An ultrastructural study of the cercarial excretory system in Bucephaloides gracilescens and Prosorhynchus squamatus

Ultrastructural characteristics of the protonephridial terminal organ and associated ducts of adult specimens of the Aspidogastrea, Digenea and Monogenea, with comments on the relationships between these groups

Transmission electron microscopical observations were made on the protonephridial terminal organ and associated ducts of three adult trematodes, the aspidogastrean Aspidogaster limacoides Diesing, 1835 and the digeneans Azygia lucii (Müller, 1776) and Phyllodistomum angulatum Linstow, 1907, and the monogenean Ancyrocephalus paradoxus Creplin, 1839. Previously unreported ultrastructural details of the terminal organ of adult trematodes include multiple contact sites (septate junctions and zonulae adherentes) between the membranes of the terminal and adjacent canal cells. Septate junctions traverse the epithelial cytoplasm of the canal wall, and the same type of septate junctions are observed within the cytoplasmic cord at the level of the tip of the flame tuft in both longitudinal and oblique sections of all three trematode species studied. In the monopisthocotylean Ancyrocephalus paradoxus, the absence of any junctions in the cytoplasmic cord and the presence of septate junction within all of the protonephridial ducts are reported. On the basis of the small number of monogenean species in which these features have been studied, in relation to the size of the group, there seems to be a high diversity in some characters of the protonephridial terminal organ. The study confirms that the Aspidogastrea and Digenea possess the same morphology of their protonephridial terminal organ and, although this differs slightly from that of most members of the Monogenea so far studied, it supports previous views on the close relationship of these groups.

Morpho-functional specialization of the branching sporocyst of Prosorhynchoides borealis Bartoli, Gibson & Bray, 2006 (Digenea, Bucephalidae)

Sporocysts of Prosorhynchoides borealis were obtained from the marine bivalves Abra prismatica and studied using transmission electron microscopy. The sporocyst body consists of a mass of branching and intertwining hollow tubules that ramify through the host's digestive gland and gonads. This study investigated the ultrastructure of the sporocyst branches which comprise alternate distended areas (brood chambers) with a relatively thin body wall, narrower portions with a thicker body wall (constricted areas) and terminal regions. Pronounced differences between these areas were revealed in the structure of their tegument and body cavity lining, as well as in the cellular composition of the subtegumental layers. Body wall composition in distended areas was consistent with the specialization for cercarial nurture in the brood chambers. The structure of the constrictions suggested a dual role of nutrient absorption and physical separation of adjacent brood chambers. Two types of terminal region were identified, one specialized for the investigation and penetration of host tissues and the other, in which the germinal cells are formed, for cercarial production. The overall structure of the sporocyst branches helps explain why this linear modular system, i.e. brood chambers and constrictions continuously growing into the host tissue, enables the sporocyst's long-term existence and can continuously produce cercariae in numbers comparable with those produced by rediae and/or daughter sporocyst infrapopulations in other digeneans. The origin of the nuclei in the outer tegumental layer of some branching bucephalid sporocysts is also discussed.

Topography and ultrastructure of the tegument of Bucephalus anguillae (Digenea: Bucephalidae), a parasite of the European eel Anguilla anguilla (Osteichthyen: Anguillidae)

The tegumental ultrastructure of the intestinal fluke Bucephalus anguillae was studied with the use of scanning and transmission electron microscopy. The surface of the tegument is covered by transverse ridges from which protrude numerous closely packed, digitated, and claw-shaped spines. Cobblestone-like units of the tegument were observed on the crescent-shaped formation of the rhynchus and at the posterior part of the body. Three types of sensory structures were examined, i.e., 2 uniciliated receptors and 1 without cilia. As anterior-posterior differences were observed, particular attention was given to spines and sensory receptors. Spine insertion zones and average cilia length are variable between anterior and posterior tegument areas. Ultrastructural study revealed that the tegument of B. anguillae has a typical syncytial organization with a distal cytoplasm lying over a basal matrix and cytons below. Cytoplasmic bridges allowed transit of secretory vesicles and granules. Diagrams of spines and sensory receptors were made to help in understanding the nature of these structures.