Date Published: February 7, 2018
Publisher: Public Library of Science
Author(s): Iveta Hodová, Radim Sonnek, Milan Gelnar, Andrea Valigurová, Claude Prigent.
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.
Monogenea Bychowsky 1937 are among the most species-rich groups of fish parasites . Monogenean parasites display a direct life cycle, lacking alternation of generations or hosts. Host specificity in the group is well defined, with morphological adaptations to the attachment organs often restricting species to a particular host and/or a very narrow niche . Blood-feeding freshwater fish gill ectoparasites of the family Diplozoidae occupy a unique position amongst monogenean taxa as they exhibit extraordinary body morphology and have a life cycle involving permanent fusion of two larval worms that subsequently transform into a single individual. As such, they represent an attractive model for evolutionary and morphological studies. The first morphological studies on diplozoids were published more than 120 years ago [3–5]. To date, the extensive work of Bovet  and Khotenovsky  still represents the most comprehensive morphological and taxonomical studies of diplozoid monogeneans. More recent reviews provide useful information on general and functional morphology of monogeneans [8,9]. Numerous studies have already targeted their life cycle and pairing process [10–17], while the other focused on molecular biological [18–22] and karyological [23,24] analyses of representatives from the family Diplozoidae. On the top of that, few immunomicroscopical observations of the diplozoid nervous system were published [14,25,26]. Recent biochemical analyses deal with the blood digestion in diplozoids [27,28].
This study focuses on individuals that have already paired and formed the juvenile/adult stages. As in other members of the family Diplozoidae, the body of the P. homoion in the adult stage typically resembles a letter X. This X-shaped body is comprised of two forebodies and two hindbodies along with the haptors of the two fused individuals (Fig 1A).
Ectoparasitic diplozoid monogeneans exhibit a range of unique biological characteristics and sophisticated functional adaptations to their bloodsucking life style. The most significant of these structures are those related to host searching, attachment, feeding/metabolism, pairing and protection against host responses [12,17]. As previous microscopy studies have shown the benefits of CLSM analysis with fluorescent labelling for detection of specific structures, we used phalloidin labelling of F-actin for visualisation of muscle structures and tubulin staining for detection of the nervous or excretory systems [40,45].