Philasterides dicentrarchi, a histophagous ciliate causing scuticociliatosis in olive flounder, Philasterides dicentrarchi--histopathology investigations

Philasterides dicentrarchi is a histophagous scuticociliate infecting Korean olive flounder farms in Jeju Island, South Korea, where it causes significant economic losses. However, the route of entry of these parasites in olive flounder is currently unknown. In the present study, we attempted experimental infection with different doses (concentrations) of P. dicentrarchi by intraperitoneal (IP) injection; maximum cumulative mortality of 95% was recorded on the administration of 2.1×10(8) ciliates/ml. In small size group (3 cm length, 24±2 gwt) the ciliates were found in connective tissues of muscle, fins, and nervous tissue while in large size group (5 cm length, 47±3 gwt), the ciliates were predominantly occurred with associated symptoms of liquefaction in the central nervous system. The major clinico-pathological manifestations were loss of scales, appearance of bleached spots that coalesced to form brownish patches, hemorrhagic, and severe dermal necrotic lesion. In addition there was degeneration of muscle fibers, necrosis of the hepatic parenchyma, and severe edema of the intestinal walls, extensive fouling, necrotic degeneration and hyperplasia in the gill bronchial epithelium. In this study, many ciliates were found in the blood vessel, brain, dorsal fins, muscle, kidney, and vertebra of infected flounder. The histopathological investigations are of significant importance in view of possible routes of entry into the host and doses of ciliates that trigger fast infection for potential utility of drugs as a strategy for the control of P. dicentrarchi in farmed olive flounder.     

Sequence conservation in the internal transcribed spacers and 5.8S ribosomal RNA of parasitic scuticociliates Miamiensis avidus (Ciliophora, Scuticociliatia)

The scuticociliate Miamiensis avidus is a histophagous parasite that causes high mortality in cultured marine fishes, with clinical signs of severe ulcers and hemorrhages in the skeletal muscle. The internal transcribed spacer (ITS) region, which is widely used in taxonomy and molecular phylogeny because of a high degree of variation, was compared for 21 cloned strains of M. avidus (Ciliophora, Scuticociliatia). These strains were isolated from olive flounder Paralichthys olivaceus, ridged-eye flounder Pleuronichthys cornutus and spotted knifejaw Oplegnathus fasciatus in Korea and Japan. The ITS1 (140 bp), ITS2 (236 bp) and 5.8S (119 bp) regions from 21 strains were identical, indicating that these regions are highly conserved in M. avidus. Phylogenic analysis of ITS2 shows that the ciliate should be included in the Philasterida with a close relationship to Pseudocohnilembus hargisi. This study exhibits the first detailed analysis of the ITS1, 5.8 S and ITS2 rRNA regions of M. avidus.     

Complete small subunit rRNA gene sequence of the scuticociliate Miamiensis avidus pathogenic to olive flounder Paralichthys olivaceus

Eight isolates of Miamiensis avidus (scuticociliates) were collected from olive flounder Paralichthys olivaceus with symptoms of severe ulcers and haemorrhages at several culture farms in 1999 and 2003. Cloned strains were produced and the complete small subunit ribosomal RNA gene (SSU rRNA) of each strain was sequenced for classification and phylogenic study. The SSU rRNA is 1759 bp in length and the sequence was deposited in the GenBank under accession number AY550080. All 8 strains exhibited the same sequence, but this sequence did not match any previously deposited scuticociliate SSU rRNA sequence. Phylogenetic analysis placed Miamiensis avidus in a sister lineage to Cohnilembus verminus, Pseudocohnilembus hargisi and P. marinus.     

Effect of dietary supplementation with Suaeda maritima on blood physiology, innate immune response, and disease resistance in olive flounder against Miamiensis avidus

