrich ciliates, Pelagostrombidium fallax (Zacharias, 1895) Krainer, 1991 and Limnostrombidium viride (Stein, 1867) Krainer, 1995, collected from a freshwater pond in northern China, was studied based on live observation and protargol staining. Currently, undescribed features in the girdle kinety in P.fallax are (a) sparsely spaced single argyrophilic basal bodies in the shoulder region and (b) a U-shape formed below the buccal peristome. An improved diagnosis for P.fallax is supplied, based on previous and present morphological descriptions. In addition, certain ontogenetic stages of P.fallax are reported for the first time. During division, two new embryonic bodies are successively generated de novo, the first one goes to the proter, the second one to the opisthe; the new canal derives from the old canal. The morphology of L.viride is redescribed in detail; the Chinese population is highly consistent with populations in Europe. The SSU rRNA gene of P.fallax was sequenced for the first time. Phylogenetic analyses, based on SSU rRNA gene sequence data, reveal that P.fallax and L.viride cluster with Strombidium species while the result of an AU test did not reject the possibility of monophyly of the family Pelagostrombidiidae.

Overview of the biodiversity and geographic distribution of aloricate oligotrich ciliates (Protozoa, Ciliophora, Spirotrichea) in coastal waters of southern China

Aloricate oligotrich ciliates are important components of marine microplankton. This paper summarizes the results of several biodiversity and faunal studies on this group in typical coastal habitats of southern China that were carried out from 2006 to 2016. The main findings are as follows: (1) a total of 39 species belonging to 17 genera, seven families and four orders of aloricate oligotrich (s. l.) ciliates were identified from coastal waters of Zhanjiang, Guangzhou, Shenzhen, Huizhou, and Hong Kong; among these 16 species, four genera, one family and one order were new to science; (2) the order Oligotrichida represents the most abundant taxon with highest species richness and its members are widely distributed in all coastal habitats; the family Strombidiidae and the genus Strombidium comprise the highest proportion of all oligotrichs at family and genus levels, respectively; (3) based on the results of our taxonomic investigations, the biodiversity of oligotrichs in five typical habitats are compared; mangrove wetland possessing the highest species richness of oligotrich ciliates represents a biodiversity hotspot habitat in the southern coast of China; (4) morphologies of 10 poorly known species, namely Pelagohalteria cirrifera, Strombidium conicum, Spirostrombidium schizostomum, Parallelostrombidium paralatum, Omegastrombidium elegans, Varistrombidium kielum, Spirotontonia turbinata, Pelagostrobilidium simile, Strombidinopsis batos, and Strombidinopsis cheshiri are redescribed based on living and protargol-stained specimens, and some new characters are analysed. Improved diagnoses of Strombidium conicum and Strombidinopsis batos are supplied.

Diversity of oligotrich ciliates (Ciliophora, Spirotrichea) in the northern coast of South China Sea as revealed in LSU rDNA sequences

Although the taxonomy of oligotrich ciliates has been widely investigated, yet the species diversity remains poorly known. We newly designed a pair of oligotrich-specific LSU rDNA primers covering the 600-bp D1/D2 region, and it was effective for detecting oligotrich species. Using the primers, we constructed the cloning libraries to investigate the species diversity of oligotrichs in the northern coastal waters of the South China Sea. In total, 165 oligotrich sequences were obtained from five widely separated sampling sites. Sixty operational taxonomic units (OTUs) were obtained at 99% similarity threshold, and low-abundance OTUs with no more than two sequences contributed most of these (about 78%). Our findings are consistent with previous morphological studies, Strombidium was found the most abundant and widely distributed genus in this area. In addition, the BLAST search in the NCBI database resulted in 95% OTUs matching with named oligotrich species in similarity below 99%. Therefore, oligotrich morphospecies diversity has been underestimated as low-abundance species, and the LSU rDNA oligotrich sequence database needs to be better promoted.

