The ultrastructure of the epidermis and associated structures in the metacercaria cercaria and sporocyst of Diplostomum phoxini (Faust, 1918)

Search, find, and penetrate: ultrastructural data of furcocercariae of Cardiocephaloides longicollis (Digenea, Strigeidae) explain their transmission and infection strategy into fish hosts

The present study provides an overview of the structures linked to fish host finding, recognition, and invasion of one of the most commonly occurring morphotypes among trematodes, furcocercariae. For this, we use free-swimming cercariae of the strigeid Cardiocephaloides longicollis (Rudolphi 1819) Dubois, 1982. Their elongated cercarial body and bifurcated tail are covered by a tegument with an irregular surface, showing numerous folds arranged in different directions and a typical syncytial organization. Both the body and the bifurcated tail are covered with short spines, rose-thorn shaped, as well as four types of sensory papillae, distinguished by the presence or absence of a cilium, its length, and their position on the cercarial body. These papillae are especially important for free-living stages that rely on external stimuli to locate and adhere to the host. A specialized anterior organ is located at the anterior part of the cercariae and is encircled by a triangle-shaped group of enlarged pre-oral spines followed by a transverse row of enlarged post-oral spines that, together with the sensory papillae, allow active finding, recognition, and penetration into fish. The ventral sucker, covered with inner-oriented spines, sensory papillae, and cilia, helps during this process. The cercariae of C. longicollis possess three types of gland cells (a head gland and two types of penetration glands), each containing different types of secretory granules that play a role in host invasion. The protonephridial excretory system consists of an excretory bladder, a system of collecting tubules, flame cells, and two excretory pores in the middle of each furcae, which serve to control osmoregulation in their marine environment, as well as to eliminate metabolic waste. Together with the four types of sensory endings, the central ganglion forms the nervous system. Our results add novel information on the ultrastructure of strigeid furcocercariae, being essential to interpret these data in relation of their functional role to better understand the transmission and penetration strategies that cercariae display to infect their fish hosts.

Molecular and morphological characterisation of Diplostomum phoxini (Faust, 1918) with a revised classification and an updated nomenclature of the species-level lineages of Diplostomum (Digenea: Diplostomidae) sequenced worldwide

We characterised morphologically and molecularly Diplostomum phoxini (Faust, 1918) based on cercarial isolates from the snail Ampullaceana balthica (L.) (Gastropoda: Lymnaeidae) and metacercariae from the Eurasian minnow, Phoxinus phoxinus (L.) (Cypriniformes: Leuciscidae), and provided molecular evidence for the identification of the snail intermediate host. Phylogenetic analyses based on the cytochrome c oxidase subunit 1 (cox1) gene depicted 44 molecularly characterised species and genetically distinct lineages of Diplostomum, and resulted in: (i) a re-identification/re-classification of 98 isolates plus D. baeri sampled in North America; (ii) re-definition of the composition of the D. baeri species complex which now includes nine molecularly characterised species/lineages; (iii) re-definition of the composition of the D. mergi species complex which now includes seven molecularly characterised species/lineages; and (iv) an updated nomenclature for the molecularly characterised species-level lineages of Diplostomum.

Effect of Praziquantel on the Tegument and Digestive Epithelium Ultrastructure of Brachylaima sp. Metacercariae Parasitizing the Edible Land Snail Cornu aspersum

The edible land snail Cornu aspersum (Pulmonata: Stylommatophora) acts as second intermediate host in the cycle of Brachylaima sp. trematode, harboring free metacercariae in its kidney. The ingestion of undercooked infected snails by humans allows metacercariae to develop to adult stage in the intestine, causing brachylaimiasis. Praziquantel (PZQ) is the drug of choice to treat trematodiasis and it is effective against Brachylaima sp. metacercariae. The objective of this work was to assess, by transmission electron microscopy (TEM), the ultrastructural changes produced on the tegument and gastrodermis of the Brachylaima metacercariae recovered from C. aspersum treated with PZQ in comparison with untreated ones. Snails naturally infected by Brachylaima sp. metacercariae were treated by PZQ both individually and in groups. Metacercariae recovered from treated and control snails were processed for TEM. The tegument of untreated metacercariae was covered by a regular and thick glycocalyx. The syncytial epithelium contained abundant T2 secretory bodies appearing as membrane-bound biconcave disk-vesicles with high electron-dense and uniform content. The T2 secretory bodies located along the external area of the syncytium were mainly arranged at right angles to the apical plasma membrane. In treated metacercariae, the content of the T2 secretory bodies appeared altered, degenerating from high to low electron density, losing its uniform appearance and forming high electron-dense accumulations scattered around the periphery of the vesicle and separated by low electron-dense spaces. The presence of clusters was detectable in the central area. The characteristic arrangement of the T2 secretory bodies observed in untreated metacercariae was lost in treated ones. Vesicles near the apical area of the tegument no longer maintained their arrangement perpendicular to the apical plasma membrane. The characteristic arrangement of T2 secretory bodies and mitochondria was lost. The T2 secretory bodies were also found altered in the tegumental cell bodies, suggesting that the alterations started at the production stage. Mitochondria were severely degenerated and located in the apical area of the tegument. The digestive system displayed a strong contraction, which included the disappearance of the intracecal lumen.

