Development of the tegument and alimentary tract in a trematode, Fellodistomum fellis

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.

Development of the alimentary tract during morphogenesis of the metacercariae of Levinseniella brachysoma

For the first time the development of the alimentary tract of Levinseniella brachysoma metacercaria (Trematoda: Microphallidae) obtained experimentally from Gammarus oceanicus has been described. The foregut primordium in 16-day-old metacercariae is represented by a syncytial cylindrical cord, resulting from the fusion of embryonic cells. Non-fused parts of the plasma membranes of adjacent cells are revealed as gap cavities within the cord. Later (30th day post infection) the lumen of the foregut is formed as a result of both partial vacuolization of the cytoplasm and by a broadening of the gap cavities, resulting from a thinning of the cytoplasmic spaces between them. Besides the usual organelles, the foregut of the mature metacercaria (42nd day p.i.) contains dense secretory granules in the apical cytoplasm region and numerous microtubules in basal areas. The cellular gastrodermis is formed later than the foregut syncytium (on day 30 p.i.); its large cells contain well-developed Golgi complexes, RER and mitochondria. A noteable inclusion of the gastrodermal cells of mature metacercariae are spherical granules of moderate electron density measuring up to 3 μm in diameter. On the basis of an analysis of the ultrastructural data the possible functioning of the metacercarial alimentary tract is discussed.

An ultrastructural study of the early cercarial development in Prosorhynchoides borealis (Digenea: Bucephalidae) with special reference to formation of the primitive epithelium

Primitive epithelium and outer tegumental layer formation during early cercarial development was studied in Prosorhynchoides borealis using electron microscopy. It demonstrated that germinal cells freely floating in the sporocyst body cavity divide to give rise to naked cell aggregates. These early embryos are highly irregular in outline and are composed of blastomeres differing in size and structure. In embryos consisting of about 12-14 cells a few (possibly only two) superficial macromeres become concave and produce thin extensions which envelop the embryonic mass before fusing to form a syncytial primitive epithelium. This primitive epithelium forms syncytial connections with underlying embryonic cells. Primordial tegumental cells become apparent in late germinal balls below the primitive epithelium. These cells expand and fuse to give rise to an embryonic nucleated tegument. The embryonic tegument is connected to peripheral embryonic cells by thin cytoplasmic bridges until the basement lamina is formed. Subsequently, the primitive epithelium is shed by the embryos and the nuclei in the embryonic tegument undergo pyknotic degeneration. These results are analysed and compared with data from studies on other trematode species and it is concluded that the primitive epithelium is derived from the embryo in at least the majority of digeneans.

An ultrastructural study of alimentary tract development in the cercariae of Diplostomum pseudospathaceum (Digenea: Diplostomidae)

Morphogenesis of the alimentary canal in developing Diplostomum pseudospathaceum cercariae has been studied using electron microscopy. The foregut primordium appears in early cercariae as a cellular cord. Later, the lumen develops within the foregut primordium, and its cells give rise to the cellular epithelium, limiting this lumen. During subsequent development, the lateral plasma membranes, separating the cells, disappear from the primary foregut epithelium as do nuclei and most of cellular organelles. These events seem to progress in several steps, and eventually, the foregut lining becomes the thin anucleate syncytial layer. In late cercariae, this layer becomes connected with nucleated cytons, producing secretory inclusions. Each of two caecal branches appears to arise from a row of large cuboid cells. The primordial gastrodermal cells are involved in synthetic and secretory activity and give rise to numerous secretory inclusions. These inclusions release their contents into the cavities which develop between adjacent primordial caecal cells. The intercellular cavities gradually increase in size and fuse to eventually form a single caecal lumen. In mature cercariae, the large caecal lumen packed with electron-dense secretory material is limited by a thin cellular gastrodermis, displaying no secretory features.

Protobranch bivalves

The subclass Protobranchia comprises more than 600 species of bivalves that occur throughout the world ocean. Mostly deposit feeders in soft sediments, they are abundant in the deep sea. Apomorphies that unite them as a group include gill structure, hinge conformation, shell microstructure, larval development, foot morphology, respiratory pigments, trophic mode and digestion. They are relatively small and highly conserved in form, originating in the Cambrian era. They may represent an ancestral, derived or paraphylectic group of the Bivalvia. The protobranchs include two orders, the Nuculoida and Solemyoida, which previously were classified separately in the subclasses Paleotaxodonta and Cryptodonta, respectively. They are of ecological interest and have a unique functional morphology. They feed mostly under the surface of the sediment with highly modified labial palps, but the degree to which they are selective in diet remains difficult to determine. They are important bioturbators in many soft-sediment assemblages; their feeding and locomotion affects sediment structure and community development. Solemyoids are unusual in inhabiting reducing environments and hydrocarbon seeps and in deriving their nutrition from endosymbiotic chemosynthetic bacteria. A variety of species of protobranchs are found in oceanic trenches, near hydrothermal vents, and in submarine caves. Protobranchs produce a lecithotrophic larval stage, the pericalymma, making their development unique among bivalves. The pericalymma remains in the plankton for a short time and presumably has low dispersal ability. Recruitment may be intermittent. Growth is rapid in post-larvae but decreases with age, though rates may not necessarily be slow, especially in continental shelf species. Life spans are commonly 1 to 2 decades, but deep-sea representatives may grow more slowly and live longer. Bottom fish, seastars and gastropods are their major predators and a few parasites and commensals have been documented. The predominance of protobranchs in deep-sea sediments may be a result of deep-sea origin or displacement from shallow waters by lamellibranchs. Their ability to deposit-feed, digest food extracellularly, and develop by means of lecithotrophic larvae make them particularly well adapted to cold and oligotrophic habitats.

