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Parasitología al día

versión impresa ISSN 0716-0720

Parasitol. día v.24 n.1-2 Santiago ene. 2000

http://dx.doi.org/10.4067/S0716-07202000000100010 

Surface ultrastructure of Stictodora fuscatum
(Trematoda: Heterophyidae) from Kuwait Bay

ASEM ABDUL-SALAM*, SREELATHA BHASKARAN NAIR* and HAMID ASHKANANI*

ABSTRACT

The surface ultrastructure of adult Stictodora fuscatum recovered from a kitten fed with mullets, Liza macrolepis was studied by scanning electron microscopy. The body was pyriform and concave ventrally. The surface of the body displayed cobblestone-like cytoplasmic processes. Scale-like multipointed spines digitated into 9-12 points covered the anterior surface of the body. The tegumental spines on the posterior surface of the body were smaller, simple-pointed and sparsely distributed. The areas dorsal to the oral sucker and around the excretory pore were spineless. Ciliated sensory papillae were disposed solitary or as conjugated groups on the rim of oral sucker and on anterolateral surface of the body. Unciliated sensory papillae appeared on the lower lip of oral sucker. The rim of oral sucker was interrupted laterally by tegumental spines extending into the inner surface. The gonotyl was armed with chitinous spines composed of 3-4 rows appearing as "cockscomb".
Key words: Stictodora fuscatum, Heterophyidae, tegument, scanning electron microscopy.

INTRODUCTION

Trematodes of the family Heterophyidae comprise small flukes found predominantly in the intestine of fish-eating birds and mammals including humans. Snails and fishes are involved in the life-cycles of these flukes as the first and the second intermediate hosts, respectively. The heterophyid, Stictodora fuscatum (Onji and Nishio, 1916) has been reported from the Far East. The adult fluke is characterised mainly by absence of the ventral sucker and presence of a gonotyl armed with 15-25 chitinous spines disposed in a semicircular configuration. In Korea, S. fuscatum metacercaria has been reported in gobies and mullets,1, 2 and the adult was collected from humans and experimental cats.2, 3

Although the tegumental ultrastructure of several heterophyids has been examined4-8, the surface of S. fuscatum has not been studied so far. The present study was undertaken to examine the surface ultrastructure of adult S. fuscatum.

MATERIAL AND METHODS

Mullets, Liza macrolepis infected with heterophyid metacercariae were caught with small mesh trap nets from the intertidal zone of Sulaibikhat, southern Kuwait Bay, in July 1998 and fed to a kitten. Adult S. fuscatum were recovered from the kitten at 7 days post-infection, washed with cold physiological saline and fixed in a solution containing 4% formaldehyde and 1% glutaraldehyde in 0.1 m phosphate buffer (pH 7.2) at 4°C. After washing, the specimens were post fixed in 1% osmium tetroxide in the same buffer for 5 min at 4°C, dehydrated in a series of anhydrous acetone and critical point dried. The specimens were coated with gold and then examined in a Jeol JSM-6300 scanning electron microscope. Type specimens are deposited in The Natural History Museum, London, British Museum (Natural History) 1999.4.15.1.

RESULTS AND DISCUSSION

The body was pyriform and concave ventrally (Figure 1). The surface of the body displayed cobblestone-like cytoplasmic processes (Figure 2), and was covered with scale-like multipointed (also designated as sawtooth or brush-shaped) tegumental spines. The tegumental spines were dorsoventrally flat and digitated into 9-12 points (Figure 3). The tegumental spines on the posterior surface of the body were smaller, simple-pointed and sparsely distributed. The area around the excretory pore was spineless (Figure 4). The morphology and distribution of the spines on the ventral surface were similar to those on the dorsal surface.

Figure 1. Ventral view showing oral sucker, excretory pore and tegumental texture of the body surface. Scale bar = 50 µm.


Figure 2. Anteroventral view of slightly contracted specimen showing cobblestone-like cytoplasmic processes. Scale bar = 20 µm.


Figure 3. Anteroventral surface between oral sucker and genital opening showing multipointed tegumental spines. Scale bar = 2 µm.


