Ultrastructural observations on the oncomiracidium epidermis and adult tegument of Discocotyle sagittata, a monogenean gill parasite of salmonids

During their different life stages, parasites undergo remarkable morphological, physiological, and behavioral "metamorphoses" to meet the needs of their changing habitats. This is even true for ectoparasites, such as the monogeneans, which typically have a free-swimming larval stage (oncomiracidium) that seeks out and attaches to the external surfaces of fish where they mature. Before any obvious changes occur, there are ultrastructural differences in the oncomiracidium's outer surface that prepare it for a parasitic existence. The present findings suggest a distinct variation in timing of the switch from oncomiracidia epidermis to the syncytial structure of the adult tegument and so, to date, there are three such categories within the Monogenea: (1) Nuclei of both ciliated cells and interciliary cytoplasm are shed from the surface layer and the epidermis becomes a syncytial layer during the later stages of embryogenesis; (2) nuclei of both ciliated cells and interciliary syncytium remain distinct and the switch occurs later after the oncomiracidia hatch (as in the present study); and (3) the nuclei remain distinct in the ciliated epidermis but those of the interciliary epidermis are lost during embryonic development. Here we describe how the epidermis of the oncomiracidium of Discocotyle sagittata is differentiated into two regions, a ciliated cell layer and an interciliary, syncytial cytoplasm, both of which are nucleated. The interciliary syncytium extends in-between and underneath the ciliated cells and sometimes covers part of their apical surfaces, possibly the start of their shedding process. The presence of membranous whorls and pyknotic nuclei over the surface are indicative of membrane turnover suggesting that the switch in epidermis morphology is already initiated at this stage. The body tegument and associated putative sensory receptors of subadult and adult D. sagittata are similar to those in other monogeneans.

Morphological and molecular characterization of larval digenean trematodes (Parvatrema: Gymnophallidae) and their pathological effects on the clam Leukoma thaca (=Protothaca thaca) (Bivalvia:Veneridae) (Molina, 1782) from northern Chile

Trematodes are one of the largest taxa of mollusk parasites. The clam Leukoma thaca is an economically exploited bivalve found along the south-eastern Pacific coast of Peru and Chile. This bivalve is parasitized by various unidentified larval stages of digeneans in the mantle, gonads and digestive gland. The aims of this study were to determine and describe the different larval stages of the digeneans based on morphological characteristics, to identify them at the species level by performing molecular analyses, and to evaluate pathologies associated with the parasites of this clam. Individuals of L. thaca were collected in San Jorge Bay (23°S), Chile, between November 2018 and February 2019. Morphological description was carried out using in vivo and fixed specimens, and analyses including histological and scanning electron microscopy were performed. Individuals were also isolated for molecular analysis using nuclear ribosomal internal transcribed spacer 1 (ITS1), including partial subunit 18S rDNA (18S) and small subunit 5.8S gene (5.8S). Morphological characteristics indicated that the metacercaria larval stage belongs to the family Gymnophallidae, genus Parvatrema, which was supported by molecular analysis. Molecular results revealed that metacercaria, sporocysts and cercaria stages found in this clam belong to the same species of Parvatrema (genetic distance 0%), evidencing that this species uses L. thaca as the first and second intermediate host. Pathologies examined in the host were similar in nature to those reported in other gymnophallids in bivalves, but high prevalence of cercariae (20%) in gonads suggested an important castrator effect on the host.

Ultrastructural patterns of the excretory ducts of basal neodermatan groups (Platyhelminthes) and new protonephridial characters of basal cestodes

BACKGROUND

The flatworms (Lophotrochozoa: Platyhelminthes) are one of the major phyla of invertebrates but their interrelationships are still not well understood including unravelling the most closely related taxon of the Neodermata, which includes exclusively obligate parasites of all main groups of vertebrates with some 60,000 estimated species. Recent phylogenomic studies indicate that the freshwater 'microturbellarian' Bothrioplana semperi may be the closest ancestor to the Neodermata, but this hypothesis receives little morphological support. Therefore, additional morphological and ultrastructural characters that might help understand interrelations within the Neodermata are needed.

METHODS

Ultrastructure of the excretory ducts of representatives of the most basal parasitic flatworms (Neodermata), namely monocotylid (Monopisthocotylea) and chimaericolid (Polyopisthocotylea) monogeneans, aspidogastreans (Trematoda), as well as gyrocotylidean and amphilinidean tapeworms (Cestoda), were studied using transmission electron microscopy (TEM).

RESULTS

The present study revealed the same pattern of the cytoarchitecture of excretory ducts in all studied species of the basal neodermatans. This pattern is characterised by the presence of septate junctions between the adjacent epithelial cells and lateral ciliary flames along different levels of the excretory ducts. Additionally, a new character was observed in the protonephridial terminal cell of Gyrocotyle urna, namely a septate junction between terminal and adjacent duct cells at the level of the distal extremity of the flame tuft. In Amphilina foliacea, a new type of protonephridial cell with multiple flame bulbs and unique character of its weir, which consists of a single row of the ribs, is described. A remarkable difference has been observed between the structure of the luminal surface of the excretory ducts of the studied basal neodermatan groups and B. semperi.

CONCLUSIONS

The present study does not provide ultrastructural support for a close relationship between the Neodermata and B. semperi.

Phylogenetic Position of Codonocephalus Diesing, 1850 (Digenea, Diplostomoidea), an Unusual Diplostomid with Progenetic Metacercariae

Codonocephalus is a monotypic genus of diplostomid digeneans and is the only genus in the sub-family Codonocephalinae. The type-species Codonocephalus urniger has an unusual progenetic metacercaria that uses frogs as intermediate hosts and can use snakes as paratenic hosts. Adult C. urniger parasitize ardeid wading birds in the Palearctic. Despite the broad distribution of Codonocephalus, no DNA sequence data are currently available for the genus. In this study, we generated sequence data for nuclear ribosomal and mitochondrial DNA from progenetic metacercaria of the type-species C. urniger from marsh frog, Pelophylax ridibundus, collected in Ukraine. We used partial sequences of the nuclear ribosomal 28S gene to examine for the first time the phylogenetic position of Codonocephalus among the Diplostomoidea.

Neotropical Turtle Blood Flukes: Two New Genera and Species from the Amazon River Basin with a Key to Genera and Comments on a Marine-Derived Parasite Lineage in South America

Two new genera and species of freshwater turtle blood flukes (TBFs) are described herein based on specimens infecting the nephritic and mesenteric blood vessels of "matamatas" (a side-necked turtle, Chelus fimbriata [Schneider, 1783] [Pleurodira: Chelidae]) from the Amazon River Basin, Peru. These taxa comprise the first-named species and the first-proposed genera of freshwater TBFs from the continent of South America. A new comparison of all TBF genera produced 6 morphologically diagnosed groups that are discussed in light of previous TBF classification schemes and a novel phylogenetic hypothesis based on the nuclear large subunit ribosomal DNA (28S). Considering external and internal anatomical features, species of the new genera (Atamatam Bullard and Roberts n. gen., Paratamatam Bullard and Roberts n. gen.) are most similar to each other and are together most similar to those of several marine TBF genera. The 28S phylogenetic analysis supported the monophyly of all 6 morphologically diagnosed groups of genera. Most notably, the freshwater TBFs of South America comprise a derived group nested within the clade that includes the paraphyletic marine TBFs. Not surprisingly in light of morphology, another marine TBF lineage (Neospirorchis Price, 1934) clustered with the freshwater TBFs of Baracktrema Roberts, Platt, and Bullard, 2016 and Unicaecum Stunkard, 1925. Our results, including an ancestral state reconstruction, indicated that (1) freshwater TBFs have colonized marine turtles twice independently and that (2) the South American freshwater TBFs comprise a marine-derived lineage. This is the first evidence that TBFs have twice independently transitioned from a marine to freshwater definitive host. Marine incursion is considered as a possible mechanism affecting the natural history of marine-derived freshwater TBFs in South America. A dichotomous key to accepted TBF genera is provided.