Microzooplankton Distribution and Dynamics in the Eastern Fram Strait and the Arctic Ocean in May and August 2014

Microzooplankton community structure, distribution, growth and herbivory were examined in the eastern Fram Strait and Arctic Ocean shelf affected by the Atlantic water inflow in May (during the spring bloom, chlorophyll up to 9 µg l-1) and August (post-bloom, 0.13 – 1.7 µg l-1) 2014. Microzooplankton were collected along two longitudinal transects at 79°N and 79.4 °N from the slope to the ice edge and crossed the Western Spitsbergen Current (WSC). In May, integrated microzooplankton biomass in the upper 100 m ranged from 0.16 g C m-2 above the slope to 2.3 g C m-2 within WSC (0.71 g C m-2 on average). Mixotrophic oligotrich ciliates from the genus Strombidium dominated in the spring and formed a surface bloom (79 x 103 cells L-1, 206 µg C L-1). This is the largest microzooplankton biomass recorded in the Arctic so far. The heterotrophic dinoflagellates Gyrodinium and Protoperidinium were abundant at the diatom-dominated stations in the marginal ice zone. In the summer, a more diverse community included a large proportion of heterotrophic and mixotrophic dinoflagellates, tintinnids, and other ciliates. Microzooplankton biomass increased to the average of 1.27 g C m-2. At the ice-covered and open water stations in the Yermak shelf and deep basin, microzooplankton grew at 0.04 to 0.38 d-1; their species-specific growth rates were up to 1.79 d-1. Microzooplankton herbivory on average removed 72% (36 to >100%) of daily primary production with the exception of Phaeocystis pouchetii colonies. These results indicate that the microbial food web plays a central role in the carbon cycle in this Atlantic-influenced polar system.

Taxonomy and phylogeny of two poorly studied genera of marine oligotrich ciliates including descriptions of two new species: Cyrtostrombidium paraboreale sp. n. and Apostrombidium orientale sp. n. (Ciliophora: Spirotrichea)

Although it is widely recognized that oligotrich ciliates are the dominant constituent of microzooplankton communities and perform key functions in energy flow and material cycling in marine microbial food webs, knowledge of their diversity is scant. In the present study, we investigate the oligotrich genera, Cyrtostrombidium and Apostrombidium, with emphasis on their morphology and evolutionary relationships. Three isolates were collected from coastal waters of northern and southern China including two new species, viz., Cyrtostrombidium paraboreale sp. n., Apostrombidium orientale sp. n., and Apostrombidium pseudokielum Xu et al., 2009. Cyrtostrombidium paraboreale sp. n. is characterized by possessing 64–98 cytopharyngeal rods and two macronuclear nodules. Apostrombidium orientale sp. n. is characterized by its somatic kinety consisting of five fragments including a horizontally orientated subterminal fragment and possessing conspicuously long dorsal cilia. Apostrombidium pseudokielum is redescribed based on the new population and a re-examination of the type material. Phylogenetic analyses were performed for the subclass Oligotrichia, incorporating SSU rRNA gene sequences of the three species investigated here. The results indicate that the genus Cyrtostrombidium is monophyletic with C. paraboreale sp. n. occupying the basal position. The genus Apostrombidium is not monophyletic as Varistrombidium kielum is nested within it.

The genetic diversity of plastids associated with mixotrophic oligotrich ciliates

AbstractMixotrophic oligotrich ciliates use plastids sequestered from algal prey to acquire energy and metabolic products from photosynthesis. We isolated mixotrophic oligotrichs from coastal waters off Massachusetts (MA) and California (CA), as well as from two freshwater ponds in MA, and identified associated plastid 23S rRNA genes. Ciliates were identified using a combination of microscopy and 18S rRNA phylogeny, and included Laboea strobila and various Pseudotontonia, Spirotontonia, and Strombidium species from marine waters, and Limnostrombidium viride from freshwater. Overall, nearly half of all plastid sequences recovered from ciliates were haptophytes, followed by 15–20% for stramenopiles and chlorophytes, and < 10% originating from cryptophyte algae. No plastid sequences were from dinoflagellates. Cells of Strombidium ‘biarmatum’ collected from coastal MA in spring and fall possessed mostly Micromonas plastids, but during spring also possessed cryptophyte sequences. During spring, L. strobila were found to have a mix of chlorophyte, cryptophyte, haptophyte, and stramenopile sequences, while a co‐occurring Strombidium sp. had mostly chlorophyte and haptophyte sequences. Mixotrophic Pseudotontonia and Spirotontonia spp. were sampled during summer from coastal MA and in Monterey Bay, CA, and cells from both populations were dominated (> 70%) by haptophyte plastids of similar phylogenetic origin. L. viride were also evaluated from two freshwater ponds and possessed mostly Chrysochromulina sp. (haptophyte) and synurid (stramenopile) sequences. These results are the first survey of the genetic diversity of plastids associated with pelagic mixotrophic oligotrich ciliates and suggest that some species may selectively retain plastids from certain algal groups, while others appear to be generalists.

