Plerocercoids of the order Phyllobothriidea were discovered in the gastrointestinal tract of the three-spined stickleback, Gasterosteus aculeatus, and White Sea cod, Gadus morhua marisalbi, in the White Sea. Based on the 28S rDNA sequence, plerocercoids from Gasterosteus aculeatus belong to Rockacestus cf. piriei. A phylogenetic tree was reconstructed to infer their evolutionary relationships. Several aspects of larval biology were described: the prevalence and intensity of infection were quantified in host fishes; movement patterns of the scolex and body were documented in vitro. The maximum infection rate of Rockacestus cf. piriei plerocercoids in Gasterosteus aculeatus was observed in June 2021. Some seasonality of fish infection by these parasites can be linked with three-spined stickleback migration. The detailed morphology of the scolex and microtriches, as well as the ultrastructure of the tegument and excretory system, have been studied using scanning and transmission electron microscopy. Unlike other cestodes, the tegument lacks rod-shaped bodies in its distal cytoplasm and exhibits uniform microtriches, specifically capilliform filitriches. Subtegumental musculature is well developed. The excretory system comprises cyrtocytes and syncytial epithelium forming protonephridial funnels, thin tubules and canals. The cytoplasm of the canal wall contains numerous electron-dense rod-shaped bodies oriented parallel to the surface. Several specific features in the ultrastructure of the tegument and excretory epithelium of the R. cf. piriei plerocercoid have been identified, contributing to the taxonomic and functional understanding of marine cestodes.

Host–parasite interactions between plerocercoids of Pyramicocephalus phocarum (Fabricius) Monticelli (Cestoda; Diphyllobothriidea) and White Sea cod (Gadus morhua marisalbi Derjugin) were studied, focusing on the tegument and secretory mechanisms and host capsule morphology.Histological and ultrastructural analyses revealed the host’s immune response to infection in the form of a granuloma. The granuloma was found to be composed of fibroblasts, mast cells, neutrophils, macrophages, and epithelioid cells. The parasite produced a protective layer, the cyst, as a structural response to the host immune reaction. Structures involved in the protection against the host included microtriches, extracellular vesicles, tegument organelles, and secretory products of neurons (neuro-exocrine secretion) and specialised eccrine glands. A unique structural adaptation of the P. phocarum plerocercoid was identified, namely the secretory poral cavities exhibiting both apocrine and neuro-exocrine secretion. These findings highlighted specialised adaptations of P. phocarum plerocercoids that may balance isolation from host immunity with access to nutrients, providing new insights into cestode–teleost relationships and the functional diversity of tegumental and neural secretions.

Изучен структурный аспект отношений «паразит-хозяин» в системе Triaenophorus nodulosus–окунь речной; выявлен ряд структурных механизмов, вовлеченных в защитную реакцию паразита против иммунного ответа хозяина. Плероцеркоиды локализуются в печени промежуточного хозяина-рыбы. В ответ на инвазию формируется паразитарная гранулема из тканей хозяина, образующих замкнутую капсулу вокруг паразита. Показано, что стенка капсулы многослойна и состоит из нескольких типов клеток. Внешний слой (фиброзный) представлен фибробластами и соединительнотканными волокнами. Внутренний слой формируется несколькими рядами уплощенных эпителиоидных клеток, плотно прилегающих друг к другу и соединенных между собой десмосомами. На внутренней поверхности стенки капсулы обнаружены индивидуальные макрофаги, проявляющие фагоцитарную активность. Структурное разнообразие клеточных элементов стенки гранулемы зависит от времени, прошедшего с момента заражения, и физиологического состояния хозяина. Впервые описан фагоцитоз апикальных частей микротрихий и тумулусов макрофагами гранулемы. Также в полости капсулы впервые обнаружены кристаллоподобные структуры неизвестной природы. Плероцеркоид находится в полости капсулы свободно, имеет сформированные крючья, которые, в отличие от кишечной стадии, покрыты цитоплазмой тегумента. Тегумент выделяет на поверхность мощный слой филаментозного матрикса, внеклеточные везикулы и вакуолизированные микротрихии. Терминали специализированных клеток выделяют секреторные продукты трех типов: секрет фронтальных желез; секрет тумулусов; нейросекрет чашевидных окончаний. Выявлены универсальные структурные адаптации, свойственные всем стадиям развития паразита, и специализированные, свойственные тканевым плероцеркоидам. К специализированным структурам, возникающим в ответ на воздействие хозяина, относятся вакуолизированные микротрихии.