Characterization of Clinostomum metacercariae using microscopic and molecular approaches

One of the fundamental aspects in understanding the biology, diversity and epidemiology of a parasite lies in its proper identification. In the present study, morphological and molecular characterization of Clinostomum metacercariae recovered from an ornamental fish, Trichogaster fasciata, was carried out in order to ascertain its identity. To serve the purpose, scanning electron micrographs and gene sequences for two commonly used molecular markers, i.e., nuclear ribosomal internal transcribed spacer 2 (rDNA-ITS2) and mitochondrial cytochrome c oxidase subunit 1 (mtCO1) were obtained. The sequences were further used for generating similarity index matrix as well as inferring phylogenies. Light and electron microscopic observations on metacercariae of the parasite revealed that it belongs to the genus Clinostomum. Identification of the same up to the level of species was made possible through sequence and phylogenetic analyses. The ITS2 sequence analyses of our species (KX758630) showed similarity to unidentified Clinostomum sp. reported from Nigeria (KY865625) and China (KP110579), and C. tilapiae recorded from South Africa (KX034048) and Nigeria (KY649353). However, the CO1 gene analyses suggested it to be highly identical to C. philippinense and the same was also corroborated in the phylogenetic analysis. Thus, morphological and molecular characterization revealed that the recovered metacercariae belong to the species C. philippinense. Additionally, a brief description of secondary structures of ITS2 of various species of Clinostomum has also been discussed.

Functional ultrastructure and cytochemistry of vitellogenesis and mature vitellocytes of the digenean Cainocreadium labracis (Dujardin, 1845), parasite of Dicentrarchus labrax (L., 1758)

Vitellogenesis and vitellocytes of Cainocreadium labracis were studied by transmission electron microscopy (TEM) and TEM cytochemistry. Four developmental stages were distinguished during vitellogenesis: (I) stem cell of high nucleo-cytoplasmic ratio; (II) early differentiation with chief activity focused on the beginning of protein synthesis and shell globule formation; (III) advanced differentiation with rapid intensification of protein synthesis, progressive fusion of single shell globules into large globule clusters, and formation of unsaturated lipid droplets surrounded by β-glycogen particles; and (IV) mature vitellocyte. Early vitellogenesis with vitellocyte maturation consists of: (1) increase in cell volume; (2) increased development of large, parallel cisternae of GER with production of proteinaceous granules; (3) development of small Golgi complexes that package granules; and (4) within vacuoles, progressive enlargement of proteinaceous granules into shell globule clusters formed during vitellogenesis. Three types of inclusions accumulate in large amounts in mature vitelline cells: (1) shell globule clusters, important component in the formation of egg shell; (2) numerous unsaturated lipid droplets. Though fewer, there are also diphasic droplets consisting of saturated and unsaturated lipids in the same droplet, and (3) a relatively small amount of β-glycogen particles, usually surround a few groups of lipid droplets. The β-glycogen and lipid droplets are nutritive reserves for embryogenesis. General pattern and functional ultrastructure of vitellogenesis greatly resemble those observed in some lower cestodes, such as bothriocephalideans and diphyllobothrideans. Variations and differences in the amount of lipids and of glycogen during vitellogenesis in lower cestodes and other trematodes are compared and discussed.

Ultrastructure of digenean trematode eggs (Platyhelminthes: Neoophora): A review emphasizing new comparative data on four European Microphalloidea

Despite their tremendous diversity and their medical and veterinary importance, details of egg ultrastructure among the digenean trematodes has been studied rather little. The available literature is spread over several decades and several species, but has not been adequately reviewed to reveal patterns of similarity and divergence. We present this review to synthesize and analyse what is known from the available literature reporting studies using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). To support our general review of existing literature, we also have synthesized our own previously published descriptions, and present herein our new previously unpublished data. From these new electron micrographs, we provide a comparative analysis of the intrauterine eggs of four digenean species, representing four genera and three families of the superfamily Microphalloidea, collected from four different host wildlife species in four European countries: 1) Mediogonimus jourdanei (Prosthogonimidae) from Myodes glareolus (Mammalia: Rodentia), collected in France; 2) Maritrema feliui (Microphallidae) from Crocidura russula (Mammalia: Soricimorpha), collected in Spain; 3) Brandesia turgida (Pleurogenidae) from Pelophylax ridibundus (Amphibia: Anura: Ranidae), collected in Russia; and 4) Prosotocus confusus (Pleurogenidae) from Rana lessonae (Amphibia: Anura: Ranidae), collected in Belarus. All were studied by preparing whole worms by various techniques for TEM, so that eggs could be studied in situ within the uterus of the parent worm. Based on the literature review and the new data presented here, we describe basic similarities in patterns of embryogenesis and egg formation among all trematode species, but substantial variations in timing of larvigenesis, sculpturing of egg shell surfaces, and some other features, especially including accessory cocoon coverings outside the egg shells of B. turgida and P. confusus. In the future, many more studies are needed to explore egg ultrastructure in other digenean taxa, to explore potential phylogenetic patterns in egg development and structure, and to correlate structure with function in the life cycle.

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

Sperm ultrastructure provides morphological characters useful for understanding phylogeny; no study was available for two basal branches of the Polyopisthocotylea, the Chimaericolidea and Diclybothriidea. We describe here spermiogenesis and sperm in Chimaericola leptogaster (Chimaericolidae) and Rajonchocotyle emarginata (Hexabothriidae), and sperm in Callorhynchocotyle callorhynchi (Hexabothriidae). Spermiogenesis in C. leptogaster and R. emarginata shows the usual pattern of most Polyopisthocotylea with typical zones of differentiation and proximo-distal fusion of the flagella. In all three species, the structure of the spermatozoon is biflagellate, with two incorporated trepaxonematan 9 + "1" axonemes and a posterior nucleus. However, unexpected structures were also seen. An alleged synapomorphy of the Polyopisthocotylea is the presence of a continuous row of longitudinal microtubules in the nuclear region. The sperm of C. leptogaster has a posterior part with a single axoneme, and the part with the nucleus is devoid of the continuous row of microtubules. The spermatozoon of R. emarginata has an anterior region with membrane ornamentation, and posterior lateral microtubules are absent. The spermatozoon of C. callorhynchi has transverse sections with only dorsal and ventral microtubules, and its posterior part shows flat sections containing a single axoneme and the nucleus. These findings have important implications for phylogeny and for the definition of synapomorphies in the Neodermata. We point out a series of discrepancies between actual data and interpretation of character states in the matrix of a phylogeny of the Monogenea. Our main conclusion is that the synapomorphy "lateral microtubules in the principal region of the spermatozoon" does not define the Polyopisthocotylea but is restricted to the Mazocraeidea.