Ultrastructure of the digestive tract of Gyliauchen nahaensis (Platyhelminthes, Digenea), an inhabitant of the hindgut of herbivorous fishes

Digenean parasites of vertebrates usually amplify the surface area of their gut by increasing the size of the absorptive caeca. Some members of the family Gyliauchenidae, however, have relatively small caeca but have a greatly expanded foregut. The morphology of the elongate gut of the digenean Gyliauchen nahaensis, an inhabitant of herbivorous fish of the family Siganidae, was examined by light and transmission electron microscopy. The extensive foregut, consisting of a mouth, pharynx, and esophagus, is lined with a syncytial tegument-like lining, which is connected to nucleated cell bodies sunken in the parenchyma. The apical cytoplasm in the mouth and anterior regions of the pharynx resembles that of the general body tegument, although some regional specialization is present. The lining of posterior regions of the pharynx is armed with large apical projections, which are thought to serve as filtration structures. The lining of the anterior and middle esophagus displays a peculiar form of surface amplification involving the formation of elongate flask-shaped invaginations of the apical cytoplasm. The cell bodies associated with these regions are rich in secretory vesicles and it is proposed that these regions of the esophagus are expanded to promote extracellular digestion. The posterior region of the esophagus lacks the invaginations of other esophageal regions, but displays instead large surface projections. The caeca consists of columnar cells lined by extensive apical microlamellae. The peculiar gut morphology of G. nahaensis, coupled with alterations in the arrangement of suckers, is interpreted to be an adaptation to the predominantly herbivorous diets of the definitive hosts.

Ultrastructure study of the intramolluscan stages of Meiogymnophallus minutus (digenea: gymnophallidae) in Scrobicularia plana (bivalvia) from Portugal

The ultrastructure of the tegument of the sporocyst and contained cercariae of Meiogymnophallus minutus found in Scrobicularia plana collected in Ria de Aveiro estuary (Portugal) were studied by scanning (SEM) and transmission electron microscopy (TEM). TEM revealed microvilli extending from the surface of the sporocyst tegument that contributed to the lacetrimmed surface morphology observed by SEM. The cercarial surface was papilliform and covered densely with single-pointed tegumental spines. On TEM, besides the spines and several mitochondria, translucid and electrondense spherical secretory vesicles were evident in the matrix of the tegument but were not seen on the sporocyst tegument. Uniciliate sensory receptors found in the cercaria of M. minutus were examined under SEM and TEM. Structural differences between the oral and the ventral sucker were observed. On the lip of the ventral sucker, large and strong spines were arranged in concentric rows, and two groups of six sensory papillae were placed concentrically and equidistantly. The oral sucker, surrounded by a crown of dense and long microvilli, exhibited a ring of uniciliate sensory papillae and some tegumental spines along the lip. The surface morphology of the M. minutus cercaria is compared with that of other Gymnophallidae cercariae.

Ontogenetic changes in helminth membrane function

During their life-cycle many parasites experience a wide range of environments including free living and those provided by a variety of intermediate and final hosts. The nutritional requirements of parasites are met by physiological processes adapted to exploit the physicochemical characteristics provided by different hosts. In helminth parasites these adaptations are frequently expressed on the tegumentary surface. As an example of adaptations within the Trematoda, the control of monosaccharide transport in Proterometra sp. is described. Environmental sodium, although not directly involved in the uptake process, nevertheless regulates the expression of transport capabilities. In the Cestoda, the uptake of monosaccharides and amino acids is described for Hymenolepis diminuta. The metacestode of this tapeworm inhabits the blood system of an arthropod, and the adult the gut of a mammal. There are quantitative and qualitative differences in the amino acids and monosaccharides in these two environments and these are reflected in the transport mechanisms exhibited by the two forms of the life-cycle. In Echinococcus granulosus the transfer of amino acids, sugars and macromolecules across the membranes of hydatid cysts and protoscoleces is described. The major difference between these two stages in the life-cycle relates to the ability of hydatid cysts to absorb macromolecules, whereas protoscoleces are impermeable to these compounds. The potential for future work is emphasized.

Effect of praziquantel on larval stages of Schistosoma japonicum

The effect of praziquantel on S. japonicum mother sporocysts, daughter sporocysts and cercariae was studied. At concentrations of 3 X 10(-7), 3 X 10(-6) and 3 X 10(-5) M and treatment times of 24 or 48 h, mother and daughter sporocysts and young cercarial embryos were not affected but nearly mature cercariae were killed and dissociated. The resistance of young cercariae could support the suggestion that the primitive cercarial epithelium arises from the sporocyst tegument. Treatment with praziquantel always stopped cercarial emission; this cessation lasted for a few days with the lowest concentration and for up to 25 d with the highest. The duration of treatment slightly affected the pattern of reappearance of cercariae but markedly affected the long-term reduction in numbers. Free cercariae treated with praziquantel lost their tails in 10 to 60 min, depending on the concentration.