The effect of Suaeda maritima enriched diet on blood physiology, innate immune response, and disease resistance in olive flounder Paralichythys olivaceus against Miamiensis avidus on weeks 1, 2, and 4 was investigated. Feeding with any enriched diet and then challenging with M. avidus significantly increased white blood cells (WBC) on weeks 2 and 4; the red blood cells (RBC) significantly increased with 0.1% and 1.0% enriched diets on week 4. The hemoglobin (Hb) and hematocrit (Ht) levels significantly increased when fed with 0.1% and 1.0% supplementation diets on weeks 2 and 4. The mean corpuscular volume (MCV) did not significantly vary with any diet and time; however the mean corpuscular hemoglobin (MCH) and mean corpuscular hemoglobin concentration (MCHC) significantly increased with 0.1% and 1.0% supplementation diets on weeks 2 and 4. The leukocytes such as lymphocytes (Lym), monocytes (Mon), neutrophils (Neu) and biochemical parameters such as total protein (TP), glucose (GLU), and calcium (CAL) levels significantly increased in 0.1% and 1.0% supplementation diet fed groups on weeks 2 and 4. The serum lysozyme activity was significantly enhanced in 0.1% and 1.0% supplementation diet fed groups from weeks 1 to 4 when compared to the control (0% herbal extract enriched diet). The scuticocidal activity and respiratory burst activity were significantly enhanced when fish were fed with 0.1% and 1.0% supplementation diets from weeks 2 and 4. The protective effect in terms of cumulative mortality (50% and 40%) was low in groups on being fed with 0.1% and 1.0% supplemented diet. Therefore the present study suggested that 0.1% and 1.0% S. maritime-supplemented diets protect the hematological and biochemical parameters, improving the innate immunity, affording protection disease from M. avidus infection in olive flounder.     

Morphology and biology of parasite responsible for scuticociliatosis of cultured olive flounder Paralichthys olivaceus.

Based on the arrangement and shape of the buccal structure, scuticociliates isolated from cultured olive flounder Paralichthys olivaceus belonged to the family Uronematidae and showed many characteristics of Uronema marinum. There was variation in the morphometry of clinical isolates taken from different organs of infected flounder. However, the isolates did not show any significant difference in morphometry under cultured conditions. The ciliates were easily maintained in in vitro medium to which antibiotic agents had been added and which had been enriched with the raw brain tissue of a healthy olive flounder. The ciliates propagated in a wide range of both temperature (6 to 30 degrees C) and salinity (10 to 35 ppt).     

The Cultivation of Symbiote-free Marine Ciliates in Axenic Medium

SYNOPSIS. Media have been developed for axenic cultivation of 10 strains belonging to 7 species of small marine ciliates. A medium containing cerophyl extract, proteose peptone, trypticase, yeast nucleic acid, biotin, calcium pantothenate folic acid, nicotinamide, pyridoxal HCl, riboflavin, thiamine HCl, and DL-thioctic acid in sea water supports the growth of Uronema nigricans strains Pc and 34/2, Parauronema virginiatum strains 2/1 and 19/1, Miamiensis avidus strains Ma and 19/3, Miamiensis sp. strain 1/1, a strain of "G" ciliate, and strains 33/8 and 34/7 of 2 unidentified species. By substituting a mixture of asolecithin, cephalin, and Tween 80 for cerophyl in the medium, luxurious growth of all except the strains of the 2 unidentified species can be obtained. A defined medium consisting of 18 amino acids, 5 purine derivatives, 8 vitamins, asolecithin, cephalin, and Tween 80 in synthetic sea water also has been developed for 6 of the strains: M. avidus Ma and 19/3, Miamiensis sp. 1/1, P. virginiatum 2/1 and 19/1, and U. nigricans Pc. In general the ciliates grow best at pH 7.2 in the dark at 27 C in media containing sea water of density = 1.015. Under these conditions maximum populations are reached in 4–5 days and range from several hundred thousand to 3 or 4 × 10⁶ depending upon the strain. Electronmicroscopic observations for the presence of endosymbiotes gave negative results.     

Evaluation of the effect of protease inhibitors on the viability of Miamiensis avidus using the WST-1 assay

Miamiensis avidus causes scuticociliatosis in cultured olive flounders (Paralichthys olivaceus), leading to economic losses in aquaculture in Korea. Quantitative evaluation of the viability of M. avidus is important to develop an effective vaccine or chemotherapeutic agent against it. We used a colorimetric assay based on the reduction of 2-(4-Iodophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium (WST-1) to quantify the viability of M. avidus. Using this method, we investigated the effect of protease inhibitors on the viability of M. avidus. The assay showed a clear difference in the optical density (OD) of over 10⁴ ciliates, and the metalloprotease inhibitors 1, 10-phenanthroline and ethylenediaminetetraacetic acid (EDTA) reduced the viability of M. avidus by more than 90% when used at concentration of 5 mM and 100 μM, respectively. However, different morphological changes in the parasite were observed when exposed to these two inhibitors. These results indicate that the WST-1 assay is a simple and reliable method to quantify the viability of M. avidus, and metalloproteases are excellent targets for the development of agents and vaccines to control M. avidus infection.     