Ultrastructural features of the tegumental surface of a new metacercaria, Nematostrigea sp. (Trematoda: Strigeidae), with a search for potential taxonomically informative characters

The tegumental surface of a new strigeid metacercaria, Nematostrigea sp., which is a parasite of the freshwater fish Channa gachua (Hamilton) in central Vietnam, is described for the first time using scanning (SEM) and transmission (TEM) electron microscopy. In addition to the general tegumental surface in various parts of the body, details of the surface of the suckers, lappets and holdfast organ are presented, as are variations in the form and distribution of the body spines. As good taxonomic criteria are few in diplostomoid metacercariae at both specific and generic levels, a number of the ultrastructural features revealed may prove to represent taxonomically informative characters. These include the presence of: two rings of dome-shaped papillae localised at different levels on the rim of the oral sucker, a single ring of ciliated papillae on the inner margin of the ventral sucker and a band of dome-shaped papillae along the lateral margins of the broad body-fold in the ventral forebody; an unarmed oral sucker and anteroventral surface of the forebody, although the latter bears protuberant secretory pores; an armed ventral sucker covered by six-pointed spines, except on its rim; multi-pointed spines along the dorsal and ventral sides of the forebody, with the number of their teeth increasing posteriorly; multi-pointed spines on the forebody which gradually transform into single-pointed, more widely distributed spines on the hindbody, disappearing completely at posterior end of the body; the surface of the lappets with a particular distribution of pores leading to three types of secretory glands and three topographical modifications (areas where the surface is smooth, bears digitiform processes or bears recurved, dagger-shaped spines); and the surface of the holdfast organ which is covered with densely packed, straight or slightly curved, simple spines on its lateral surface but is smooth medially.

Developmental changes in the tegument of four microphallid metacercariae in their second (crustacean) intermediate hosts

The morphology of the tegument of four microphallid metacercariae from the stage of invasive cercariae to their maturation as encysted metacercariae inside their crustacean second intermediate hosts is described. The tegument of the metacercariae developed surface lamellae and projections which, along with coated vesicles in the surface syncytium, indicated that the tegument had an absorptive function. The disappearance of secretory granules from the tegument at the same time as the appearance of the first cyst wall suggested that the tegument had a role in primary cyst production. Following this, the metacercariae continued to grow and seemingly retained their absorptive ability. The tegument was also involved in the transport of material into the perimetacercarial lumen prior to its eventual inclusion in the developing inner cyst layers. It appeared that this material originated in tegumental cells located amongst the parenchymal cells beneath the tegumental syncytial layer. On completion of the secondary cyst layers there was a gradual degeneration of structures associated with absorption and a progressive accumulation of dense discoid granules traceable to underlying tegumental cells. All four microphallid species (Maritrema arenaria,M. subdolum,Levinseniella brachysomaandMicrophallus claviformis) demonstrated the same developmental pattern but the period spent in each stage differed depending on the time spent migrating to encystment sites. The pattern of tegumental development described is thought to be applicable to all microphallid metacercariae and possibly to other metacercariae which undergo growth and development in their second intermediate hosts.

Regional specializations of the tegument of the metacercaria ofTimoniella imbutiforme

The ultrastructure of the spinous body tegument of the metacercaria ofTimoniella imbutiforme(Molin, 1859) has recently been described. Other regions of the metacercarial tegument, including those of the oral sucker, pharynx, and nephridiopore, demonstrate considerable specializations. The oral sucker tegument had an aspinous outer syncytial layer that possessed a pimpled apical surface as well as enclosing two types of secretory bodies. The pharyngeal tegument likewise lacked spines, but possessed only one type of secretory body, and a smooth but folded outer surface. The nephridiopore tegument, however, showed the greatest degree of specialization possessing a single type of secretory body specific only to this region of the tegument. Also associated with the syncytium here was a prominent long filamentous glycocalyx, and microtubules which were observed for the first time in this region of the tegument.

The origin and development of the epidermis and associated structures in the cercaria of Cryptocotyle lingua (Creplin) (Digenea: Heterophyidae) from Littorina littorea (L.)

Cercariae of Cryptocotyle lingua develop from intraredial germinal cells which divide to form ‘naked’ cell aggregates and later germ balls covered, first, by a syncytial primitive epithelium and later a syncytial epidermis formed, in sequence, from superficial cells of the embryo. The primitive epithelium is soon lost. The original nuclei of the syncytial epidermis degenerate when the first series of epidermal cell bodies, formed immediately below and having the characters of protein synthesizing cells, become connected with it. The first cell bodies are replaced by a series of five types of epidermal (secretory) cell bodies developing in the parenchyma and giving off cytoplasmic processes which become connected, in sequence, with the outer cytoplasmic layer. Secretion bodies from four of the five types are discharged into the outer cytoplasmic layer, before the cercaria leaves the molluscan host, and remain there in the free swimming cercaria. The secretions of the fifth type are retained in their epidermal (secretory) cell bodies. The arrangement of the secretion bodies in the outer cytoplasmic layer and their histochemical reactions suggest possible functions concerned, later, with entry into and encystment within the second intermediate fish host.

An electron microscope study of the tegument of the metacercaria and adult of Leucochloridiomorpha constantiae (Trematoda: Brachylaemidae)

The tegumentary ultrastructure of Leucochloridiomorpha constantiae metacercariae and adults has been described. A filamentous glycocalyx invests the tegument of metacercariae and large numbers of biconcave, disk-shaped secretion vesicles are found in its outer zone, cytoplasmic bridges and cytons. Mitochondria within the outer tegumentary zone of metacercariae are restricted to its basal half. The transformation of metacercariae to adults involves the loss of the glycocalyx, a decrease in the number of disk-shaped secretion vesicles, a more uniform distribution of mitochondria through the outer tegumentary zone, an apparent degeneration of cytons, a thinning of the fibrous basal lamina and a decrease in the number of visible cytoplasmic bridges traversing it. The significance of these events for a metacercaria which does not encyst within the molluscan host is discussed. In addition, crystalline, spine-like inclusions in the outer tegumentary zone of adult L. constantiae are described and their possible function discussed.