The ultrastructure of the integument and proventriculus in the raptorial rotifer Cupelopagis vorax (Monogononta: Collothecaceae: Atrochidae)

Members of the sessile rotifer species Cupelopagis vorax are unusual ambush predators that live permanently attached to submerged freshwater plants. Previous light microscopical research has revealed several uncommon features in this species including a stellate‐patterned integument and an expansive foregut region called the proventriculus. In this study, we apply transmission electron microscopy to explore the ultrastructure of both the integument and foregut to determine how they differ from other rotifers. Our results reveal that the integument is covered by a thick glycocalyx and is patterned with tubercles that originate from the intracytoplasmic lamina (ICL) within the syncytial epidermis. The ICL forms an apical layer within the syncytium, is electron dense and mostly amorphous, and forms tubercles up to 2.3 μm; these tubercles probably account for the patterned appearance of the integument and are similar to what has been found in other gnesiotrochan rotifers. The basal cytoplasm is highly granular and contains two types of membrane‐bound vesicles: large ovoid vesicles (320–411 nm) with amorphous, opaque contents, and secretory bulbs (110–264 nm) with electron‐lucent cores and occasionally electron‐dense contents. Only the secretory bulbs were observed to form connections to the apical plasmalemma, and so are probably exocytotic. Internally, the proventriculus is a large distensible sack that connects the anterior pharyngeal tube to the posterior mastax. The proventricular epithelium is a thin syncytium mostly covered with a dense glycocalyx and a strong brush border of microvilli underlain by a thin terminal web. The cytoplasm contains few organelles and there is no evidence that it is either secretory or has features (e.g., ICL) that might aid in maceration. We hypothesize that the thick glycocalyx might serve a protective function against the movements of live prey and/or against enzymes released from the rotifer's gastric glands that become regurgitated during feeding.     

The phylogeny of rotifers: molecular,ultrastructural and behavioural data

This work discusses the nature and significance of molecular, ultrastructural, and behavioural characters that can be used in phylogenetic analyses of rotifers.Recent molecular research has demonstrated the presence of very small amounts of 4-hydroxyproline in rotifers, probably arising from acetyl-cholinesterase or glycoproteins. Thus, rotifers appear to be the first known Metazoa without collagen.Ultrastructural work also has made some interesting discoveries. (1) The myelinic cuticle of the integument and pharynx of gastrotrichs is present in the pharynx of at least two rotifers (Philodina and Brachionus) and some Annelida. (2) The intracytoplasmic lamina (IL) of the syncytial ingestive integument of Acanthocephala is similar to the IL of the syncytial stomach of Bdelloidea. (3) The fibrous terminal web of primitive epidermal ciliated cells may have evolved in the skeletal IL of the syncytial, aciliated integument of rotifers. (4) Using the ultrastructural features of the skeletal, IL of the integument, I derived two possible dendograms of rotifer evolution. (5) These models and other ultrastructural data predict that Bdelloidea should be separated from Monogononta, while Seison has several characters which suggest that it should be more closely aligned to the Monogononta than previously proposed.Molecular and ultrastructural data suggest that rotifers are primitive Metazoa, probably derived by neoteny from ancestral, ciliated larvae. Finally, I argue that information on sensory organs and the behaviour of rotifers may offer unique insights into the evolution of the phylum.     

