Monocotylid Monogeneans Research Articles

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

BackgroundThe 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.MethodsUltrastructure 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).ResultsThe 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.ConclusionsThe present study does not provide ultrastructural support for a close relationship between the Neodermata and B. semperi.

The first ultrastructural observations of the egg-forming complex of Calicotyle affinis, an endoparasitic monocotylid monogenean which lacks a uterus.

This ultrastructural study of the female reproductive system of Calicotyle affinis, a monogenean without a uterus, demonstrates, for the first time in a monogenean, the presence of accessory cells located in the peripheral region of the ovary and a single large cell whose surface is penetrated by deep invaginations which restrict the passage of oocytes through the lumen of the distal extremity of the ovary. The cytoarchitecture of the epithelial lining of the two vaginae is a syncytium formed by an anucleate epithelial lining with sunken epithelial perykaria. The wall of the seminal receptacle is an enlarged prolongation of the vaginal epithelial lining. Fertilization occurs in the fertilization chamber and fertilized oocytes retain cortical granules within their cytoplasm, a characteristic known to occur in free-living flatworms. Our study also highlights concentrations of two distinct groups of Mehlis' gland cell ducts on either side of the proximal end of the ootype distinguished by ultrastructural characteristics of their secretory granules. The epithelial wall of the ootype is formed by a single layer of regular, columnar, glandular epithelial cells; these cells are closely adjacent, conjoined towards their apical region by septate junctions and produce rounded, electron-dense granules which are discharged into the ootype lumen via a merocrine or holocrine mechanism. Released granules concentrate around the eggshell and form an additional fibrous coat. The morphological diversity observed in the female reproductive system of the Monogenea is commented on it relation to clarifying patterns in monogenean evolution and for understanding the phylogeny of the Neodermata.

Ultrastructural characteristics of the male ducts and terminal genitalia of an endoparasitic monogenean, Calicotyle affinis Scott, 1911 (Monopisthocotylea: Monocotylidae), with the first detailed description of a copulatory stylet in a monogenean.

This investigation of Calicotyle affinis is the first ultrastructural description of the male reproductive system of a monocotylid monogenean. It has revealed numerous characteristics which are either new or clarify those determined from previous light microscopical observations on the morphology of Calicotyle spp. These include numerous individual testes, each surrounded by its own basal lamina; an ejaculatory bulb delimited by a basal lamina surrounded by two independent internal and external bands of muscles; a bipartite, but not fully divided, muscular chamber (an internal seminal vesicle) in the proximal region of the ejaculatory bulb; well-developed unicellular prostatic glands localised outside the ejaculatory bulb, the cytoplasmic extensions of which pass into the ejaculatory bulb, to form the ducts which open into the lumen of ejaculatory duct; and a male copulatory organ in the form of a coiled penis tube containing throughout its lumen a long, hollow stylet. The penis tube appears to be a modified prolongation of the ejaculatory duct and is supported by a basal lamina and muscle layers. The stylet wall consists of a homogenous, dense material with a narrow, denser inner lining; it is funnel-shaped proximally but narrows imperceptibly towards the distal end of the penis tube, and appears to be a derivative of the basal laminae of the ejaculatory bulb and distal ejaculatory duct. A comparative analysis of the nature of the stylet in relation to those found in free-living platyhelminths forms part of a search for characters useful for understanding monocotylid origins and relationships.

Spermiogenesis and sperm ultrastructure inCalicotyle affinisScott, 1911 (Platyhelminthes, Monogenea, Monopisthocotylea, Monocotylidae)