Morphological and molecular characterisation of three Indian Neascus-type metacercariae (Digenea: Diplostomidae)

Diplostomid digeneans are important parasites whose larval stages infect freshwater snails and fish as first and second intermediate hosts respectively. Diplostomid digeneans as adults are parasitic in many fish-eating birds and mammals. Our understanding of the species diversity of diplostomid digeneans remains incomplete, especially in the Indian sub-continent. Here, we describe three Neascus-type metacercariae (N. hanumanthai, N. gussevi, and N. xenentodoni) from freshwater fish specimens of Channa punctata (Bloch 1793), Trichogaster fasciata (Bloch and Schneider 1801) and Xenentodon cancila (Hamilton, 1822) respectively, collected in India. Next, we characterised these metacecariae using nuclear (28S and ITS1) and mitochondrial DNA (cox1) to determine their systematic and phylogenetic position. Molecular identification using interspecific variation for all three molecular markers revealed a closer relationship between N. hanumanthai and N. gussevi (1.9%-2.4%) than either of N. hanumanthai and N. gussevi to N. xenentodoni (3.1%-3.7% and 4.4%-4.0% respectively). In phylogenetic analyses, estimated by neighbour-joining (NJ) and maximum likelihood (ML) methods, N. gussevi and N. hanumanthai nested as sister groups of Posthodiplostomum Dubois, 1936 for all three markers used; N. xenentodoni, however, nested as a sister taxa of all other diplostomes when using 28S and ITS1 and nested as a sister taxa of Bolbophorus, Alaria and Neodiplostomum using cox1. These findings suggest that N. hanumanthai and N. gussevi are members of Posthodiplostomum, but that N. xenentodoni belongs to a separate and unknown genus. Similarly, by proteomics, we found that the cox1 protein sequences and structures were similar between N. hanumanthai and N. gussevi but distinct for N. xenentodoni.

Gobioecetes biwaensis n. g., n. sp. (Monogenea: Dactylogyridae) from the gills of a freshwater gobiid fish, Rhinogobius sp. BW Takahashi & Okazaki, 2002, with a redescription of Parancyrocephaloides daicoci Yamaguti, 1938

A description is given of Gobioecetes biwaensis n. g., n. sp. (Monogenea: Dactylogyridae) on the gills of a freshwater gobiid fish, Rhinogobius sp. BW Takahashi & Okazaki, 2002 from Lake Biwa, Shiga Prefecture and Rhinogobius sp. from Sento Imperial Palace, Kyoto Prefecture, Japan. This monogenean is most similar to Parancyrocephaloides rhinogobius Ling, 1973, proposed here as Gobioecetes rhinogobius n. comb., collected from the gills of Rhinogobius giurinus (Rutter, 1897) in Hupei Province, China, but can be differentiated from it in that ventral hamuli of G. biwaensis are larger than those of G. rhinogobius and in that in G. biwaensis, ventral hamuli are larger than dorsal ones, but in G. rhinogobius, ventral and dorsal hamuli are almost equal in size. The new genus is characterized by the absence of a bar supporting dorsal hamuli. Parancyrocephaloides daicoci Yamaguti, 1938, the type species of the genus, is redescribed based on newly collected specimens from a marine dactylopterid, Dactyloptena peterseni (Nyström, 1887) and the type specimens, revealing that it has a pair of vestigial bars supporting dorsal hamuli. The generic diagnoses of Gobioecetes n. g. and Parancyrocephaloides are given and systematic position of these genera in Dactylogyridae is discussed in this paper.

The first record of Gyrodactylus corleonis Paladini, Cable, Fioravanti, Faria & Shinn, 2010 (Monogenea: Gyrodactylidae) from the wild

Specimens of Gyrodactylus corleonis Paladini, Cable, Fioravanti, Faria & Shinn, 2010 (Monogenea: Gyrodactylidae) were collected from the body and fins of Gobius cobitis Pallas in the western Mediterranean Sea off northwestern Sardinia. This is the first finding of this species in the wild and also represents a new host record. A morphological comparison of the new material with the type description of G. corleonis is presented. Since G. corleonis was found in the wild on representatives of the Gobiidae, a comparison with species of Gyrodactylus von Nordmann, 1832 parasitising gobies is included. The occurrence of G. corleonis on different host species and its phylogenetic relationships with gyrodactylids from sand gobies are discussed.

Biliary cirrhosis caused by Campula spp. in a dolphin and four porpoises

Biliary cirrhosis produced by Campula spp. is described in 1 striped dolphin Stenella coeruleoalba and 4 harbour porpoises Phocoena phocoena. The hepatic lesions consisted of severe proliferation of fibrous connective tissue with loss of the lobular pattern, nodular regeneration of the hepatic tissue, bile duct hyperplasia and severe inflammatory infiltrate composed of eosinophils, macrophages, lymphocytes and plasma cells. These lesions were associated with severe infestation by Campula spp. Although inflammatory and degenerative hepatic lesions are frequently found in stranded dolphins, biliary cirrhosis has not been previously reported in cetaceans. Massive infestation by these parasites should be included as a cause of hepatic failure resulting in stranding of marine mammals.

The study of the sporocyst broodsacs coloring in Leucochloridium paradoxum (Trematoda: Brachylaemidae)

The secretory cells were found in the subtegument of the sporocysts Leucochloridium paradoxum by histological assay. Pigment granules are formed by these cells. The movement of granules from secretory cells to the tegument external layer was observed. These pigment granules provide the yellow color of sporocysts broodsacs and the brown color of protuberant spots in the terminal part of broodsacs. It was shown that the pigment granules did not contain proteins, nucleotides, lipids and carbohydrates. The positive result was received while staining on bile pigments. The question on the nature of the green pigment remains open. The paletot on the surface of sporocyst formed by spreading hemocytes was observed. This structure was not described before in brachylaemid parthenites.

Morphology and histology of the adult Paramphistomum gracile Fischoeder, 1901

In the present study, we evaluated the histological morphology of the adult Paramphistomum (P.) gracile. Adult flukes with bodies 5~15 mm in length and 2~7 mm in width were subjected to histological analysis. Longitudinal and transversal serial-sections were stained with hematoxylin and eosin, and examined. The body surface and longitudinal section of P. gracile were also assessed using scanning electron microscopy. In this species, the anterior sucker and posterior sucker (acetabulum) were present on an anterior and posterior part of the body, respectively. The major folds were located in the areas of the anterior sucker, genital canal, and posterior sucker. The fluke membrane was spineless at the tegument surface and in the tegument tissue. Histological data showed structural-systematic characteristics of the digestive tract, reproductive tract, excretory tract, copulatory organs, connective tissues, and muscle tissues. We attempted to elucidate the histological characteristics of P. gracile that might increase the knowledge and understanding of rumen fluke morphology.

Euclinostomum heterostomum infection in guppies Poecilia reticulata cultured in Southern Thailand

During April to June 2009 and February 2010 to February 2011, numerous digenetic trematode metacercariae were observed embedded in the muscles of guppies Poecilia reticulata cultured at Kidchakan Supamattaya Aquatic Animal Health Research Center, Songkhla, southern Thailand. A total of 424 guppies were examined to identify the parasite and to study its pathogenicity. Based on conventional parasitological techniques, the parasitic digenean found in the guppies was identified as Euclinostomum heterostomum Rudolphi, 1809. Histological analysis revealed numerous metacercariae embedded in the fish muscle. A life cycle study indicated that the snail Indoplanorbis exustus was the first intermediate host, with the guppies serving as the second intermediate host. No E. heterostomum metacercariae were found in cohabited fish species, giant sailfin molly Poecilia velifera or platy Xiphophorus maculatus, which indicated that the guppy was the only suitable fish host present.