Neuromuscular organization of the freshwater colonial rotifer, Sinantherina socialis, and its implications for understanding the evolution of coloniality in Rotifera

Coloniality among rotifers is rare, and while the adaptive significance of the lifestyle has been explored previously, there are few details about how it may have influenced the morphology of colony members. In this study, we use confocal laser scanning microscopy combined with cyto- and immunohistochemistry to determine if the colonial rotifer, Sinantherina socialis, differs in neuromuscular organization relative to other colonial and solitary forms. Our observations indicate that the patterns of serotonergic neurons and somatic muscles in S. socialis are broadly similar to other rotifers. At the neuronal level, the distribution of serotonergic (5HT-IR) neurons in S. socialis is most similar to colony-forming species of Conochilus: (1) paired nerve cords extend the length of the body; (2) the cerebral ganglion innervates the corona with paired neurites that form a neuronal ring; and (3) a single pair of neurites innervates the ventral sensory antennae. Unique to S. socialis is the presence of two additional neuronal rings that supply the corona and may function to modulate ciliary beat. At the muscular level, S. socialis displays a typical pattern of somatic muscle organization similar to other rotifers, but diverges from the pattern in three significant ways: (1) somatic circular muscles form complete rings; (2) circular muscles are distinguishable based on their size and position in the body; and (3) transverse muscles are present within the corona and presumably function to modify its shape for feeding and defensive purposes. These differences in neuromuscular organization may be adaptations to the colonial and/or sessile lifestyle, both of which are characteristic of S. socialis.     

Epizoic and parasitic rotifers

Many rotifer species live in close association with plants or other animals. Most of these associations are of a commensal or synoecious nature, some rotifer species having lost the ability to live independently. Few rotifers are true parasites, actually harming their hosts.The Seisonidae, Monogononta and Bdelloidea include epizoic and parasitic species. The most widely known are probably the parasites of colonial and filamentous algae (e.g. Volvox, Vaucheria). However, rotifers are also found on a wide range of invertebrates: colonial, sessile Protozoa; Porifera; Rotifera; Annelida; Bryozoa; Echinodermata; Mollusca, especially on the shells and egg masses of aquatic gastropods; Crustacea, including the lower forms (e.g. Daphnia, Asellus, Gammarus) and in the gill chambers of Astacus and Chasmagnathus; the aquatic larvae of insects. There appear to be few records of epizoic or parasitic rotifers among vertebrates, apart from Encentrum kozminskii on carp, Limnias ceratophylli on the Amazonian crocodile, Melanosuchus niger, and an unidentified Bdelloid apparently living as a pathogenic rotifer in Man.     

Sessile rotifers (Rotifera) exhibit strong seasonality in a shallow,eutrophic Ramsar site in Mexico

To better understand the sessile rotifer community of a shallow, eutrophic lake, we monitored over a 1‐year period the monthly changes in the density and diversity of rotifers on the roots of the common water hyacinth, Eichhornia crassipes, which served as a representative hydrophyte. Selected physical and chemical parameters of the water were also determined. Mean annual density of all sessile species was 630 individuals per ml. Species of Collothecacea (n = 7) were most abundant. Of the 12 species of Flosculariaceae (Flosculariidae), Sinantherina socialis occurred seasonally, while Ptygura beauchampi was most common across all seasons. The Shannon–Wiener diversity index was highest (H′ = 2.8) in April, but decreased in January (H′ = 1.3). Applying the concept of the guild ratio (functional‐based analysis) we found that raptorial species (Collothecacea) were dominant over most of the year (June–February), while microphagous Flosculariidae were dominant during the warm, drier season (March–May). BEST (optimal matching of biota to environment, including stepwise search) analysis and canonical correspondence revealed that temperature, dissolved oxygen, pH, turbidity, and transparency strongly influenced sessile rotifer dynamics. We also found significant correlations between the density of some species and physicochemical variables. These species included Collotheca ambigua, C. coronetta, C. ornata, C. campanulata, C. tenuilobata, and Stephanoceros millsii in the Collothecacea; and Beauchampia crucigere, Limnias melicerta, L. ceratophylli, Ptygura crystallina, P. melicerta, and Sinantherina ariprepes in the Flosculariidae. Our study shows that the communities of sessile rotifers were structured by abiotic factors and that the guild ratio is useful for understanding the relationship between sessile rotifers and their substrata.     

