Architecture of Paradiplozoon homoion: A diplozoid monogenean exhibiting highly-developed equipment for ectoparasitism.

Diplozoidae (Monogenea) are blood-feeding freshwater fish gill ectoparasites with extraordinary body architecture and a unique sexual behaviour in which two larval worms fuse and transform into one functioning individual. In this study, we describe the body organisation of Paradiplozoon homoion adult stage using a combined approach of confocal laser scanning and electron microscopy, with emphasis on the forebody and hindbody. Special attention is given to structures involved in functional adaptation to ectoparasitism, i.e. host searching, attachment and feeding/metabolism. Our observations indicate clear adaptations for blood sucking, with a well-innervated mouth opening surrounded by sensory structures, prominent muscular buccal suckers and a pharynx. The buccal cavity surface is covered with numerous tegumentary digitations that increase the area in contact with host tissue and, subsequently, with its blood. The buccal suckers and the well-innervated haptor (with sclerotised clamps controlled by noticeable musculature) cooperate in attaching to and moving over the host. Putative gland cells accumulate in the region of apical circular structures, pharynx area and in the haptor middle region. Paired club-shaped sacs lying laterally to the pharynx might serve as secretory reservoirs. Furthermore, we were able to visualise the body wall musculature, including peripheral innervation, the distribution of uniciliated sensory structures essential for reception of external environmental information, and flame cells involved in excretion. Our results confirm in detail that P. homoion displays a range of sophisticated adaptations to an ectoparasitic life style, characteristic for diplozoid monogeneans.

Eudiplozoon nipponicum in focus: monogenean exhibiting a highly specialized adaptation for ectoparasitic lifestyle.

Developmental stages of the diplozoid monogenean Eudiplozoon nipponicum, comprising oncomiracidium, diporpa, juvenile, and adult, were investigated using light and scanning electron microscopy in conjunction with confocal scanning laser microscopy in order to examine body organization and identify explicit morphological adaptations to the ectoparasitic life in each stage. The parasite exhibits a complex digestive tract well equipped for hematophagous feeding. It consists of a mouth opening with prominent buccal suckers, eversible pharynx with adjacent glandular structures, and a blind-ending gut with cecal lining. Glandulo-muscular organs, located apically and opened into the mouth corner, are considered to be a part of the digestive tract. Based on our observations of pharynx eversion and in light of the presence of several glandular or gland-like structures, we propose a new hypothesis on the possibility of extracorporeal digestion of this parasite. The hindbody bears an attachment apparatus, comprising haptor, lobular extensions, and tegumental folds, responsible for the parasite's firm attachment to the host gills. The possibility of buccal suckers assisting in the parasite's translocation while searching for an optimal niche or their temporary attachment function during feeding is discussed. The body of each compound adult (i.e., permanent copula) is almost completely filled by two complete reproductive tracts comprising the female as well as male organs. Such a reproductive strategy, in which two independent heterogenic individuals fuse into a single hermaphrodite organism without the need to search for mating partner, represents a high specialization of diplozoids to their parasitic life.