Ultrastructural observations of the attachment organs of the monogenean Rajonchocotyle emarginata (Olsson, 1876) (Polyopisthocotylea: Hexabothriidae), a gill parasite of rays

Abstract

The present study uses scanning and transmission electron microscopical observations to examine the haptoral features of the hexabothriid polyopisthocotylean monogenean Rajonchocotyle emarginata from the gills of the elasmobranch Amblyraja radiata. The haptor possesses six equal, pedunculate suckers, each armed with a large, curved, hooked sclerite, and an appendix bearing two small, terminal suckers and a pair of minute hamuli. The outer side of the distal region of the hooked sclerite forms a large bulge along the antero-lateral side of each sucker and the proximal region forms a small bulge on the opposite postero-lateral side; part of the large bulge, which covers the small sclerite hook, surmounts the sucker rim. The sucker sclerite is situated outside the main sucker muscle complex of the sucker wall and is surrounded by its own muscular envelope; close to the sclerite itself, long, tightly packed lamella-like processes of sarcoplasm are present. A muscular sphincter surrounds the distal region of the sclerite shaft. The sclerite consists of three heterogeneous layers, but its hook region is distinguished from the shaft by the presence of an additional homogeneous layer. Hamuli within the appendix consist of three distinct layers loosely surrounded by sarcoplasmic lamellae. The luminal surface of the appendicular suckers is covered with unusual, long, thin, interconnected surface protrusions. In all other regions of the haptor, the tegumental surface is smooth, with numerous shallow pores penetrating the "terminal web" of the syncytial cytoplasm. Unicellular gland cells are localized close to the base of the appendix, with their ducts, containing non-homogenous secretory bodies, opening onto the anterior haptoral surface. Two types of sense receptors are visible on the haptoral surface. The ultrastructural features of the haptor are discussed in relation to our understanding of their function and the evolutionary relationships of the basal polyopisthocotylean monogenean groups Hexabothriidae and Chimaericolidae.

An ultrastructural study of the surface and attachment structures of Paradiplozoon homoion (Bychowsky & Nagibina, 1959) (Monogenea: Diplozoidae)

“…Our findings on P. homoion correlate with observations on the clamps of other heteronchoinean monogeneans in that the clamp frame is usually formed of several electron-dense sclerites bound by radial muscle fibers, the surface is covered with a very thin outer syncytial layer, and nerve fibers are located close to the terminal and basal parts of the sclerites [26–28]. Association of nerve plexuses with the clamps has also been observed in E. nipponicum (a species closely related to P. homoion ) using immunostaining and microscopic observation [54].…”

“…These clamps are morphologically distinct and can be used for species determination [19–22]. While the ultrastructure of heteronchoinean clamps has been investigated previously [23–28], ultrastructural data for diplozoids are presently missing. In general, the clamp wall surrounding the inner sclerites is formed of a specialized muscle complex with radially oriented muscle fibers [22].…”

Spermiogenesis and spermatozoon ultrastructure in basal polyopisthocotylean monogeneans, Hexabothriidae and Chimaericolidae, and their significance for the phylogeny of the Monogenea

“…In this paper we present, for the first time, observations on two species of the family Hexabothriidae (Diclybothriidea) and one of the family Chimaericolidae (Chimaericolidea), thus filling the gaps in our knowledge of sperm ultrastructure in the Monogenea. These observations complement previous studies on the tegument [ 88 ], attachment organs [ 84 , 85 ], reproductive organs [ 83 , 87 ], and digestive system [ 86 ] of the same species.…”

Parasitic flatworms infecting thorny skate,Amblyraja radiata:infection by the monogeneansAcanthocotyle verrilliandRajonchocotyle emarginatain Svalbard

“…Measurement are given in micrometers. [105] off Bergen, Norway [106] Chaleur Bay, Canada [29] off Newfoundland, Canada Rajonchocotyle emarginata Chaleur Bay, Canada [29] Barent Sea Review in [23] Patagonian Shelf [111] off Tromsø, Norway [112] Adventfjorden estuary, 921 Table 3: Uncorrected pairwise genetic distances (%) between Acanthocotyle spp. based on 844bp of 28S rDNA gene portion.…”