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

Taxonomy, Morphology and Phylogeny of a New Oligotrich Ciliate–Omegastrombidium hongkongense n. sp. (Protozoa: Ciliophora) from Clear Water Bay, Hong Kong

One new marine oligotrich ciliate, Omegastrombidium hongkongense n. sp., was isolated from a bloom of Noctiluca scuntillans near Port Shelter, Hong Kong. The morphology and infraciliature of this new species were studied on both living and protargol-stained specimens. Its phylogenetic position was discussed based on the sequence of the small subunit rRNA gene. O. hongkongense is different from its congeners with special characters. The cells are usually heart-shaped, and the cell size usually is (20–35) × (20–30) μm in vivo. Its deep buccal cavity extends obliquely to about 1/2 of cell length. It shows prominent apical protrusion. The adoral zone of membranelles is divided into 17–19 collar membranelles and four buccal membranelles. It has one ball-like macronucleus. The girdle kinety forms a closed loop which obliquely surrounds the body. The ventral kinety and thigmotactic membranelles are not observed. The SSU rRNA sequence of O. hongkongense was close to those of Strombidium paracalkinsi and Varistrombidium kelum with approximately 99% similarity. In the phylogenetic trees, O. hongkongense can be grouped with O. elegans and V. kielum species with very low support (16% ML).

Oil Spills and Dispersants Can Cause the Initiation of Potentially Harmful Dinoflagellate Blooms ("Red Tides").

After oil spills and dispersant applications the formation of red tides or harmful algal blooms (HABs) has been observed, which can cause additional negative impacts in areas affected by oil spills. However, the link between oil spills and HABs is still unknown. Here, we present experimental evidence that demonstrates a connection between oil spills and HABs. We determined the effects of oil, dispersant-treated oil, and dispersant alone on the structure of natural plankton assemblages in the Northern Gulf of Mexico. In coastal waters, large tintinnids and oligotrich ciliates, major grazers of phytoplankton, were negatively affected by the exposure to oil and dispersant, whereas bloom-forming dinoflagellates ( Prorocentrum texanum, P. triestinum, and Scrippsiella trochoidea) notably increased their concentration. The removal of key grazers due to oil and dispersant disrupts the predator-prey controls ("top-down controls") that normally function in plankton food webs. This disruption of grazing pressure opens a "loophole" that allows certain dinoflagellates with higher tolerance to oil and dispersants than their grazers to grow and form blooms when there are no growth limiting factors (e.g., nutrients). Therefore, oil spills and dispersants can act as disrupters of predator-prey controls in plankton food webs and as indirect inducers of potentially harmful dinoflagellate blooms.

Molecules illuminate morphology: phylogenomics confirms convergent evolution among 'oligotrichous' ciliates.

'Oligotrichous' ciliates have been traditionally placed in a presumed monophyletic taxon called the Oligotrichia. However, gene sequences of the small subunit rRNA gene, and several other genes, suggest that the taxon is not monophyletic: although statistical support for this is not strong, the oligotrich Halteria grandinella is associated with the hypotrich ciliates and not with other oligotrich genera, such as Strombidium and Strombidinopsis. This has convinced some taxonomists to emphasize that morphological features strongly support the monophyly of the oligotrichs. To further test this hypothesis of monophyly, we have undertaken a phylogenomic analysis using the transcriptome of H. grandinella cells amplified by a single-cell technique. One hundred and twenty-six of 159 single-gene trees placed H. grandinella as sister to hypotrich species, and phylogenomic analyses based on a subset of 124 genes robustly rejected the monophyly of the Oligotrichia and placed the genus Halteria as sister to the hypotrich genera Stylonychia and Oxytricha. We use these phylogenomic analyses to assess the convergent nature of morphological features of oligotrichous ciliates. A particularly 'strong' morphological feature supporting monophyly of the oligotrichs is enantiotropic cell division, which our results suggest is nevertheless a convergent feature, arising through the need for dividing ciliates to undertake rotokinesis to complete cell division.