河鲈锚首吸虫体壁的超微结构观察

寄生鳜鳃部的河鲈锚首吸虫的体壁由表皮合胞体、基板、环肌、纵肌和表皮细胞核周体所组成。合胞体顶部质膜起伏形成表皮的嵴纹,基部质膜折叠形成指状突起伸入到合胞体中。合胞体表面覆盖着一层糖萼。河鲈锚首吸虫的表皮中含有四类分泌体,即电子致密的分泌颗粒、中等电子致密的分泌颗粒、有膜包围的电子稀疏的分泌体和多囊体．可见分泌体和合胞体基质通过胞吐作用排到体外,未见吞饮小泡,推测表皮的主要功能在于分泌和渗透压调节而非营养吸收。在外侧头瓣的乳突状结构所在处,合胞层较薄,基板平滑,在实质组织中的一腔体样结构中可见囊状体、电子致密度各异的颗粒体、泡状体和电子致密的基质团,神经突起分布于腔体周围,这类乳突可能代表一类新的非纤毛感受器类型。     

Fine structure of the subitaneous eggshell of the sessile rotifer Stephanoceros millsii (Monogononta) with observations on vesicle trafficking in the integument during ontogeny

Rotifers that engage in cyclical parthenogenesis produce two types of eggs: subitaneous eggs that hatch as clonal females and meiotic eggs that hatch as haploid males, or if fertilized, as females after a period of diapause (resting eggs). The ultrastructure of resting eggshells is known for some motile species, but there are limited data on subitaneous eggshells, and no data on any eggshells of sessile rotifers. Here, we investigated the ultrastructure of the subitaneous eggshell of the sessile rotifer Stephanoceros millsii and its potential origins of secretion, the maternal vitellarium and embryonic integument. We also explored secretory activity in the larval and adult integuments to determine whether activity changes during ontogeny. The eggshell consists of a single layer with two sublayers: an external granular sublayer apparently derived from the maternal vitellarium, and an internal flocculent sublayer secreted by the embryonic integument that may form a hatching membrane or glycocalyx. Secretory activity remains high in both the larva and adult and appears to be the source of the thickening glycocalyx. Altogether, the subitaneous eggshell of S. millsii is the thinnest among monogonont rotifers. Thin eggshells may have evolved in response to the added protection provided by the mother’s extracorporeal tube.     

Studies on the ultrastructure of the integument of the rotifer Habrotrocha rosa Donner (Aschelminthes)

Summary The integument of the rotifer Habrotrocha rosa Donner is provided with pores and formed by an extrasyncytial cuticle and a syncytial hypodermis. The hypodermis peripherally contains 3 layers of dense cytoplasm and borders the cuticle by an asymmetric cell membrane. The wall of the pores is stiffened proximally like an annulus. The pores lead into cytoplasmic invaginations which are surrounded by vesicles. Close to and also beneath the condensed cytoplasmic layers microbodies are found, which are interpreted as microperoxisomes. Subhypodermal layers of muscles are connected with the cytoplasm of the hypodermis by desmosome-like structures.I am indebted to Dr. H. Breucker, Anatomisches Institut Hamburg, and Prof. Dr. W. Becker, Zoologisches Institut Hamburg, under whose direction this work was carried out. Supported by the Deutsche Forschungsgemeinschaft (Be 464/10)     

Fasciola hepatica: development of tegument during migration in mouse.

T-0 granules of the type 0 tegumental cells of newly excysted juveniles appear in the syncytium of juveniles recovered from the abdominal cavity of mice 12 hr postinfection (p.i.). They undergo exocytosis and/or add their contents to the glycocalyx of the syncytial apical plasma membrane. While in the abdominal cavity the syncytial ground cytoplasm has an increased electron density. After arrival in the liver the type 0 cells metamorphose into type 1 cells of the adult and begin to synthesize T-1 granules. The type 2 cells of the adult arise by differentiation of embryonic cells in the parenchyma, 2–3 days p.i., and subsequently form protoplasmic tubular connections with the syncytium. On arrival in the bile ducts, 4 wk p.i., T-2 granules, formed in the type 2 cells, congregate in the apical cytoplasm of the thickened syncytium and the apical plasma membrane becomes much invaginated. The discussion correlates the development of the tegument with the changes in environment and a mechanism of spine growth is proposed.     

