The digenean complex life cycle includes various morphological forms with different locomotory and behavioral activities, and the functional specialization of their nervous system is of importance for the transmission of these parasites. Adult digeneans acquire many adaptive features associated with the final settlement in a vertebrate host. Our study describes the general morphology and ultrastructure of the nervous system of the adult renicolid digenean Renicola parvicaudatus parasitizing the renal tubules of herring gulls. Using immunocytochemical and electron microscopic methods, we identified the distinctive characteristics of ganglia and synapses in the studied species. A comparative analysis of the organization of the nervous system of adult individuals and their continuously‐swimming stylet cercariae revealed a number of stage‐related differences in the composition of ganglia, the distribution of serotonin‐ and FMRFamide‐immunoreactive neurons, the cytomorphology of neuron somata and free sensory endings. Thus, in adults, the presence of FMRFamide‐positive neuron somata, accessory muscle bundles in the ganglionic cortex, and eight types of neuronal vesicles was detected, but no glia‐like elements were identified. Their neurons are characterized by a larger volume of cytoplasm and also show greater ultrastructural diversity. Although the sensory papillae of adults do not vary in their external morphology as much as those of larvae, their sensory bulbs are more diverse in cytomorphology. Following our previous data on the “support” cell processes related to various tissues of the larvae and considered as glia‐like structures, we also briefly present the identified features of the parenchyma, attachment organs and excretory system of adult individuals. The excretory system of adult R. parvicaudatus is characterized by the presence of unique terminal cells with several flame tufts, which are not typical either for the larvae of this species or for other digeneans studied so far. We also used molecular phylogenetic analysis to clarify species identification.

Microanatomy and ultrastructure of the nervous system of adult Renicola parvicaudatus (Digenea: Renicolidae)

This paper focuses on the fine structure of mother sporocysts, the least studied stage of the digenean life cycle. The ultrastructure of the tegument of mature mother sporocysts of Bunocotyle progenetica was examined by transmission electron microscopy and described in detail. The tegument of these parthenitae is represented by an outer anucleate syncytium connected with the deeper-lying tegumental cells by cytoplasmic bridges. Its outer plasma membrane forms minute leaf-like outgrowths and numerous deep invaginations in the shape of interconnected channels. These channels, which pass in various directions and permeate almost the entire outer syncytium, considerably amplify its surface area. The cytoplasm of the outer layer of the tegument contains large mitochondria, microtubules and rare dense secretory granules, whose contents are discharged into the lumen of the channels. Numerous pinocytotic vesicles originate from the plasma membrane of the channels. Small endocytic vesicles are transported along the cytoplasmic bridges to the tegumental cells, where endocytosed food material is sorted and broken down. These cells are characterized by a well-developed Golgi apparatus, which is represented by multiple stacks of cisternae, and the presence of numerous endosomes at different maturation stages and residual bodies. Some steps of endosomal maturation in the tegumental cells were traced. In addition to their digestive activity, tegumental cells produce secretory granules, which are transported to the outer syncytium after their maturation. It was shown for the first time that in mature parthenitae, the population of tegumental cells could be renewed at the expense of a reserve pool of undifferentiated cells. The ultrastructural features of the tegument of mother sporocysts of B. progenetica are discussed in light of the concept of the enhanced trophic function of the tegument in sporocysts lacking the alimentary canal. 

pdf/tegument-ultrastructure-in-mother-sporocysts-of-bunocotyle-sfhj2jthgx.pdf

Tegument ultrastructure in mother sporocysts of Bunocotyle progenetica (Markowski, 1936) Chabaud et Buttner, 1959 (Digenea: Hemiuridae)

Species of the family Prosthogonimidae are considered the most pathogenic trematodes of poultry and wild birds worldwide, causing heavy economic losses in many countries. Prosthogonimosis was a common parasitic disease of Grus japonensis (Müller, 1776) which caused inflammation of the cloaca and bursa of Fabricius and even death. Morphological identifications of Prosthogonimus species are easily confusing; therefore, molecular characterization is used for discrimination. The present study was conducted to identify Prosthogonimus species at Zhalong National Nature Reserve, northeast of China. Considering the morphological variability and wide host range of individual Prosthogonimus species, a combination of both morphological and molecular analyses is indispensable for the valid identification of this parasite and the internal transcribed spacer (ITS) region was amplified for the sequence analysis and phylogenetic analysis. The results of molecular analysis together with phylogenetic reconstruction indicated that the Prosthogonimus pellucidus (von Linstow, 1873) in this study form a single cluster with P. pellucidus, revealing potentially high diversity within the genus Prosthogonimus. Classification of Prosthogonimus species seems to be unrelated to the host and may be related to geographical location. These data provide a significant resource of molecular markers for studying the taxonomy, population genetics, and systematics of Prosthogonimidae. 