Effects of phenanthrene on growth and basic physiological functions of the olive flounder, Paralichthys olivaceus

Flounder were exposed to waterborne phenanthrene (0.5, 1 and 2 μM) for 4 weeks to test effects of waterborne phenanthrene on growth and hematological properties of the olive flounder (Paralichthys olivaceus). The average weight gain (WG) of flounder was significantly decreased in fish exposed to phenanthrene at 2.0 μM for 2 weeks, whereas WGs of fish treated by 1.0 and 2.0 μM phenanthrene for 4 weeks were significantly decreased. However, hepatosomatic index (HSI) and condition factor (CF) of flounder were not significantly affected by phenanthrene exposure. Red blood cell (RBC) count, hemoglobin (Hb) concentration, hematocrit (Ht), the mean corpuscular hemoglobin (MCH) and the mean corpuscular hemoglobin concentration (MCHC) mean levels were decreased with an increase in exposure time of phenanthrene to the fish, but the level of the mean corpuscular volume (MCV) was increased. Plasma bilirubin concentrations were significantly increased following exposure to waterborne phenanthrene (2.0 μM) for 2 and 4 weeks; however, there were no significant changes in plasma total cholesterol in fish of all treated groups compared to control. The phenanthrene-exposed groups (≥1.0 μM) showed significantly higher mean plasma lysozyme activity. Kidney lysozyme activity of fish exposed to phenanthrene (≥1.0 μM) was also significantly higher than that of control fish. The central finding from these data is that olive flounder exposed to waterborne phenanthrene at more than 1.0 μM are likely to experience negative impacts on fish health and basic physiological functions.     

Dexiotrichides pangi n. sp. (protozoa,ciliophora, scuticociliatia), a new marine ciliate from the north China sea

The morphology, infraciliature, and silverline system of a new marine scuticociliate, Dexiotrichides pangi n. sp. were investigated. The new species is characterized by: size about 45-65 x 20-25 microm in vivo with kidney-like body shape and obliquely truncated semicircle-shaped apical plate; cytostome at bottom of conspicuously depressed oral cavity, which is located at the cell equatorial level; paroral membrane extending anteriorly to membranelle 3; scutica multi-rowed; 33-38 somatic kineties; contractile vacuole near ventral side and subcaudally positioned, opening at posterior end of somatic kinety 3; one oval macronucleus and one small micronucleus; caudal cilium positioned in a small pouch; marine habitat. Based on the data obtained, an improved diagnosis for the genus Dexiotrichides is suggested: body with circular cross-section and conspicuous cilia-free apical plate; buccal cavity conspicuously depressed with cytostome located near or at equatorial level; three membranelles transversely orientated each with 2-3 rows; paroral membrane zigzaging structure, extending to about half of the length of buccal field; multi-rowed scutica; somatic kinety one strongly shortened and terminating anteriorly at posterior end of buccal field; basal bodies in equatorial region arranged usually in circular pattern, while in the anterior portion of somatic kinety 2, basal bodies characteristically in pairs and separated from the posterior part of kinety 2; one caudal cilium.     

Ultrastructure of the Somatic and Buccal Cortex of the Tetrahymenine Hymenostome Glaucoma chattoni

SYNOPSIS The membranes, epiplasm, and fiber systems are described in the somatic cortex of Glaucoma chattoni strain HZ-1. Kinetodesmal fibers, postciliary and transverse microtubular ribbons, basal microtubules, transverse fibers and transverse accessory material are associated with kinetosomes. Longitudinal microtubular ribbons and mitochondria occur interkinetally. In the buccal cortex, the membranes, epiplasm and fibers of the 3 membranelles, the paroral kinety, the ribbed wall, and the cytostome are described. Comparisons between G. chattoni and other ciliates reveal ultrastructural differences of possible systematic significance. In the somatic cortex of this and other tetrahymenines. Iongitudinal microtubular ribbons and basal microtubules occur concurrently. In the buccal cortex, alveoli are absent in tetrahymenine membranelles. A table is presented of the fiber systems associated with single somatic kinetosomes of various ciliates whose cortical ultrastructure has been studied to date.     