The ultrastructure of the nervous system has been studied in sexually mature Nybelinia surmenicola (Cestoda: Trypanorhyncha) from the intestine of a shark Lamna ditropis. The central nervous system (CNS) reveals a complex organization within cestodes and corresponds to the trypanorhynch pattern of brain architecture. The brain of N. surmenicola is differentiated into nine clearly defined lobes and semicircular, median, and X-shaped cruciate commissures. A specific feature is the presence of a powerful extracellular capsule that surrounds the brain lobes with the cortical glial cells. Moreover, the architecture of the anterior lobes clearly distinguishes the species of Tentacularioidea. The neurons of the anterior lobes form compact groups looking like frontal horns. There are approximately 120 neurons in the anterior lobes and a preliminary estimate of more than 300 perikarya in the brain. Several ultrastructural types of neurons have been identified, differing in the size and shape of the soma, the density of the cytoplasm, and the ultrastructure of synaptic vesicles. Numerous synapses involving clear and electron-dense vesicles have been observed in neuropils. Two types of glial cells have been found in the brain that participate in neuronal metabolism and wrap around the giant axons, brain lobes, neuropil compartments, and the main nerve cords. Such a powerful extracellular fibrillar brain capsule has not been observed in the brain of other studied cestodes and has been demonstrated in this study for the first time. The differentiation of the brain lobes reveals the important role of the rhyncheal system in the evolution of cestodes and correlates with their behavior. The anterior nerves arising from the anterior lobes innervate the radial muscles stabilizing the position of the tentacle sheaths and movements of the attachment organs. The nervous system anatomy and the brain architecture may reflect the morphofunctional aspects of the tapeworm evolution.

A structural aspect of the parasite–host interactions in the Triaenophorus nodulosus– European perch system was studied; a number of structural mechanisms involved in the parasite’s protective reaction against the host’s immune response were identified. Plerocercoids are localized in the liver of the intermediate fish host. In response to invasion, a parasitic granuloma is formed from the host tissues forming a closed capsule around the parasite. It was demonstrated that the capsule wall is multilayered and consists of several cell types. The outer layer (fibrous) is represented by fibroblasts and connective tissue fibers. The inner layer is formed by several rows of flattened epithelioid cells tightly adjacent to each other and connected by desmosomes. Individual macrophages exhibiting a phagocytic activity were found on the inner surface of the capsule wall. Structural diversity of the cellular elements of the granuloma wall depends on time elapsed since infection and physiological state of the host. Phagocytosis of apical parts of microtriches and tumuli by granuloma macrophages was described for the first time. Crystal-like structures of unknown origin were also found for the first time in the capsule cavity. The plerocercoid is located freely in the capsule cavity and has formed hooks that (unlike the intestinal stage) are covered with the tegument cytoplasm. The tegument secretes a thick layer of filamentous matrix, extracellular vesicles, and vacuolized microtriches onto the surface. Terminals of specialized cells secrete three types of secretory products: secretion of frontal glands, secretion of tumuli, and neurosecretion of cup-shaped terminals. Universal structural adaptations common to all stages of parasite development and specialized ones common to tissue plerocercoids were detected. Vacuolized microtriches belong to the specialized structures that arise in response to the host immunity.

Free endings of peripheral neurosecretory neurons (NNs) were found in the tegument of plerocercoids of five species of parasitic cestodes of fish in an ultrastructural study. The free terminals secreted vesicles on the tegument surface and into the host body. Secretion was experimentally shown to increase in response to the host fish blood serum. In the cestode body, NNs form paracrine-type contacts near the cell membranes of the frontal glands, the tegument, and muscles, functioning as endocrine glands. Simultaneously, NNs function as exocrine glands and secrete the so-called manipulative factors, which influence the physiology of the host.