Review of haploporid (Trematoda) genera with ornate muscularisation in the region of the oral sucker, including four new species and a new genus

Species of the Haploporidae Nicoll, 1914 with elaborate muscularisation of the oral sucker belong in three trematode genera, including three new species and a new genus from the intestine of fishes in Australian waters. Spiritestis Nagaty, 1948 is resurrected and S. herveyensis n. sp. is described from the mullet Moolgarda seheli (Forsskål) collected in Hervey Bay, Queensland, Australia; the latter differs from S. arabii Nagaty, 1948 in that the position of the genital pore is pharyngeal rather than post-pharyngeal and the geographical range is off Australia rather than the Red Sea. A new genus is proposed for two new species, with a uniquely ornamented oral sucker, which infect Australian scatophagids. Members of Capitimitta n. g. are distinguished from Waretrema Srivastava, 1937, species of which have a simple oral sucker with six radially arranged anterior muscular lobes, in that their oral sucker is V-shaped with six embedded muscular finger-like structures in the anteroventral portion. The relatively small C. darwinensis n. sp., collected from Selenotoca multifasciata (Richardson) at Darwin, Northern Territory, Australia, is distinguished from C. costata n. sp., collected from Scatophagus argus (Linnaeus) in the same locality and S. multifasciata off Brisbane, Australia, and by having smaller eggs, a vitellarium commencing at a level close to the ventral sucker rather than at greater than one ovarian length posterior to the ventral sucker, and shorter tegumental body spines. Sequence data of a c.2,500 bp region of the 3' end of 18S, the entire ITS region and the 5' end of the 28S revealed that Spiritestis and Capitimitta are not as closely related as some morphological features would suggest and are probably not the closest relative of each other. What has been reported as Waretrema piscicolum Srivastava, 1937 probably consists of several species, some in different genera, and one, based on material collected by Dr Masaaki Machida, is proposed as Spiritestis machidai n. sp. from Crenimugil crenilabis (Forsskål) off Japan. Phylogenetic hypotheses, based on analysis of an alignment of partial 28S sequences with other haploporids, provide a framework for the evaluation of interrelationships within the Haploporidae. These analyses show that: (1) Spiritestis and Capitimitta are supported within the Haploporidae; (2) branches to Forticulcita Overstreet, 1982, Saccocoelioides Szidat, 1954, Spiritestis and Capitimitta create a clade that is sister to haploporines from the Mediterranean Sea; (3) the branch to Saccocoelioides, Spiritestis and Capitimitta create a polytomy; and (4) the two new species of Capitimitta, plus an immature specimen of an unnamed species, form a monophyletic clade.

[The influence of the degree of infestation with trematode parthenites on the structure of penial glands in the mollusks Littorina saxatilis of different age]

Histological and confocal microscopy studies of the structure of penial glands in the Littorina saxatilis males were carried out. The examined mollusks belong to two age groups and were at different stages of spontaneous infection with a trematode from the pygmaeus species group (Microphallus piriformes). Based on comparative analysis of microscopic sections of copulative organs in infested and non-infested mollusks, data on the modifications in histological structure ofpenial glands were obtained. From these data we can suggest that the infestation have an influence on all parts of the gland. Decrease of secret production and reduction of muscular capsule (down to its disappearance) were observed. We suppose that changes in the penial glands structure prevent their normal functioning. In the mollusks infested on reaching the sexual maturity, gradual reduction of distal part of reproductive system was observed, while in the mollusks infested before the sexual maturity, development of ancillary part of reproductive system was blocked.

[The nature of the germinal mass (gonad) in trematode parthenites (Trematoda)]

Dynamics of reproduction of the Notocotylus imbricatus (Notocotylidae), Echinostom caproni (Echinostomatidae), Sphaeridiotrema globulus and Psilotrema tuberculata (Psilostomatidae) rediae was examined. Forming of germinal mass in trematodes is considered and discussed on the base of literary and original data.

[Germinal mass of the rediae of Trematoda]

Dynamic of the reproduction of the trematodes Notocotylus imbricatus (Notocotylidae), Echinostoma caproni (Echinostomatidae), Sphaeridiotrema globulus and Psilotrema tuberculata (Psilostomatidae) parthenites was observed. The formation of generative cells takes place only as a result of undifferentiated cells proliferation and following differentiation. These processes and early stages of embryogenesis are taking place only in the special reproductive organ--germinal mass. The germinal mass is always formed at the posterior part of the body. The process of reproduction had been finishing to the beginning of the generating of cercariaea of new age by parthenites. Rediae of different generations in fact stop producing new generative cells with the beginning of the appearance of a new generation.

Morphology and ultrastructure of the digenetic trematode Calicophoron microbothrium from a cow in Taif, Saudi Arabia

Several digenetic trematode flukes belonging to the family Paramphistomidae were recovered from a cow slaughtered at Taif abattoir KSA. Parasites were identified as Calicophoron microbothrium (Family Paramphistomidae) .The surface tegumental structures and the anatomical details of the flukes were studied by making sagittal hand sections in the fluke and observations were made by scanning electron microscopy, which is a very useful technique in case of paramphistomes. This included the description of tegumental surface of the fluke, mouth opening and pharynx, acetabulum, genital atrium, caecum and eggs. Adult C. microbothrium is described for the first time using SEM from Saudi Arabia.

Two new species of Parspina Pearse, 1920 (Digenea: Cryptogonimidae) from freshwater fishes (Gymnotiformes) of the Paraná River basin in Argentina

Two new species of the cryptogonimid genus Parspina Pearse, 1920 are described from gymnotiform fishes of the Paraná River basin, P. carapo n. sp. from the banded knifefish Gymnotus carapo Linnaeus and P. virescens n. sp. from the glass knifefish Eigenmannia virescens (Valenciennes). Parspina carapo differs from P. virescens in the number of oral spines (32-39 vs 30-33) and their length (28-47 vs 16-28 μm), the distribution of tegumental spines and their anchorage, the types of sensory papillae on the body surface (three vs two types), the extent of body length posterior to the caeca (5 vs 13% of the total body length), the dimensions of the pars prostatica (52 × 34 vs 24 × 10 μm), and in the absence of a gonotyl (vs presence). Both P. carapo and P. virescens differ from P. bagre Pearse, 1920 and P. argentinensis (Szidat, 1954) in the number of oral spines (20-21 and 21-28 in the latter pair) and their length (28-32 and 35-60 μm), and in total body length. Additionally, the two new species differ from P. argentinensis in the arrangement of the vitelline follicles (one continuous band vs two groups on each side of the body) and in having a smaller pars prostatica (149 × 49 μm in the latter). Parspina carapo is the fifth intestinal helminth found in G. carapo, and P. virescens is the first found in E. virescens.

[Muscular system of the Microphallus pygmaeus metacercaria (Trematoda: microphallidae)]

The musculature of body wall, attachment organs (oral and ventral suckers), and internal organs (alimentary tract, excretory bladder, and ducts of reproductive system) of the Microphallus pygmaeus metacercaria are described. An unusual arrangement of longitude and diagonal muscles was found in the hind part of the metacercaria body. Longitude fibers of dorsal and lateral body surfaces are bow-shaped and bend round the excretory pore along its ventral margin. An additional group of diagonal fibers is situated in the hind part of ventral body surface.