Figure 4. Posterior end showing minute simple-pointed spines and a spineless area around the excretory pore. Scale bar = 10 µm

Two types of sensory papillae were observed on the body. Type I papillae consisted of tegumental swelling with a short apical cilium (Figure 5). Type I papillae were found solitary or as conjugated groups encompassing up to six papillae (Figure 6), arranged in bilaterally symmetrical pattern on the rim of oral sucker and on lateral surface of anterior half of the body (Figure 7). Type I papillae were also present sparsely on the dorsal surface of the body. Type II papillae of unciliated tegumental swelling were confined to the lower lip of oral sucker (Figure 7). Oral sucker was subterminal and its rim was interrupted laterally by tegumental spines extending into the inner surface of the lip (Figure 8). The surface of the lip of the oral sucker was corrugated radially and devoid of spines. The area dorsal to the oral sucker was spineless (Figure 7). The genital opening appeared as a round or slit-like depression of the tegument at anterior one-third of the body. The gonotyl was armed with chitinous spines composed of 3-4 rows of spines appearing as "cockscomb" (Figure 9). The gonotyl was seldom observed protruding from the genital opening. The excretory pore was devoid of spine and located at the posterior end of the body (Figures 1 ,4).

Figure 5. Ventrolateral surface middle of the body showing scale-like multipointed spines and ciliated dome-shaped papillae (type I) disposed solitary or in conjugated groups. Scale bar = 5 µm.


Figure 6. Anterodorsal region showing multipointed spines and a conjugated form of type I papillae. Scale bar = 2 µm


Figure 7. Anterior end showing arrangement of the tegumetal papillae and spines around the oral sucker. Note a spineless area just anterior to the oral sucker. Scale bar = 10 µm.


Figure 8. The oral sucker showing two types of sensory papillae on the lips, type I and rough domes without cilia (type II). Note the rim is interrupted laterally by tegumental spines extending into the inner surface of the upper lip. Scale bar = 10 µm.

Figure 9. The genital opening showing a protruded gonotyl with finger-like round-ended spines and sperms tangled. Scale bar = 10 µm.

The surface ultrastructure of adult S. fuscatum compliments the light microscopic observation of specimens from experimentally infected cats.1, 2 Observations on the surface ultrastructure of adult heterophyids are available for species representing the genera Centrocestus,7, 9 Cryptocotyle,10 Haplorchis,4 Heterophyes,6, 8 Heterophyopsis,5 Metagoni-mus11 and Pygidiopsis.12 The tegumental ultrastructure of S. fuscatum was generally similar to those of other heterophyids in shape of the spines, and types and distribution pattern of the sensory papillae. However, the oral sucker armed with prominent tegumental spines inner surface of the upper lip was characteristic for S. fuscatum. The multipointed spines inside the oral sucker are probably involved in abrasion of host tissues for feeding, while the tegumental spines on the anterior half of body may help with anchoring the trematode to the crypts of the small intestine. The proposed involvement of the tegumental spines in the attachment process may substitute for the absence of a proper ventral sucker in S. fuscatum.

Ciliated (type I) and unciliated (type II) sensory papillae observed on S. fuscatum are common on the tegumental surfaces of metacercaria and adult of heterophyids. The exact function of these papillae is still obscure, although functions as tango- and chemo-receptors have been inferred from their shapes and positions on the body, and from the results of transmission electron microscopy showing sensory bulbs associated with both types of the papillae.13 As speculated for other heterophyids, the papillae abundant on and around the oral sucker of S. fuscatum are probably tactile sensory receptors involved in mechanisms of feeding and attachment. Studies on the development of heterophyids have shown that during transformation from the metacercaria to the adult stage changes occur in shape, structure and density of the tegumental spines and sensory papillae, and in the on the appearance of the tegumental cytoplasmic processes. These changes have been tentatively explained as morphological adaptations to different hosts environs.5, 6, 9, 10, 11 The present study revealed for the first time the ultrastructural topography of the chitinous spines on the gonotyl of a Stictodora species. The spines resemble those reported on the gonotyl of Heterophyes heterophyes.8 It is imperative to note that the number and arrangement of the gonotyl spines are considered one of the most important characteristics for the identification of Stictodora species.14

Acknowledgement. We wish to thank the staff of the Electron Microscopy Unit, Faculty of Science, Kuwait University for their help and guidance with the SEM aspects of the study.

* Department of Biological Sciences, Kuwait University, P.O. Box 5969, Safat, 13060, Kuwait. E-mail: research@kfas.org.kw

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