Morphology and Ecology of Siroloxophyllum utriculariae (Penard, 1922) N. G., N. Comb. (Ciliophora, Pleurostomatida) and an Improved Classification of Pleurostomatid Ciliates

ABSTRACT. The morphology and infraciliature of Siroloxophyllum utriculariae (Penard, 1922) n. g., n. comb. were studied in live cells, with the scanning and transmission electron microscope, as well as in specimens impregnated with protargol and silver carbonate. The new genus, Siroloxophyllum , belongs to the Loxophyllidae and has a specific combination of characters, viz. an oral bulge surrounding almost the entire cell, three perioral kineties, a single brush kinety, and a single right dorsolateral kinety. The ecology and faunistics of S. utriculariae are reviewed. It is a rare and infrequent predator preferring clean freshwaters. The somatic monokinetid of S. utriculariae has typical haptorid ultrastructure, including two transverse microtubular ribbons. The oral bulge is patterned string-like with riffles containing the transverse microtubular ribbons originating from the oral kinetids. Perioral kineties 1 and 2 consist of dikinetids having one basal body each ciliated; the nonciliated basal body is associated with a nematodesmal and a transverse microtubular ribbon. Perioral kinety 3 consists of ciliated monokinetids having a fine structure similar to the somatic kinetids; they form triads with the dikinetids from perioral kinety 2. The classification of pleurostomatid ciliates is reviewed. Two suborders (Amphileptina, Litonotina) and three families (Amphileptidae, Litonotidae, Loxophyllidae n. fam.) are recognized and defined.     

The fine structure of the paralabial organelle in the rumen ciliate Ophryoscolex purkinjei Stein, 1858

The paralabial organelle of the rumen ciliate Ophryoscolex purkinjei, located on the ventral side of the ciliophor, is a highly specialized part of the somatic cortex. It consists of alternating rows of short modified cilia and thin pellicular folds which form a ridge-like structure. The central "top kinety" is composed of monokinetids which bear cilia with 9 + 2 axonemes and 2 microns in length. The top kinety is accompanied by a comb-shaped fold on its distal side and by a broad wedge-shaped fold on its proximal side. To both sides there follow two or three lateral kineties made of dikinetids. The anterior kinetosome of each pair bears a clavate cilium, only 0.5-0.7 micron in length and with a 9 + 0 axoneme while the cilium of the posterior kinetosome is even shorter. Lateral folds with numerous microtubules cover these lateral kineties and rows of barren basal bodies. The fine structure of this supposed sensory organelle show a basic pattern in four other ophryoscolecids, and its increasing complexity parallels the suggested phylogenetic line of evolution of these ciliates.     

Description of the Infraciliature and Morphogenesis in the Ciliate Urotricha ondina N. Sp. (Prorodontida, Urotrichidae)

Recent works on prostomatid ciliates show that some genera of this group have a differentiated oral infraciliature and that their stomatogenesis during division involves the proliferation of only a few somatic kineties. These findings have significant implications regarding the iaxonomic status of these genera and also on the terminology used for the oral structures. In Urotricha ondina , the oral infraciliature consists of (1) a paroral kinety formed of paired kinetosomes that encircle the cytostome at the anterior pole of the cell and (2) 3 adoral organelles, each formed of 2 rows of kinetosomes, ventral in position and obliquely disposed, lying above 3 short somatic kineties that do not reach the anterior pole of the cell. This oral ciliature —formerly known as the corona and brosse, respectively—originate during stomatogenesis from the proliferation of 4 somatic kineties that lie posterior to the adoral organelles of the parental cell.     

The Fine Structure of the Paralabial Organelle in the Rumen Ciliate Ophryoscolex purkinjei Stein, 1858

The paralabial organelle of the rumen ciliate Ophryoscolex purkinjei , located on the ventral side of the ciliophor, is a highly specialized part of the somatic cortex. It consists of alternating rows of short modified cilia and thin pellicular folds which form a ridge-like structure. The central "top kinety" is composed of monokinetids which bear cilia with 9 + 2 axonemes and 2 μm in length. The top kinety is accompanied by a comb-shaped fold on its distal side and by a broad wedge-shaped fold on its proximal side. To both sides there follow two or three lateral kineties made of dikinetids. The anterior kinetosome of each pair bears a clavate cilium, only 0.5–0.7 μm in length and with a 9 + 0 axoneme while the cilium of the posterior kinetosome is even shorter. Lateral folds with numerous microtubules cover these lateral kineties and rows of barren basal bodies. The fine structure of this supposed sensory organelle show a basic pattern in four other ophryoscolecids, and its increasing complexity parallels the suggested phylogenetic line of evolution of these ciliates.     

