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

Abstract

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.

First evidence of serotoninergic components in the nervous system of the monogenean Chimaericola leptogaster (Chimaericolidae, Polyopisthocotylea), a gill parasite of the relict holocephalan fish

“…In our study, muscle fibres intensively stained with phalloidin were observed around the mouth opening of C. leptogaster as well as in the attachment organ and around the reproductive pore and vaginae. The data obtained correspond to those for other monogenean species G. rysavyi, M. congolensis and Paradiplozoon homoion (Bychowsky et Nagibina, 1959), in which muscle fibres were detected in the attachment organs, as well as in the ducts of the digestive and reproductive systems (Arafa et al 2007, Hodová et al 2018.…”

“…According to Cable et al (1996), two pairs of serotoninergic neurons have been found in the haptor of the monogenean Discocotyle sagittata (Leuckart, 1842). The innervation of the attachment organs by serotoninergic nerve fibres was also observed in E. nipponicum, P. integerrimum, P. homoion and M. congolensis (McKay et al 1991, Zurawski et al 2001, Hodová et al 2018.…”

“…The cholinergic, peptidergic and aminergic elements in the nervous system of a number of monopisthocotylean and polyopisthocotylean monogeneans were identified using immunocytochemical and histochemical methods (Arafa et al 2007, Petrov et al 2016, Hodová et al 2018.…”

Molecular Effects of Silver Nanoparticles on Monogenean Parasites: Lessons from Caenorhabditis elegans

“…The tegument is the epidermis of monogeneans; it has several functions, such as absorption and secretion of substances, osmoregulation, mechanical support, and protection against xenobiotics [ 99 ]. The parasite’s tegument is the first contact site for AgNPs, which cause damage at the ultrastructural level [ 12 ].…”

Eudiplozoon Nipponicum: Morphofunctional Adaptations of Diplozoid Monogeneans for Confronting Their Host

“…The prominent annular ridges and extensions of the E. nipponicum hindbody are considered to be important for attachment and securing the position of the parasite among the secondary gill lamellae, somewhat analogous to a zip fastener [11]. Less prominent tegumentary folds and three highly mobile lobes have been described on the haptor of P. homoion [15].…”