The phenomenon of exocrine secretion via nervous cells into the host tissue has been discovered in cestodes. In five cestode species of different orders specialized "cup-shaped" free nerve endings located in the tegument have been found. Their ultrastructure is characterized by the presence of a septate junction, a thin support ring and neurosecretory vesicles 90-110 nm in diameter, which are secreted onto the surface of the tegument through a thin pore. The phenomenon is referred to in this article as the neuro-exocrine secretion. We observed a direct relationship between neurosecretory processes in the deep subtegument and free endings in a series of ultrathin sections in two species. The peripheral neurosecretory neurons of species studied are characterized by similar ultrastructural features: size and location; diameter of neurosecretory granules; absence of microtubules and mitochondria in the neurites. The size of neurosecretory granules has been found to decrease from perikaryon towards neurosecretory terminals that lead to the tegument. In two species, we examined the neurosecretion during incubation in the host's blood serum. Depending on the time of incubation we have shown the changes a) in the diameter of the cup-shaped endings, b) in the number of secretory vesicles in the endings; c) changes in number and diameter of neurosecretory vesicles in the processes of neurosecretory neurons in the subtegument. The detected changes differ in D.dendriticus and L.interrupta and, taken together, indirectly confirm the secretory specialization of the cup-shaped endings. Supposed targets for the neurosecretory neurons in the studied cestodes are the following: (a) eccrine frontal gland ducts, especially their terminal regions involved in the release of secretory products; (b) longitudinal and circular muscles in the subtegument region; (c) the basal membrane of the tegument. Besides the discovered secretion vesicles through the cup-shaped terminals, we observed vacuoles derived from the basal membrane of the tegument containing extracellular substances released into the host tissue. Their possible role in the release of neurosecretory substances is discussed. Considering the data acquired via immunocytochemical methods, an assumption about involvement of FMRFamide-like related peptides (FaRPs) in the neuro-exocrine secretion is proposed. Possible functions of the neuro-exocrine secretion are discussed in the context of host-parasite interactions.

Parasitic 'turbellarians' are known from various animals such as echinoderms, crustaceans, annelids, bivalve and gastropod molluscs. So far, however, no 'turbellarians' have been reported from cephalopods. In this paper we report a parasitic 'turbellarian' from the giant Antarctic octopus, Megaleledone setebos. We dissected two specimens of M. setebos caught in the Ross Sea (Antarctica) and found numerous worms in their intestine and liver. The worms were spherical or oblong and had two morphologically different poles. The frontal pole bears a small conical protrusion containing large elongated pear-shaped frontal glands and large polygonal cells. The ducts of the frontal glands open terminally to form the frontal organ. The caudal pole has an opening shaped as a folded tube connected by the genital pore with a common genital atrium, which continues into a canal with a muscular sheath. The worms were identified as 'turbellarians' from the family Notenteridae (Fecampiida). This family contains only one species, Notentera ivanovi, reported from the gut of a polychaete at the White Sea. The worms that we found in the gastrointestinal tract of the octopuses were morphologically similar to N. ivanovi but differed from it in several important respects. Phylogenetic analysis based on 28S rDNA gene showed that the newly found worm clustered together with other fecampiids in a highly supported clade and was closely related to N. ivanovi. On the basis of these morphological and molecular data, we described a new species, Octopoxenus antarcticus gen. nov., sp. nov. (Fecampiida: Notenteridae), establishing a new genus to accommodate it and provided an updated diagnosis of the family Notenteridae. This is the first report of a parasitic 'turbellarian' from a cephalopod mollusc.

А novel type of a complex neuro-glandular brain structure including both nervous and glandular elements and associated with sensory ones is detected in Pyramicocephalus phocarum plerocercoid (Cestoda: Diphyllobothriidea), parasite of Gadus morua from the White Sea. The brain has two lateral lobes connected by a long cellular median commissure. The brain is tightly surrounded by glandular cells, which receive numerous synapses from the brain neurons. A complex of sensory organs associated with ducts and terminal pores of the frontal glands lies in the scolex tegument. Serotonin, FMRFamide- and GABA-like immunoreactive (IR) neurons are found in the brain, the main nerve cords, and the plexus of the plerocercoid. The innervation of the frontal gland ducts by FMRFamide-IR neurites is detected for the first time proving that they function under control of the nervous system and thus evidencing the eccrine nature of the secretion mechanism. Ultrastructural data show that light, dark and neurosecretory neurons are present in the brain lobes. The median commissure consists of loosely arranged thin parallel axons and several giant and small neurons. The commissure is stratified and penetrated by frontal glandular cells and their processes. Such neuro-glandular morpho-functional brain complex is suggested as a model for Diphyllobothriidae family. Five structural types of sensory organs are described in the scolex of P. phocarum; their colocalization with eccrine gland terminals is supposedly specific for Diphyllobothriidae family. Within the order Diphyllobothriidea, there are significant differences in the architecture of the plerocercoid brain at the family level. We suppose homology of giant commissural neurons among Diphyllobothriidea. Differences between diphyllobothriidean nervous system and that of other cestodes are discussed.