Ultrastructural features of the tegumental surface of a new metacercaria, Nematostrigea sp. (Trematoda: Strigeidae), with a search for potential taxonomically informative characters

The tegumental surface of a new strigeid metacercaria, Nematostrigea sp., which is a parasite of the freshwater fish Channa gachua (Hamilton) in central Vietnam, is described for the first time using scanning (SEM) and transmission (TEM) electron microscopy. In addition to the general tegumental surface in various parts of the body, details of the surface of the suckers, lappets and holdfast organ are presented, as are variations in the form and distribution of the body spines. As good taxonomic criteria are few in diplostomoid metacercariae at both specific and generic levels, a number of the ultrastructural features revealed may prove to represent taxonomically informative characters. These include the presence of: two rings of dome-shaped papillae localised at different levels on the rim of the oral sucker, a single ring of ciliated papillae on the inner margin of the ventral sucker and a band of dome-shaped papillae along the lateral margins of the broad body-fold in the ventral forebody; an unarmed oral sucker and anteroventral surface of the forebody, although the latter bears protuberant secretory pores; an armed ventral sucker covered by six-pointed spines, except on its rim; multi-pointed spines along the dorsal and ventral sides of the forebody, with the number of their teeth increasing posteriorly; multi-pointed spines on the forebody which gradually transform into single-pointed, more widely distributed spines on the hindbody, disappearing completely at posterior end of the body; the surface of the lappets with a particular distribution of pores leading to three types of secretory glands and three topographical modifications (areas where the surface is smooth, bears digitiform processes or bears recurved, dagger-shaped spines); and the surface of the holdfast organ which is covered with densely packed, straight or slightly curved, simple spines on its lateral surface but is smooth medially.

Comparative topography and ultrastructure of tegument in a polyopisthocotylean parasite (Metamicrocotyla cephalus) and a monopistho-cotylean parasite (Dactylogyrus extensus)

Comparative topography and ultrastructure was done for the tegument architecture between a polyopisthocotylean parasite (Metamicrocotyla cephalus) and a monopisthocotylean parasite (Dactylogyrus extensus) for the first time. The result showed that the tegument of M. cephalus is characterized by many foldings producing closely packed annular corrugations superimposed with a complex configuration of many robust papillae bearing microvillus-like projections, thin terminal web just underneath the apical plasma membrane, membranous projections from the basal plasma membrane inside the syncytium, one type of tegumental cell producing two tegumental secretory bodies inside the syncytium (electron-lucent bodies "ts1" and electron-dense bodies "ts2") and the syncytium is thick (l.3-3 microm). The tegument of D. extensus was characterized by the weak annulations and many minute folds like microvilli, some invaginations in the apical plasma membrane form small vacuoles, eruption of some secretory bodies through the apical plasma membrane, three kinds of secretory bodies inside the syncytium from two different tegumental cells (small electron-dense granules "td1" and large fusiform moderately electron-dense bodies containing fibrous threads "td2" from one cell type and thin dumbbell-shaped electron-dense bodies "td3" from other cell type) and the syncytium is thin (1.4-1.5 microm). The possible functions of the distinctive features of the tegument whether presence or absence have been discussed, especially the proposed role played during the life of the parasite and the possibility of making them as characteristic features of the polyopisthocotyleans than the monopisthocotyleans.

Mode of attachment of a polyopisthocotylean parasite, Metamicrocotyla cephalusto the gills of Mugil cephalus through the study of haptoral compartments

The monogenean parasite, Metamicrocotyla cephalus uses the ventrally located haptor provided with two symmetrical rows of clamps in its attachment to the middle regions of the gill filaments of Mugil cephalus. Several factors are reported that protect the parasite from dislodgement by the breathing water current of the host. Firstly, the body of adult parasite produces 45 degrees with the longitudinal axis of the gill filament and its length is nearly equal to the free distal region of the filament located anterior to the site of attachment. So, the parasite did not bend to loop around the gill filaments as exhibited in mature ones with bodies longer than the gill filaments. Secondly, clamps can tightly grip two successive secondary gill lamellae in alternate positions which are directed inwardly towards the longitudinal axis of the haptor. Thirdly, the number of clamps depends on the age of the parasite where it increases as the age increases. Fourthly, the middle clamps are the largest while the anterior ones are the smallest. Fifthly, there is an exocytotic movement of electron-dense secretory bodies through the thin syncytium in the interior surface of the clamp. Sixthly, the clamp sclerites are embedded into the radial muscle fibers and directly inserted into the sclerite walls especially the median ones. Seventhly, the lateroposterior sclerite II is able to penetrate the gill tissue with its tip under the controlled action of three groups of muscle fibers. Thus, the protrusion and retraction of clamp are controlled by longitudinal haptoral muscle strands and extrinsic muscle fibers forming a highly organized array inside the haptor.

A stereological analysis of the tegumental mitochondria in Corrigia vitta (Trematoda: Dicrocoeliidae)

The tegumental mitochondria from the anterior, mid-ventral and mid-dorsal regions of the body surface of Corrigia vitta have been analysed using stereological and statistical methods. Mitochondria from the ventral tegument were more numerous and contained a greater density of cristae than either the anterior or dorsal tegumental mitochondria. A close association between the size and numbers of mitochondria in the anterior regions of tegument appears to be important in maintaining a relatively constant mitochondrial volume fraction. Results are discussed with respect to worm surface function in the intact host-parasite system.

Impact of several control measures on the encysted metacercariae of heterophyids

The present study evaluated the effect of temperature, chemicals and microwave irradiation on the viability and infectivity of encysted metacercariae (EMC) of Pygidiopsis genata in fish. Flesh of infected fish was divided into two groups; control (GI) and experimental (GII). GII was subdivided into IIa: exposed to -4 degrees C & -20 degrees C for different periods, IIb: marinated in sodium chloride, acetic and citric acid at different concentrations and exposure times and IIc: subjected to microwave irradiation for 20 seconds. Assessment of the effect of these factors was by studying the viability and infectivity of EMC after isolation from the fish. The ultrastructure change was studied by scanning electron microscope (SEM), in comparison with control. The results showed that the highest significant reduction in the EMC viability and infectivity was detected in those exposed to microwave irradiation (100%). SEM examination showed distortion in the shape of microwave-treated EMC with change in the surface nature. Freezing of EMC at -4 degrees C & -20 degrees C for 10 days at either temperature was sufficient to abolish their infectivity, although some were remained viable. Soaking of flesh in salt or acids reduced EMC viability and infectivity. So, microwave irradiation proved prophylactic for EMC.

Ultrastructure and some pathological pictures of Gastrodicus aegyptiacus (Cobbold, 1876) in Egyptian horses

The surface ultrastructure (SEM) of adult worms and eggs of the equine caecal fluke, Gastodiscus aegyptiacus, together with some pathological observations of the caecum and colon of infected horses were studied. SEM showed that G. aegyptiacus had a disc-like body with anterior cone-like part. The oral sucker was surrounded by numerous dome-shaped papillae. The cone-like part possessed tubercle-like papillae at its anterior third. The large ventral sucker was subterminal with small papillae at its rim. The ventral surface was covered with large cauliflower-like papillae. Microvilli were densely arranged on the egg shell surface. The pathological changes were proved by severe haemorrhagic colitis.

[Two new species of virgulate cercariae from the freshwater molluscs Melanopsis praemorsa (L.)]

The new species of virgulate cercariae, Cercaria agstaphensis 6 and C. agstaphensis 14, were described from the freshwater mollusc Melanopsis praemorsa in the Kura River basin, Azerbaidjan. Secretory apparatus, excretory system, arming of tegument, and other structures of virgulate cercariae are characterized.