SEM of internal structures of Brachionus plicatilis (Rotifera)

Examination by scanning electron microscopy of sectioned rotifers provides views on their internal structures which complement the results of other techniques. Thus, additional information has been obtained for example, on the digestive tract, on nerve connections and on the morphology of the mastax. Our observations confirm that nearly all organs are connected in some way to the integument, suggesting that integument structures may be responsible for the holding together of the whole rotifer body.     

Clearance rates of sessile rotifers: in vitro determinations

We measured laboratory clearance rates of 10 rotifer and one unidentified bryozoan species from 3 different lakes using ³²P labeled algae (Chlamydomonas) or yeast (Rhodotorula). Clearance rates for all rotifers fed yeast ranged from < 2.0 to > 260 µl · animal^–1 · h^–1 depending on species. The in vitro clearance rates of two sessile rotifers (Ptygura crystallina and P. pilula) were not significantly different from previously measured in situ rates (Wallace and Starkweather 1983). Clearance rates for 5 rotifers fed algae ranged from < 5.0 to > 90.0 µl · animal^–1 · h^–1. Ptygura beauchampi, P. crystallina, P. pilula, Floscularia conifera, and F. melicerta ingested both cell types but their clearance rates varied substantially among species and between cell types. There was a substantial time-dependent loss of 32P from formalin-fixed animals (Sinantherina socialis) awaiting processing. This loss stabilized at approximately 20 hours and was estimated to be about 40% of the initial ingested label. Clearance rates for the bryozoan fed yeast or algae were highly variable, ranging from < 1.0 to > 3 000 µl · animal^–1 · h^–1.     

Structural Changes Associated with Resistance of Soybean to Heterodera glycines

Subcellular responses to infection by Race 3 of Heterodera glycines in susceptible (''Lee'') and resistant (''Forrest'' and ''Bedford'') soybean cultivars were compared. Syncytial formation, initiated in susceptible as well as resistant soybean cultivars, was characterized by wall perforations, dense cytoplasm, and increased endoplasmic reticulum, In susceptible plants, syncytia developed continuously until nematode maturity. This included hypertrophy of nuclei, increase of rough endoplasmic reticulum in early stages of infection, and formation of wall ingrowths at a late stage of infection. In the resistant reaction in Forrest, a necrotic layer surrounded syncytium component cells demarcating them from surrounding normal cells and leading to syncytial necrosis. Wall appositions were prominently formed near the necrotic layer, and the cytoplasm of the syncytium component cells was extremely condensed. The whole syncytium became necrotic at a late stage of infection. Bedford had nuclear degeneration prior to cytoplasmic degradation. Chromatin was often scattered throughout the syncytial cytoplasm. Finally the whole syncytium became degenerated with plasmalemma completely detached from the syncytial cell walls. The differences in resistant responses reflect a difference in genetic composition of the soybean cultivars tested.     

Effect of Food Quantity and Quality on Population Growth Rate and Digestive Activity in the Euryhaline Rotifer Brachionus plicatilis Müller

