Electron and light microscopic study of various glands and the secretions released into the environment by the turbellarian Urastoma cyprinae

Abstract

The turbellarian Urastoma cyprinae (Graff) occurs on the gills of various bivalve species including the mussel Mytilus galloprovinciallis, where it is known to cause serious damage. More recently, it has been shown that the worms are strongly attracted to the gill of the American oyster (Crassostrea virginica) and are capable of inducing changes to the composition of proteolytic enzymes of the host mucus. Such changes may be attributable to secretory products released by the worms. Mucous glands (11-18 mum in diameter) produce minute spherules (0.7-0.9 mum in diameter) tightly bound together. The glands occupy approximately 20% of the body volume and are the most voluminous secretory organs in the worm. The smaller rhabdoid glands are unevenly distributed throughout the peripheral parenchyma and contain secretory granules of 0.35-1.2 mum in diameter. The latter occur most prominently along the distal margins of the epidermis. The frontal pole of U. cyprinae consists of a complex assembly of mucous and rhabdoid gland cells as well as other glandular structures. Collectively, these bodies release their contents to the outside via narrow gland necks. The overall organization is consistent with the frontal gland previously described for other free-living turbellarians, including other rhabdocoels. A variety of secretory products, displaying variations in staining properties, have likewise been identified in association with the body wall from other regions of the worm. This work attempts to gain a better appreciation of the secretory structures associated with the worm tegument, focusing primarily on the widespread mucous and rhabdoid glands. The secretions play a role in host-parasite interactions.

Reaction of the mussel Mytilus galloprovincialis (Bivalvia) to Eugymnanthea inquilina (Cnidaria) and Urastoma cyprinae (Turbellaria) concurrent infestation

“…Our results speak in favor of harmful interaction of the turbellarian with the host gills, the later in turn succeeding, even when parasite numbers are very high, to sequestrate turbellarian inside a cyst originating from mussel hemocytes, disabling its movements and consequent gill damage. Since we were not able to evidence any absorption processes through turbellarian epidermis as described by Bataller et al (2003), we suggest that food consumption within hemocyte capsule is limited. Such capsule was composed of hemocytes, whose role resides in internal defense, such as recognition and phagocytosis or encapsulation of nonself materials (Foley and Cheng, 1974), and coagulation (Moore and Lowe, 1977;Nakayama et al, 1997) that could be engaged in the encapsulation process of U. cyprinae.…”

“…Such capsule was composed of hemocytes, whose role resides in internal defense, such as recognition and phagocytosis or encapsulation of nonself materials (Foley and Cheng, 1974), and coagulation (Moore and Lowe, 1977;Nakayama et al, 1997) that could be engaged in the encapsulation process of U. cyprinae. Encapsulation of the turbellarian within ''small mucus-looking covering'' or ''mucus coat'' has been previously described (Winstead et al, 2004;Crespo-González et al, 2010), but authors have suggested that the envelope derived from the secretion of turbellarian mucus glands (Bataller et al, 2003). Although the later have evidenced mucus secretion by TEM, turbellarians were not fixed in situ with bivalve gills in order to prove that the secreted mucus formed turbellarian envelope, whilst other authors (Winstead et al, 2004;Crespo-González et al, 2010) did not use TEM to define cyst origin.…”

Virus-like particles in Urastoma cyprinae, a turbellarian parasite of Mytilus galloprovincialis

“…They are arranged in large paracrystalline arrays that, in some cases, extended practically throughout the whole cytoplasm. The morphology of the cells containing VLPs, the electrondensity pattern of their intracytoplasmic granules, and the abundant endoplasmic reticulum observed in their cytoplasm suggest that they are probably rhabdoid cells that belong to the glandular system of U. cyprinae (Noury-Sraïri et al 1990, Bataller et al 2003. The VLPs are isometric, not enveloped, about 24 to 30 nm in diameter and present a typical icosahedral symmetry (Fig.…”

“…During the course of our investigations on the morphology of U. cyprinae, we detected the presence of virus-like particles (VLPs) in the cytoplasm of some subepidermal cells of adult turbellarians. The worms were isolated from the gills of the mussels M. galloprovincialis collected from the rocky intertidal zone in the south of the Vigo estuary (NW Spain), and processed for transmission electron microscopy (TEM) according to the protocol described by Bataller et al (2003). Briefly, the worms were fixed for 2 h in 6% glutaraldehyde in 0.4 M cacodylate buffer and postfixed for 2 h in 1% osmium tetroxide in the same buffer.…”