The structural response and plasticity of the cestode tegument in response to the influence of the host organism is not yet well understood. The main aims of our in vitro study were to analyse the ultrastructural mechanisms and kinetics of tegumental secretion in two cestode species, Dibothriocephalus dendriticus and Ligula interrupta, in response to the influence of fish host blood serum. The incubation of plerocercoids in the culture medium, which contained fish host blood serum, resulted in an increased number of secretory products on the tegumental surface. Our study is the first to experimentally demonstrate the formation of plerocercoid protective layers influenced by the host's internal environment factors. The mechanism of the generation of the protective layer included the following: the intensive formation of organelles in the tegumental cytons and their transfer to the distal cytoplasm of the tegument; increases in extracellular vesicles and vacuoles released on the tegumental surface; arrangement of secretory products and fine-dispersed extracellular matrix in layers; and formation of the protective layer. The structural tegumental response included increases in the glycocalyx layer and structural changes. Our study revealed that the universal mechanism of protective layer formation was intrinsic to different tapeworms. We hypothesize that plerocercoids of cestodes parasitizing fish may use tegumental secretion in the formation of a protective layer and in the release of immunoregulator molecules to evade the host's immune response.

The excretory system ultrastructure and immunocytochemistry have been investigated in the plerocercoid Pyramicocephalus phocarum. It has been shown that P. phocarum has independent terminal cells, cyrtocytes. The entire canal system is a single undivided syncytium, which includes nephridial funnels of the terminal tubules, and peripheral and central canals. The nephridial funnel and cyrtocyte form a filtration complex of the protonephridial type. In the caudal region, several peripheral canals open into a deep fold of the tegument, the urinary bladder. The excretory pores are separated from the tegument by annular septate desmosomes. There are no cell junctions inside the excretory system. The presence of the F-actin ring and the expression of non-synaptic serotonin in the collar area have been detected in cyrtocytes by immunocytochemistry methods.

The brain architecture in four species of tapeworms from the order Trypanorhyncha has been studied. In all species, the brain consists of paired anterior and lateral lobes, and an unpaired central lobe. The anterior lobes connect by dorsal and ventral semicircular commissures; the central and lateral lobes connect by a median and an X-shaped crisscross commissure. In the center of the brain, five well-developed compact neuropils are present. The brain occupies a medial position in the scolexpars bothrialis. The ventral excretory vessels are situated outside the lateral lobes of the brain; the dorsal excretory vessels are located inside the brain and dorsal to the median commissure. The brain gives rize four anterior proboscis nerves and four posterior bulbar nerves with myelinated giant axons (GAs). The cell bodies of the GAs are located within the X-commissure and in the bulbar nerves. Highly developed serotonergic neuropils are present in the anterior and lateral lobes; numerous 5-HT neurons are found in the brain lobes including the central unpaired lobe. The X-cross commissure consists of the alpha-tub-immunoreactive and 5-HT-IR neurites. Eight ultrastructural types of neurons were found in the brain of the three species investigated. In addition, different types of synapses were present in the neuropils. Glial cells ensheath the brain lobes, the neuropils, the GAs, and the bulbar nerves. Glia cell processes form complex branching patterns of thin cytoplasmic sheets sandwiched between adjacent neural processes and filling the space between neurons. Multilayer myelin-like envelopes and a mesaxon-like structure have been found in Trypanorhyncha nervous system. We compared the brain architecture of Trypanorhyncha with that of an early basal cestode taxon, that is, Diphyllobothriidea, and present a hypothesis about the homology of the anterior brain lobes in order Trypanorhyncha; and the lateral lobes and median commissure are homologous brain structures within Eucestoda.

Жанетта Вячеславовна Корнева - доктор биологических наук, активный исследователь тонкого строения и тканевой организации паразитических плоских червей, скончалась 12 сентября 2017 г. на 55-м году жизни. Вся трудовая деятельность Жанетты Вячеславовны связана с Институтом биологии внутренних вод им. И. Д. Папанина Российской академии наук (ИБВВ РАН, пос. Борок, Ярославская обл.).