We investigated the nutritional effects of both food quantity and quality on Brachionus plicatilis. Decomposition of particulate and dissolved organic matter by rotifer digestive enzymes play a crucial role in rotifer nutrition. Among other enzymes, rotifers produce phosphatases, non‐specific enzymes that allow for the release of orthophosphate from a variety of organic phosphorus compounds. Phosphatase saturation was measured in B. plicatilis homogenates using the spectrofluorimetric method. We examined population growth rate, reproduction and phosphatase activity in the homogenate of rotifers (PA_RH) fed by nutrient‐replete algal food supplied at different quantities. Population growth rate, number of eggs per individual and PA_RH were affected by food quantity. Growth rate and number of eggs per individual significantly increased in rotifers fed by food supplied at the highest quantity. The highest population growth rate was reached by rotifers fed by nutrient‐replete food, while it did not significantly differ between rotifers fed on nitrogen (N)‐depleted and phosphorus (P)‐depleted food. The number of eggs per individual was more affected by N than P supply. PA_RH and rotifer RNA content were not influenced by different food quality. The results indicate that B. plicatilis is not able to regulate its digestive apparatus in terms of efficiently getting access to essential nutrients when scarce, but do this when nutrient‐replete food is available in different quantity. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enzyme activity measurements on isolated organs of Brachionus plicatilis (Rotifera)

A method is described by which the integument of Brachionus plicatilis, together with its intracellular lamina, is quickly dissolved before other parts or tissues of the animal are destroyed. After removing the integument several parts of the body can be separated and fractionated in a more or less intact state by centrifugation in a Percoll gradient. The measurement of enzyme activities has indicated that this procedure might provide a way of localizing enzymes within the rotifer body.     

On relation to substrate in sessile rotifers

Information on the distribution of 45 sessile rotifers from diverse freshwaters in south and central Sweden was analyzed to reveal possible relationships to their substrates. Almost all of them showed an exclusively periphytic occurrence. The highest number of species was found on Utricularia, but otherwise it was not possible to correlate rotifer presence to any special macrophyte or category of plants (e.g., fine-leaved or floating-leaved species).     

Topography and ultrastructure of the tegument of Lecithochirium musculus (digenea: Hemiuridae), a parasite of the European eel Anguilla anguilla (osteichthyes: Anguillidae)

The tegumental ultrastructure of the stomach fluke Lecithochirium musculus was studied using scanning and transmission electron microscopy. The surface of the tegument was smooth and covered by transverse cytoplasmic ridges. Cobblestone‐like units of the tegument were observed on the ventral surface. Invagination and evagination of the ecsoma induced variations in the tegumental surface. The ultrastructural study revealed that the tegument of L. musculus had a typical syncytial organization with a distal cytoplasm lying over a basal matrix and cytons. Two types of intra‐tegumental sensory structures were observed. Type 1 sensory receptor was a domed‐like fusiform structure consisting of a smooth elevation of the tegument. Four receptors of this type were observed on the anterior dorsal surface of the fluke. Three nerve bulbs filled with electron‐lucent material and mitochondria composed this receptor. Hemispherical electron‐dense collars were observed at the top of the nerve bulbs. Striated rootlets laid just beneath the hemispherical electron‐dense collars. Type 2 sensory receptor presented two morphological variations, i.e., a bulb‐like monolobed structure, and a bulb‐like bilobed structure observed at two different degrees of evagination. For both variations, the nerve bulb enclosed mitochondria, electron‐lucent material, and a conical electron‐dense collar from which extended a striated rootlet. Numerous sensory receptors of this type were observed around the ventral sucker. J. Morphol., 2012. © 2011 Wiley Periodicals, Inc.     

An ultrastructural study of alimentary tract development in the cercariae of Prosorhynchoides borealis (Digenea,Bucephalidae)