The filterability of leukocytes in undiluted blood

A filtrometer is described for measuring the flow of fluids through microfilters. The flow of Newtonian fluids through the filters can be predicted from the diameter, length and number of pores. There are no physical artefacts such as turbulent flow or a significant lag period before steady-state flow is achieved. The instrument has been used as a viscometer and has been used to record and analyse the flow of undiluted blood through 5 microns polycarbonate filters. The calculated viscosity of Newtonian fluids agrees well with those measured by a more conventional viscometer (Ostwald). Flow profiles of blood have been analysed to give both the numbers and the flow properties of a small population of slow leukocytes which equate numerically with the monocytes. They are subdivided into three distinct sub-populations, according to their rheological properties, and these are termed SL1, SL2 and PB. The concentration of these cells, in blood, are 0.12 +/- 0.02 x 10(6) ml-1, 0.11 +/- 0.02 x 10(6) ml-1, 0.09 +/- 0.02 x 10(6) ml-1 in young females aged about 25 years. The transit time of these cells, through 5 microns pores, is 34.8 +/- 1.4 s, 147.5 +/- 2.5 s and > 300 s, respectively. Analysis of blood from older men (53-79 years) gives essentially the same results although the concentration of SL1 is slightly higher at 0.19 +/- 0.09 x 10(6) ml-1.     

Evaluation of non-specific immune components from the skin mucus of olive flounder (Paralichthys olivaceus)

The innate immune system, particularly the external body surface, plays a frontier role in protecting fish under intensive aquaculture and at prolonged low temperatures from relevant infections due to inadequate adaptive immune responses. In the present study we aimed to understand the mucosal immunity of an economically important mariculture fish, olive flounder (Paralichthys olivaceus) by evaluating the immune components from its skin mucus. The activities of lysozyme (233.33+/-171.82 units mg(-1)), trypsin-like protease (42.84+/-1.249 units mg(-1)), alkaline phosphatase (0.376+/-0.005 units mg(-1)) and esterase (0.170+/-0.006 units mg(-1)) were detected in the skin mucus. Transferrin was identified by MALDI-TOF/MS analysis. ELISA and immunoblot assays using anti-flounder IgM monoclonal antibody showed the presence of a significant level (1.80+/-0.001, n=3) of monomer immunoglobulin M (IgM) with approximate molecular weight of 160 and 25 kDa under non-denaturing and denaturing states, respectively. Skin mucus showed strong antibacterial activity against tested fish pathogenic bacteria. In addition, skin mucus successfully agglutinated (HA titre 2(8)), but completely failed to haemolyse, rabbit erythrocytes. In conclusion, the major immune components of the skin mucus, identified in the present study, are possibly involved in the broad spectrum non-specific immunity of olive flounder.     

Comparison of toxic effects of nitric oxide and peroxynitrite on Uronema marinum (Ciliata: Scuticociliatida)

To discover the effects of nitric oxide (NO) and peroxynitrite on Uronema marinum (a ciliate responsible for systemic scuticociliatosis in cultured olive flounder Paralichthys olivaceus), the dose-dependent inhibitory effect of NO donors, S-nitroso-N-acetylpenicillamine (SNAP) and 3-morpholinosydnonimine (SIN-1) on the proliferation and survival of U. marinum was investigated. The inhibitory effects of exogenous superoxide dismutase (SOD) and catalase on the toxicity of SIN-1 were also investigated. After 24 h of incubation in the presence of 0.2 mM SNAP, the number of ciliates was not statistically different from that of the controls, whereas incubation in the presence of 0.5 mM SNAP reduced the number of parasites significantly to 59.1% of controls. Concentrations of SNAP higher than 0.5 mM resulted in greater reductions in the number of ciliates, but levels of generated NO far exceeded physiological ranges. The number of viable ciliates incubated for 24 h with 0.2 mM SIN-1 was reduced significantly to 25.0%, and all ciliates were killed by incubation in concentrations above 0.5 mM SIN-1. Although SOD decreased the toxic effect of SIN-1 on U. marinum, protection was not complete and did not improve after increasing the SOD concentration from 50 to 400 U ml(-1). Addition of catalase ranging from 500 to 10000 U ml(-1) completely protected U. marinum from SIN-1 toxicity. Ciliates exposed to catalase alone or catalase plus SIN-1 showed significantly higher and dose-dependent proliferation rates compared to controls. Addition of haemoglobin, ranging from 0.5 to 2.0 mg ml(-1), also protected U. marinum from SIN-1 toxicity, and increased the proliferation rate dose-dependently. In conclusion, resistance of U. marinum to oxidative and nitrative stress may allow this pathogen to withstand the NO- and oxygen-radical-dependent killing mechanisms of phagocytic cells.     