Genetical and Morphological Identification of Prosthogonimus pellucidus (Digenea, Prosthogonimidae) in Grus japonensis

The trematode Acrolichanus auriculatus is a widely distributed intestine parasite of acipenserid fishes. For the first time the localization and distribution of the serotonergic nerve elements in A. auriculatus was studied using immunocytochemical method and confocal laser scanning microscopy. The study revealed the presence of biogenic amine, serotonin, in the central and peripheral nervous systems of A. auriculatus, that is in the neurons and neurites of the brain ganglia, brain commissure, the longitudinal nerve cords, and the connective nerve commissures. The innervation of the attachment organs, pharynx, oesophagus and distal regions of the reproductive system by the serotonergic nerve elements is observed. The distribution of serotonergic neurons in A. auriculatus is schematically marked. The comparative analysis of findings obtained in A. auriculatus with those recorded for other digeneans reveals the presence of both conservative and distinctive features in the organization of the serotonergic nervous system in various representatives of trematodes. 

Serotonergic elements in the nervous system of parasite of acipenserid fishes, Acrolichanus auriculatus (Digenea: Allocreadiidae)

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First Evidence on Serotonergic Elements in the Nervous System of Parasite of Acipenserid Fishes, Acrolichanus Auriculatus (Digenea: Allocreadiidae), a Confocal Microscopic Study

Digenea is a group of widespread parasitic flatworms with a complex life cycle including a successive change of parthenogenetic and hermaphroditic generations. Daughter sporocysts are among the least studied parthenitae in terms of the ultrastructure of their body wall and nervous system. Here we present an ultrastructural and immunocytochemical study of Podocotyle sp. daughter sporocyst (Opecoelidae), parasitizing in Littorina obtusata Linnaeus, 1758 from the White Sea. Our results focus on the structure of the body wall, birth pore, excretory and nervous systems, and include new data on the taxonomic affiliation based on the partial cox1 mtDNA sequence. The daughter sporocyst of Podocotyle sp. has ultrastructural similarities with different plagiorchiid and diplostomid digeneans and also possesses some specific features. In the studied species, the birth canal is epithelialized; the body wall is composed of nine types of somatic cells and includes a well-developed central nervous system. Neurons of the latter form specific hemidesmosome-like attachment sites, which we found in digeneans for the first time. The obtained results are necessary to complement the comparative morphological analysis of daughter parthenogenetic generations in different digenean lineages.

Ultrastructure of an opecoelid daughter sporocyst, Podocotyle sp. (Digenea: Opecoelidae): comparative analysis of the somatic tissues and new insights into the organization of the nervous system

: Xiphidiocercariae is a non-taxonomic group of digenean larvae. These small cercariae are characterized by complex morphology, which complicates their accurate identification. Recently, we have devoted a series of papers to the least studied groups of xiphidiocercariae — Cercariae virgulae and Cercariae microcotylae . Here we present the morphological description and molecular phylogenetic analysis of the new microcotylous cercaria. We have identified morphological features that allow us to distinguish this larva from all previously described cercariae. This species lacks virgula organ and possesses three pairs of penetration glands and Y-shaped excretory bladder. Molecular phylogenetic analysis performed with 28S rDNA and cox 1 mtDNA sequences made it possible to confirm the taxonomic position of this cercaria within the family Prosthogonimidae, and to determine its species identity as

pdf/morphological-description-and-phylogenetic-position-of-3k3ck403.pdf

Morphological description and phylogenetic position of xiphidiate cercaria of Prosthogonimus pellucidus (Trematoda: Digenea)

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Ultrastructure of an opecoelid daughter sporocyst, Podocotyle sp. (Digenea: Opecoelidae): comparative analysis of the somatic tissues and new insights into the organization of the nervous system

Morphological description and phylogenetic position of xiphidiate cercaria of Prosthogonimus pellucidus (Trematoda: Digenea)

Microanatomy and ultrastructure of the nervous system of adult Renicola parvicaudatus (Digenea: Renicolidae)