Ultrastructure of Karyakartia egyptensis Abdou, Dronen and Blend 2006 (Digenea: Lepocreadiidae) from the Red Sea fish, Terapon jarbua

The tegumental surface of Karyakartia egyptensis Abdou, Dronen and Blend 2006, infecting the Red Sea fish, Terapon jarbua, is described using scanning and transmission electron microscope. Scanning electron microscopy showed that the mouth is surrounded by a single row of 14 digitform tegumental extensions that are seen protruding from the oral sucker. The tegumental surface is covered with posteriorly-directed, multidigitated, scale-like spines that arise from the tegumental enfoldings. Multidigitated spines with 11 to 22 digits are densely distributed on the first and second thirds of the body. Spines with 5 digits are less densely distributed on the tegumental surface of the ventral sucker. Uniciliated knob like sensory papillae and smooth tegumental ridges are observed on the tegument. These are particularly dense on the tegumental surface of the suckers. Transmission electron microscopy revealed that the distal cytoplasm contains tegumental spines at different developmental stages. Three types of membrane-bounded vesicles containing electron dense material: disc-shape opaque vesicles, ovoid electron opaque vesicles and electron lucent vesicles were described.

A new parasitic metacercaria from the land snail Monacha obstructa pfeiffer1842 with critical review on relevant metacercariae belonging to the genus Brachylaima Dujardin 1843

Out of 328 specimens of the land snail Monacha obstructa, 105 (32%) were infected with a new metacercaria belonging to the genus Brachylaima Dujardin 1843. The present snail is considered as a new host for this parasite in Egypt. Morphological and morphometric patterns were elucidated. Number of recovered infective metacercariae ranged between 4-35 per kidney of snail host. Metacercariae inhabited only the kidney of infected snails. Total length of mature metacercariae ranged from 1800mu-2050mu (1940mu). The anterior extremity is dome shaped and ceca were highly expanded, undulating, and filled with masses of brown cells and renal tissues. Metacercariae were fully mature, unencyted and highly active. They were either roaming in pathways inside the kidney or firmly anchored to renal tissues via oral sucker. Oral sucker length/total body length is 1:8.5. The histopathological effects involved disintegration of renal lamellae and formation of numerous necrotic masses. Invading metacercariae faced no host resistance and they were surrounded by a parasitovorus vacuole as a mean of immune-evasion. SEM studies revealed that dorsal surface of ventral sucker area is covered with rows of minute spines, Oral suker also is provided with a median tongue-like shaped septum. The rest of integument is aspinose. A critical review for relevant metacercariae of genus Brachylaima is provided.

Tegumental ultrastructure of adult Gynaecotyla squatarolae (Digenea: Microphallidae)

Gynaecotyla squatarolae (Digenea: Microphallidae) adult flukes were recovered from experimental chicks at day 4-6 post-infection and their tegumental ultrastructure was observed with a scanning electron microscopy. They were pyriform in shape, and their anterior halves were concaved ventrally. The whole body surface was covered with tegumental spines, which were wide and 16-17 digitated between oral and ventral suckers. The density of spines and number of digits decreased posteriorly. The oral sucker was subterminal and the excretory pore was at the posterior end of the worm. Two ventral suckers were similar in appearance and protruded near midline of the worm. The genital atrium was dextral to the small ventral sucker. The dorsal surface was covered with tegumental spines, but the spines were sparser than on the ventral surface. On the middle portion of the dorsal surface, a small opening presumed to be the Laurer's canal was seen. From these findings, it has been confirmed that the adult G. squatarolae has unique characteristics in the surface ultrastructure.

[Carmyerius marchandi, a new species of trematode (Gastrothylacidae), a parasite of cattle in Senegal]

Carmyerius marchandi n. sp., a new species of trematode (Paramphistomoidea, Gastrothylacidae), has been described in Senegal. It has an average length of 13 +/- 1 mm and an average diameter of 3 +/- 0.4 mm. The brood pouch, bearing muscular invaginations and a ventrolateral swelling, makes up two thirds of the body length. There is a Carmyerius-type acetabulum. The ratio between the diameter of the acetabulum and body length is 1:7.4. The trematode has a Gastrothylax-type pharynx. The ratio between pharynx length and body length is 1:13.3. The ratio between pharynx length and acetabulum diameter is 1:1.8. The caeca extend beyond the mid section of the parasite. The lobed testicles are in a lateroposterior position. The ovary is situated between the two testicles and the uterus is in a mediodorsal position. The vitelline glands take up a large area in the ventrolateral swelling of the brood pouch. The small genital atrium is of the elongatus type. Using scanning electron microscopy, the parasite's body shows transverse ridging and tegumentary papillae around the oral aperture, genital pore and acetabulum. All of these characteristics indicate that this Gastrothylacidae belongs to the Carmyerius genus. To judge by the other species in the genus previously described, this appears to be a new species. We suggest that it should be named C. marchandi n. sp., as a token of gratitude to Professor Bernard Marchand.

The biology of gyrodactylid monogeneans: the "Russian-doll killers"

This article reviews the history of gyrodactylid research focussing on the unique anatomy, behaviour, ecology and evolution of the viviparous forms while identifying gaps in our knowledge and directions for future research. We provide the first summary of research on the oviparous gyrodactylids from South American catfish, and highlight the plesiomorphic characters shared by gyrodactylids and other primitive monogeneans. Of these, the most important are the crawling, unciliated larva and the spike sensilla of the cephalic lobes. These characters allow gyrodactylids to transfer between hosts at any stage of the life cycle, without a specific transmission stage. We emphasise the importance of progenesis in shaping the evolution of the viviparous genera and discuss the relative extent of progenesis in the different genera. The validity of the familial classification is discussed and we conclude that the most significant division within the family is between the oviparous and the viviparous genera. The older divisions into Isancistrinae and Polyclithrinae should be allowed to lapse. We discuss approaches to the taxonomy of gyrodactylids, and we emphasise the importance of adequate morphological and molecular data in new descriptions. Host specificity patterns in gyrodactylids are discussed extensively and we note the importance of host shifts, revealed by molecular data, in the evolution of gyrodactylids. To date, the most closely related gyrodactylids have not been found on closely related hosts, demonstrating the importance of host shifts in their evolution. The most closely related species pair is that of G. salaris and G. thymalli, and we provide an account of the patterns of evolution taking place in different mitochondrial clades of this species complex. The host specificity of these clades is reviewed, demonstrating that, although each clade has its preferred host, there is a range of specificity to different salmonids, providing opportunities for complex patterns of survival and interbreeding in Scandinavia. At the same time, we identify trends in systematics and phylogeny relevant to the G. salaris epidemics on Atlantic salmon in Norway, which can be applied more generally to parasite epidemiology and evolution. Although much of gyrodactylid research in the last 30 years has been directed towards salmonid parasites, there is great potential in using other experimental systems, such as the gyrodactylids of poeciliids and sticklebacks. We also highlight the role of glacial lakes and modified river systems during the ice ages in gyrodactylid speciation, and suggest that salmon infecting clades of G. salaris first arose from G. thymalli in such lakes, but failed to spread fully across Scandinavia before further dispersal was ended by rising sea levels. This dispersal has been continued by human activity, leading to the appearance of G. salaris as a pathogen in Norway. We review the history and current status of the epidemic, and current strategies for elimination of the parasite from Norway. Finally, we consider opportunities for further spread of the parasite within and beyond Europe.

Parvatrema chaii n. sp. (Digenea: Gymnophallidae) from mice experimentally infected with metacercariae collected from surf-clam, Mactra veneriformis

Parvatrema chaii n. sp. (Digenea: Gymnophallidae) is described using the worms recovered from experimentally infected mice in Korea. The metacercariae were collected from surf-clams, Mactra veneriformis, from a tidal flat in Sochon-gun, Chungchongnam-do. The metacercariae were elliptical (0.262 x 0.132 mm), and the genital pore had an anterior arch of 16-17 sensory papillae in scanning electron microscopic view. Adult worms were ovoid to foliate (0.275-0.303 by 0.140-0.150 mm), and their characteristic features included the presence of lateral lips, short esophagus, genital pore located some distance anterior to the ventral sucker, club-shaped seminal vesicle, a compact to slightly lobed vitellarium, elliptical eggs (0.018-0.020 by 0.010-0.013 mm), and absence of the ventral pit. This gymnophallid is classified as a member of the genus Parvatrema because of the location of the wide genital pore some distance from the ventral sucker, and the absence of the ventral pit. It differs from previously reported Parvatrema species, including the type species, P. borinquenae. In particular, the morphologies of the vitellarium and the genital pore with an anterior arch of 16-17 sensory papillae are unique features. Therefore, we propose it as a new species, Parvatrema chaii n. sp. (Digenea: Gymnophallidae).