A comparison between the axon terminals of octopaminergic efferent dorsal or ventral unpaired median neurons in either desert locusts (Schistocerca gregaria) or fruit flies (Drosophila melanogaster) across skeletal muscles reveals many similarities. In both species the octopaminergic axon forms beaded fibers where the boutons or varicosities form type II terminals in contrast to the neuromuscular junction (NMJ) or type I terminals. These type II terminals are immunopositive for both tyramine and octopamine and, in contrast to the type I terminals, which possess clear synaptic vesicles, only contain dense core vesicles. These dense core vesicles contain octopamine as shown by immunogold methods. With respect to the cytomatrix and active zone peptides the type II terminals exhibit active zone-like accumulations of the scaffold protein Bruchpilot (BRP) only sparsely in contrast to the many accumulations of BRP identifying active zones of NMJ type I terminals. In the fruit fly larva marked dynamic changes of octopaminergic fibers have been reported after short starvation which not only affects the formation of new branches (“synaptopods”) but also affects the type I terminals or NMJs via octopamine-signaling (Koon et al., 2011). Our starvation experiments of Drosophila-larvae revealed a time-dependency of the formation of additional branches. Whereas after 2 h of starvation we find a decrease in “synaptopods”, the increase is significant after 6 h of starvation. In addition, we provide evidence that the release of octopamine from dendritic and/or axonal type II terminals uses a similar synaptic machinery to glutamate release from type I terminals of excitatory motor neurons. Indeed, blocking this canonical synaptic release machinery via RNAi induced downregulation of BRP in neurons with type II terminals leads to flight performance deficits similar to those observed for octopamine mutants or flies lacking this class of neurons (Brembs et al., 2007).

The spectrum of immunomodulating molecules produced by tapeworms is not yet well understood. The aims of this study, on the tapeworm Diphyllobothrium dendriticum, were: 1) detection and quantification of prostaglandins (PGs) E2 and D2 by high performance liquid chromatography; 2) visualization of PGE2 and PGD2 in specific cells, using methods of immunocytochemistry and confocal laser scanning microscopy; and 3) investigation of the ultrastructure of the cells potentially producing PGE2 and PGD2. The PGE2 immunoreaction (IR) was found in the apical terminals of the frontal glands and sensory organs in the tegument and in small neurons belonging to the main cords and commissures. PGE2-IR partly coincided with α-tubulin-IR. PGD2-IR occurred in the muscle fibers of longitudinal and transverse body muscles and coincided with phalloidin TRITC staining. Both PGE2 and PGD2 were found in the flame cells of the excretory system. Ultrastructural study of the tegument revealed two types of structures that potentially produce PGE2: ciliated and unciliated free nerve endings and frontal gland terminals reinforced with neurotubules. In the main nerve cords, small neurons were identified as potentially exhibiting PGE2-immunoreactivity. In homogenates of the plerocercoids, the measured content of PGE2 and PGD2 was 33.15ngmg-1 and 1.94ngmg-1 of fresh tissue weight, respectively. We found evidence of PGE2 and PGD2 in D. dendriticum parasitizing Coregonus autumnalis (fish) and proved excretion of PGE2 and PGD2 in response to C. autumnalis blood serum. Prostaglandins produced by D. dendriticum probably play a dual role: 1) PGE2 and PGD2 potentially modulate the fish antiparasitic immune response; 2) PGE2 is presumably necessary for proper development and function of the nervous system, and PGD2 can act as an antagonist against mediators causing muscle contraction.

The ultrastructure of the tegument, glands and sensory organs of Pyramicocephalus phocarum (Cestoda: Diphyllobothriidea) have been studied.Three types of the microthriches are found, which have a specific distribution on the scolex and body.A welldeveloped basal lamina contained radial anchoring fibrils; they are associated with the microfibrills of the lamina reticularis and form regular cross-links.In the tegument, we have found six types of sensory organs and also terminal pores of the frontal glands.It has been shown that sensory endings and secretory pores are co-localized in the bothria tegument: 30 pores and 50 sensory endings were found in one section.Frontal glands are located in the parenchyma of the scolex and body; glands are well-developed and have intensive eccrine secretion.Comparative ultrastructural analysis of four diphyllobothriidean species shows similarity in the ultrastructure of microtriches and frontal glands in the plerocercoids of P. phocarum and Diphyllobothrium latum.