The apertospathulidae, a new family of haptorid ciliates (protozoa, ciliophora)

We studied the morphology of three rare haptorid ciliates, using live observation and silver impregnation: Apertospathula verruculifera n. sp., Longispatha elegans n. gen., n. sp., and Rhinothrix porculus (Penard, 1922) n. gen., n. comb. Simple ethanol fixation (50-70%, v/v) is recommended to reveal the ciliary pattern of "difficult" ciliates, such as R. porculus, by protargol impregnation. The three genera investigated have a distinct feature in common, viz., a lasso-shaped oral bulge and circumoral kinety, where the right half is slightly to distinctly longer than the left and the circumoral kinety is open ventrally. Thus, they are united in a new spathidiid family, the Apertospathulidae n. fam., which probably evolved from a Bryophyllum-like ancestor by partial reduction of the oral bulge and circumoral kinety. Apertospathula verruculifera has a wart-like process, the palpus dorsalis, at the anterior end of the dorsal brush. The right branch of the circumoral kinety is only slightly longer than the left one. Longispatha elegans has a straight oral bulge and circumoral kinety, the right branch of which extends to the posterior end of the body while the left branch ends in the anterior third of the body. Rhinothrix porculus, a curious ciliate with a snout-like dorsal elongation of the oral bulge, the palpus oralis, has a highly characteristic ciliary pattern: the oral pattern is as in Longispatha, but the bulge and circumoral kinety extend spirally to the posterior end of the body while the somatic kineties course meridionally. This is achieved by inserting some shortened kineties in the curves of the oral bulge.     

Effect of different densities of live and dead Chlorella vulgaris on the population growth of rotifers Brachionus calyciflorus and Brachionus patulus (Rotifera)

In order to maintain rotifer populations during periods of low algal production, it is necessary to offer alternate diets, some of which include forms of preserved algae. The present work is based on the effect of live and dead Chlorella vulgaris on the population growth of Brachionus calyciflorus and Brachionus patulus. The experimental design consisted of three algal levels (0.5 x 10(6), 1.5 x 10(6) and 4.5 x 10(6) cells ml-1) offered in three forms (living, frozen and heat-killed). The maximal population density values for B. calyciflorus ranged from 55 +/- 1 ind. ml-1 (at 0.5 x 10(6) cells ml-1) to 471 +/- 72 ind. ml-1 (at 4.5 x 10(6) cells ml-1) with live Chlorella, but was much lower (6 +/- 1 to 26 +/- 6 ind. ml-1) with frozen or heat-killed alga under comparable food levels. However, the maximum population density of B. patulus under live or or heat-killed Chlorella was similar at comparable algal levels but when offered frozen algae it was four times less. The highest mean peak population density was 1,277 +/- 83 ind. ml-1 under 4.5 x 10(6) cells ml-1. The rate of population increase for B. calyciflorus varied from 0.50 to 0.79 using live Chlorella, but under comparable conditions, this range was lower (0.21 to 0.31) for B. patulus. Results have been discussed in light of possible application for aquaculture.     

A New Species of the Genus Metacystis (Ciliophora,Prostomatida, Metacystidae) from a Wastewater Treatment Plant

ABSTRACT. Unusual prostomatid specimens were found in the biological reactor of a wastewater treatment plant in a health resort in Valencia, Spain. These ciliates were attached to flocs unlike other free‐swimming prostomatid ciliates described to date in the mixed liquor of activated sludge plants. The morphological study of this species led to a typically different combination of characteristics: elongated cell shape, 20–30 somatic kineties, 2 perioral kineties, and 1 circumoral kinety, 1 large vacuole protruding at the terminal end, a lorica tapered toward the aperture with a smooth neck, and 11–16 annular ridges. These characteristics place this representative as a new species of the genus Metacystis—Metacystis galiani n. sp. This species became the dominant population within the biological reactor when high values of conductivity (4,244 mS/cm) and temperature (26.8 °C) were recorded.