Proctophantastes brayi, n. sp. (Digenea: Zoogonidae) parasite of the deep-sea fish Polymixia Lowe, 1838 from Vanuatu

Proctophantastes brayi n. sp. (Digenea: Zoogonidae; Lepidophyllinae) has been found in the intestine of two species of deep-sea fish Polymixia (silver eye fish) near the island of Erromango in Vanuatu at a depth ranging from 720 to 830 m. Specimen whole mounts, histological and scanning electron microscopy preparations showed that P. brayi differs from the five known species of the genus Proctophantastes (P. abyssorum, P. gillissi, P. glandulosum, P. infundibulum and P. nettastomatis) by the following morphological characters: (i) a slit in the anterior part of the oral sucker, (ii) Laurer's canal is absent, (iii) a more extended periatrial gland than the ones in the other species of Proctophantastes, consisting of divided masses of cells and that form a conspicuous multilobated structure which does not have a membrane-bounded sac, (iv) the distal part of the metraterm has vesicle-like processes which we refer to as metratermal sacs, in addition to atrial sacs, (v) a long extension of the glandular cells surrounding the saccular bladder which extends posteriorly to the excretory pore.

Ultrastructural study of the spermatozoon of Nicolla testiobliquum (Digenea, Opecoelidae) parasite of brown trout Salmo trutta (Pisces, Teleostei)

The present paper describes the spermatozoon of Nicolla testiobliquum (Digenea, Opecoelidae), an intestinal parasite of brown trout Salmo trutta, studied by transmission electron microscopy. The mature spermatozoon possess two axonemes of 9 + "1" pattern, two mitochondria, a nucleus, external ornamentation of the plasma membrane, spine-like bodies and cortical microtubules. A comparative study between N. testiobliquum and Nicolla wisniewskii shows mainly two different dispositions of the two mitochondria in these two species of a same genera. Moreover, the comparison of each of these spermatozoon features with others digeneans, in general, and other Opecoelidae in particular, is also presented. It appears that the organisation of the posterior part of the spermatozoon, the disposition of cortical microtubules and external ornamentation associated with spine-like bodies could be interesting elements for phylogeny.

Ultrastructural study of the spermatozoon of Pronoprymna ventricosa (Digenea, Baccigerinae), parasite of the twaite shad Alosa fallax Lacepede (Pisces, Teleostei)

In this paper, we describe the ultrastructural organisation of the spermatozoon of a Digenea Baccigerinae Pronoprymna ventricosa. This spermatozoon possesses the elements found in the others digeneans: two axonemes with 9 + "1" pattern, a mitochondrion, a nucleus, cortical microtubules and external ornamentations. However, this spermatozoon presents two particularities. The first is the presence of a single field of cortical microtubules disposed in the ventral side (mitochondrion side) of the spermatozoon. In the other digeneans, two fields of cortical microtubules are observed between the two axonemes. The second is the presence of external ornamentations not associated with cortical microtubules. These two points have, to our knowledge, never been described in another digenean. Moreover, a separation in two groups of the digenean spermatozoa is proposed in relation to the localisation of the external ornamentations. We propose to distinguish a first group exhibiting these elements in the proximal part of the spermatozoon and a second one with external ornamentations in a second part of the gamete.

SPERMIOGENESIS AND SPERMATOZOON ULTRASTRUCTURE OF NICOLLA WISNIEWSKII (DIGENEA: OPECOELIDAE), AN INTESTINAL PARASITE OF BROWN TROUT SALMO TRUTTA (PISCES: TELEOSTEI)

Spermiogenesis and ultrastructure of spermatozoon of Nicolla wisniewskii (Digenea, Opecoelidae), an intestinal parasite of Salmo trutta, were studied by electron microscopy. Spermiogenesis follows the general pattern found in the Digenea. It begins with the formation of a differentiation zone, including striated rootlets associated with 2 centrioles and an intercentriolar body. The flagella undergo a rotation of greater than 90 degrees. Then, their fusion with the median cytoplasmic process is proximodistal and asynchronous. A peculiarity was observed before the fusion of flagella, i.e., the attachment zones joined as 2 pairs by an electron-dense bridge. The mature spermatozoon is characterized by 2 axonemes, cortical microtubules, a nucleus, 2 mitochondria, external ornamentation, and spinelike bodies. At the posterior end of flagella, the spermatozoon is also characterized by the presence of a central element of the axoneme and without the 9 microtubule doublets. These results were compared with those of the other digeneans and, in particular, with other species of Opecoelidae. It appears that the number of cortical microtubules and their localization in the spermatozoon may be an interesting feature of their phylogeny.

A new pronocephalid, Pleurogonius tortugueroi n. sp. (Digenea), from the intestine of green sea turtles (Chelonia mydas) in Costa Rica

A new species of trematode, Pleurogonius tortugueroi n. sp. (Digenea: Pronocephalidae) is described from the lower intestine of green sea turtles (Chelonia mydas) from Tortuguero National Park, Costa Rica. The new species differs from all other species of Pleurogonius by having a short oesophagus and oval testes close to lateral posterior limit of the body. It differs from all other species, except P. malaclemys Hunter 1961, by having an ovary between the testes; moreover the latter species is a parasite of freshwater turtles. All others members of the genus have a long oesophagus, testes placed to some distance from the posterior end, and the ovary is pretesticular. The new species appears most closely related to P. linearis Looss, 1901 but differs from it by having a different body shape, lappets of the head collar close at the cecal bifurcation level, a longer vitellarian field, different testis shape and position, ovary intertesticular, and different egg size.

ULTRASTRUCTURAL STUDY OF SPERMIOGENESIS AND THE SPERMATOZOON OF CREPIDOSTOMUM METOECUS (DIGENEA: ALLOCREADIIDAE), A PARASITE OF SALMO TRUTTA (PISCES: TELEOSTEI)

Spermiogenesis and the spermatozoon of Crepidostomum metoecus, an intestinal parasite of brown trout Salmo trutta, were studied by transmission electron microscopy. Spermiogenesis begins with the formation of a differentiation zone in front of 2 centrioles associated by an intercentriolar body. Each centriole is linked to a striated rootlet, and gives rise to a flagellum. The rotation of flagella is greater than 90 degrees; their fusion with the median cytoplasmic extension is proximodistal and asynchronous. The spermatozoon is formed after constriction of arched membranes. The spermatozoon possesses 2 axonemes of the 9 + "1" pattern, a nucleus, mitochondria, and glycogen. A major feature is the presence, in the anterior part, of external ornamentation and a lateral expansion associated with spinelike bodies. Another attribute is the presence of 2 mitochondria rather than just 1, as in most of the digenean spermatozoa. To our knowledge, this study is the first undertaken with a species of the Allocreadiidae.