Podvyaznaya I. 2011. An ultrastructural study of alimentary tract development in the cercariae of Prosorhynchoides borealis (Digenea, Bucephalidae). —Acta Zoologica (Stockholm) 92 : 170–178. The development of digestive system in Prosorhynchoides borealis cercariae was studied using transmission electron microscopy. The foregut and caecum primordia arise in early cercarial embryos as two adjoining cellular cords. The primordial pharynx appears as a cluster of myoblasts in the mid‐part of the foregut primordium whose proximal end abuts onto the ventral embryonic tegument. Later, a lumen develops within the gut primordia and their component cells form the embryonic cellular epithelium with an essentially similar structure in the foregut and caecal regions. Subsequently, the foregut epithelial cells merge to form a syncytium. This process proceeds asynchronously and the most proximal foregut area remains cellular for the longest time. The syncytial lining of the foregut establishes syncytial connections with secretory cytons differentiating in the surrounding parenchyma. These cytons produce secretory granules, which are transported through cytoplasmic connections to the foregut syncytium. Before cercariae reach maturity, their foregut epithelium becomes anucleate and continuous with the external tegument. By the end of cercarial development, numerous short lamellae appear on the luminal surface of the caecal epithelium. The caecal cells become involved in secretory activity as indicated by the presence of Golgi‐derived secretory bodies in their cytoplasm.     

Movements in rotifers: correlations of ultrastructure and behavior

In this review, original observations on rotifer cilia and muscles are used to explain the mechanism of their movements. The following topics are discussed: (1) cilia and infraciliature of the cingulum, (2) muscle insertions into the infraciliature, (3) innervation of the muscles and non-innervation of the ciliated cells, (4) recent data on the location of gap junctions which link some muscles, (5) cilia and infraciliature of the cirri of the pseudotrochus of Brachionus calyciflorus, and the insertion of muscles onto their infraciliature, (6) the relation between muscles and integument in the foot of Philodina roseola, (7) the relation between muscles and integument in the joint present at the base of the tail of Trichocerca rattus, and (8) hypotheses on the first articulated (muscle-integument) movements which exist alongside ciliary motion in certain rotifers.Neuro-ethological summaries, based on data of the effectors and on observations on behavior and the sensitivity of rotifer sensory receptors (reviewed in Clément et al., 1983), are attempted. A more complete analysis is provided in the following instances: (1) swimming and the mechanism of turning, (2) retraction of the rotatory apparatus, (3) feeding behavior in Trichocerca rattus involving the succession swimming, crawling and nutrient intake.Other behavioral sequences, for which the ethological and ultrastructural data are incomplete, are presented and briefly discussed.Lastly, the argument is put forward that rotifers constitute an exceptional model for comparative, neuroethological studies which may enable us to better understand the mechanisms of evolution. Carrying out detailed observations, aimed at understanding how certain movements occur, should enable us to gain insights into the question of why they exist.     

Behavior,metamorphosis, and muscular organization of the predatory rotifer Acyclus inquietus (Rotifera,Monogononta)

Abstract. The atrochid rotifer, Acyclus inquietus, is a sedentary predator that lives within the colonies of its prey, the rotifer Sinantherina socialis. After larvae infiltrate and become associated with the colony, they secrete a permanent gelatinous tube and undergo metamorphosis to the adult stage. We followed settlement and metamorphosis using bright-field microscopy to document specific larval behaviors after eclosion, and used epifluorescence and confocal microscopy of phalloidin-labeled specimens to visualize some of the morphological changes that occur during metamorphosis. Upon eclosion, larvae possess paired eyespots and a ciliated corona that functions strictly in locomotion. After leaving the parent's gelatinous tube, larvae eventually settle on unoccupied colonies of S. socialis or on other substrates if colonies are unavailable. Settlement involves a period of gliding among colony members before attachment with the foot and the secretion of a gelatinous tube. After settlement, there is a drastic reconfiguration of the corona that involves loss of the eyespots, loss of the coronal cilia, and the formation of the cup-shaped infundibulum, a deep depression in the anterior of the head that leads to the mouth. The development of the infundibulum involves the expansion of tissues around the mouth and is accompanied by a reorientation of the underlying musculature that supplies the infundibulum and allows its use in prey capture. The arrangement of the muscles in the trunk and foot regions, which contain outer circular (complete and incomplete) and inner longitudinal bands, remains unchanged between ontogenetic stages, and reflects the condition characteristic of other rotifers.