Spermiogenesis and sperm ultrastructure of Deropristis Inflata Molin, 1859 (Digenea, Deropristidae), a parasite of Anguilla anguilla

To our knowledge, this paper describes the first ultrastructural results on spermiogenesis and the spermatozoon of a Digenea belonging to the family Deropristidae, Deropristis inflata. Spermiogenesis follows the usual pattern found in the digeneans, but a single noteworthy characteristic concerns the centriole, which presents a "prominent pear-shaped electron-dense region." Spermiogenesis in D. inflata begins with the formation of a differentiation zone. The two centrioles give rise to flagella. These two flagella undergo a rotation of 90 degrees and fuse with the median cytoplasmic process. The proximo-distal fusion occurs at the level of attachment zones. The mature spermatozoon of D. inflata possesses five regions and presents all the features found in the digenean gamete. However, several characteristics allow us to distinguish the spermatozoon of D. inflata from other digenetic trematoda, namely the external ornamentations of the plasmic membrane, the anterior and posterior extremities. Furthermore, the distal part of the nucleus in front of those of the mitochondria appears such a distinctive criterion in the spermatozoon of this species. Our ultrastructural investigations produce new data on the reproduction of D. inflata, which may be useful for the understanding of the phylogenetic relationships in the Deropristidae family.

A new gymnophallid trematode from the intestine of mice infected with metacercariae from the razor clam Sinonovacula constricta

Meiogymnophallus sinonovaculae n. sp. (Digenea: Gymnophallidae) is described from metacercariae found in the razor clam, Sinonovacula constricta, in the Republic of Korea, and adults recovered from the small intestine of experimentally infected mice. The worms are characterized by paired clusters of vitelline follicles, a well-developed pars prostatica with abundant prostate cells, a voluminous undivided seminal vesicle (in adults and metacercariae), and large, grouped, domelike sensory papillae on the ventral surface anterior to the ventral sucker (in metacercariae). This new species resembles the type species Meiogymnophallus affinis, but differs in having compact, elliptical, and 4-5-lobed vitellaria, and an excretory vesicle with bicornuated anterior arms reaching to the oral sucker. This is the second documentation of the presence of a species of Meiogymnophallus in the Republic of Korea.

Revision of Potamotrygonocotyle Mayes, Brooks & Thorson, 1981 (Platyhelminthes: Monogenoidea: Monocotylidae), with descriptions of four new species from the gills of the freshwater stingrays Potamotrygon spp. (Rajiformes: Potamotrygonidae) from the La Plata river basin

The only known monocotylid genus to parasitise Neotropical freshwater stingrays (Potamotrygonidae) is Potamotrygonocotyle Mayes, Brooks & Thorson, 1981, a monotypic genus erected to accommodate P. tsalickisi Mayes, Brooks & Thorson, 1981. For more than 20 years, no other species has been recognised in this genus, but new efforts to survey the diversity of parasites inhabiting potamotrygonids have revealed the existence of new species and the need to redefine the genus. Here, the generic diagnosis of Potamotrygonocotyle is amended, P. tsalickisi is redescribed and four new species are recognised and described based on samples collected from the gills of freshwater potamotrygonids from the La Plata river basin: Potamotrygonocotyle chisholmae n. sp. and P. dromedarius n. sp. from Potamotrygon motoro; Potamotrygonocotyle eurypotamoxenus n. sp. from Potamotrygon cf. motoro (type-host), P. castexi, P. falkneri and P. histrix; and Potamotrygonocotyle uruguayensis n. sp. from Potamotrygon brachyura. Potamotrygonocotyle is characterised by species possessing: (1) slightly sinuous sclerotised ridges on all septa; (2) two pairs of the dorsal haptoral accessory structures associated with the four posterior peripheral loculi and with anterior dorsal haptoral accessory structure bilobate or semicircular; and (3) male copulatory organ without an accessory piece.

Histopathological and ultrastructural observations of metacercarial infections of Diplostomum phoxini (Digenea) in the brain of minnows Phoxinus phoxinus

The spatial distribution and histopathological changes induced by metacercariae of the digenean trematode Diplostomum phoxini (Faust, 1918) in the brains of European minnows Phoxinus phoxinus (L.) from the River Endrick, Scotland, were studied by light and electron microscopy. Post-mortem examination of a sample of 34 minnows revealed that 50% (n = 17) of the population was infected with 13.7 +/- 2.6 (mean +/- SE; range 1 to 38) metacercariae per infected host. Serial histological sections of the infected minnow brains revealed that the metacercariae were unevenly distributed throughout the brain, with aggregations occurring in the cerebellum, the medulla oblongata and the optic lobes. In fish with highest intensities of infection, over 40% of the cerebellar area and about 30% of the medulla oblongata area were occupied by larvae. Metacercariae disrupt the integrity of brain tissue, with individuals being found in small pockets surrounded by cellular debris. Metacercariae were rarely encountered on the surface of the brain. Electron microscopic examination of infection sites revealed that the granular layer surrounding metacercariae was necrotic, exhibited nuclear degradation and was marked by vacuolation of the cytoplasm. Rodlet cells, the only inflammatory cell types recorded in this study, were found only in parasitized brains and in close proximity to the teguments of metacercariae. It is hypothesised that secretions released from the teguments of metacercariae are a counter response to protect the metacercariae from the fish brain's cellular defence mechanisms.

Development of Centrocestus armatus in different final hosts

In this study, Centrocestus armatus metacercariae were fed orally to hamsters, albino rats, mice, and chicks. Animals were sacrificed and dissected at 1, 3, 5, 7, and 14 days post-infection to determine the development and recovery rate of worms. Results indicated that the average worm recovery rate in hamsters was 25% on the first day post-infection and recovery continued until the 14th day with a gradual decrease in the percentage. Worms were also recovered from mice and albino rats from the first until the third day post-infection, but no worms were recovered thereafter. In chicks, worms were not observed on first day but recovery was positive at 12 h post-infection. Among the four animal hosts, feces from hamsters were the only ones positive for eggs; these were initially observed from the third day and recovery continued until 14 days post-infection. In our study, hamsters are the animal model most suitable for the study of C. armatus when compared to rats, mice, and chicks.

Life cycle and description of a new species of brachylaimid (Trematoda: Digenea) in Spain

The life cycle of Brachylaima llobregatensis n. sp. (Trematoda: Brachylaimidae) is elucidated. Embryonated, operculated, asymmetric eggs (30.9 x 18.2 microm) are eliminated with feces of natural hosts wood mouse Mus spretus; white-toothed shrew, Crocidura russula; and an experimental host, domestic mouse, Mus musculus var. domesticus. The eggs are ingested by the helicid gastropod Helix (Cornu) aspersa, the only natural and experimental first intermediate host. The miracidium hatches from the egg, infects the snails, and develops into a branched sporocyst in the digestive gland. Microcaudate cercariae emerge from this snail and develop into unencysted metacercariae in the kidney of second intermediate host snails H. (C.) aspersa and Otala punctata (natural hosts) and Theba pisana (experimental host). Ingestion of infected snails leads to the infection of definitive hosts, with the adults inhabiting the middle part of the small intestine. There is a chaetotaxic pattern specific on the acetabular (S(II) 5-6 papillae) and body (papillae absent on P(II)) levels. Three types of cercaria papillae were observed by scanning electron microscopy: argentophilic papillae with fingerlike processes (cephalic, body, and acetabular levels); argentophilic papillae with opening (2 papillae in the M body level); and nonargentophilic dome-shaped papillae (on the cephalic C(II) level, alternating with argentophilic S(II) papillae on the ventral sucker). Suckers are subequal, with the acetabulum located in the posterior part of the anterior third of body. Vitellaria extend from anterior margin of acetabulum to between middle level and anterior margin of anterior testis.