SummarySpermatological characteristics ofCalicotyle affinisScott, 1911, an endoparasitic monocotylid monogenean from the cloaca of a holocephalan fishChimaera monstrosaL, have been investigated by means of transmission electron microscopy for the first time. Spermiogenesis exhibits features basically similar to those of the congenericCalicotyle kroyeriandCalicotyle australiensis, but there are some new findings with respect to the formation and fine structure of the spermatozoon including the remarkable complex end-piece (EP). Morphogenesis of the EP, which is located at the anterior (proximal) region of the late spermatid, includes two stages: (1) the centriolar region is continuous with a cytoplasmic mass of the zone of differentiation, the electron-dense surface of the spermatid undergoes significant changes in the sculpturing and the inner core of developing spermatid is electron-lucent; (2) after central fusion of the arching membranes a definitive structure of the EP is subsequently evolved, finally comprising 3 – 4 electron-dense discs attached to a central common electron-lucent column. The EP is considered as a synapomorphy of the generaCalicotyle+Dictyocotyle. The mature spermatozoon ofC.affiniscomprises the EP, two parallel axonemes of almost equal lengths with the 9 + “1” trepaxonematan pattern, mitochondrion, nucleus, and a reduced number of parallel cortical microtubules (1 – 3). The posterior (distal) extremity of the mature spematozoon contains a single tapering axoneme. Ultrastructural characteristics of the mature spermatozoon ofC. affiniscoincide mostly with those of congenericC. australiensis. Variations of the spermatological characters within the genusCalicotyle, betweenCalicotyleand enigmaticDictyocotyleas well as other monocotylid monogeneans are discussed.

Spermatological characteristics of the enigmatic monogenean Dictyocotyle coeliaca Nybelin, 1941 (Monopisthocotylea: Monocotylidae) reveal possible adaptation to endoparasitism.

Spermatological characters of Dictyocotyle coeliaca Nybelin, a unique endoparasitic monogenean from the body cavity of the ray Amblyraja radiata (Elasmobranchii: Rajidae), was investigated by means of scanning and transmission electron microscopy for the first time. The process of the spermatozoon formation begins with the appearance of the differentiation zone which contains two centrioles. Subsequently developed two free flagella rotate to lie parallel before their fusing. After fusion, both the mitochondrion and nucleus migrate alongside the axonemes. Spermatids are formed within parallel cytoplasmic canals. During late spermiogenesis, an electron-dense spiral end-piece is formed proximally to the centriolar region of some spermatids. At the end of spermiogenesis, this end-piece is separated by a plasma membrane from the late spermatid and remains in the residual cytoplasm when spermatozoa are released into the testis lumen. The mature spermatozoon of D. coeliaca contains two parallel axonemes with the 9 + "1" trepaxonematan pattern, mitochondrion, nucleus, parallel cortical microtubules, and electron-dense granules. Especially the presence of electron-dense granules, occurring frequenly in the spermatozoa of endoparasitic flatworms, may indicate an adaptation to endoparasitic lifestyle in D. coeliaca. The anterior extremity of the male gamete contains one centriolar derivative, a small component of the second centriolar derivative and anterior extremity of the mitochondrion. The posterior extremity of the mature spematozoon exhibits tubular elements of the disorganized axoneme. Variations of the spermatozoon ultrastructural characters within monocotylid monogeneans and possible evolutionary significance of glycogen in sperm to the endoparasitic habit are discussed.

Thaumatocotyle (Monogenea: Monocotylidae) From Dasyatidae (Elasmobranchii) of the North American Atlantic Coast: Current Issues Resolved

The study of 4 dasyatid stingrays- Dasyatis centroura (Mitchill, 1815), D. americana Hildebrand and Schroeder, 1928, D. say (Lesueur, 1817), and D. sabina (Lesueur, 1824)-from the northwestern Atlantic and the Gulf of Mexico showed the presence of monocotylid monogeneans Thaumatocotyle Scott, 1904 in their nasal fossae. Specimens collected from D. centroura from the northern Atlantic were identified as T. dasybatis (MacCallum, 1916). This confirmed the incorrect type host attribution of D. pastinaca (Linnaeus, 1758), which does not occur in the geographic area where T. dasybatis was described. Monogeneans collected from D. americana and D. say in the western Atlantic and the Gulf of Mexico were identified as T. longicirrus Hargis, 1955 and T. retorta Hargis, 1955, respectively, which corresponds to the opposite hosts attributed for each parasite species compared with their original description in the Gulf of Mexico. The species T. roumillati de Buron and Euzet, 2005 described from D. sabina in the northwestern Atlantic is reported for the first time in D. sabina from the Gulf of Mexico. Results clarify confusion in the literature regarding the hosts and habitats of Thaumatocotyle from dasyatids of the western Atlantic and Gulf of Mexico and support the oioxenous specificity of these monogeneans as well as their specificity for the nasal fossae of their hosts.

Effects of various treatments on egg hatching of Dendromonocotyle pipinna (Monogenea: Monocotylidae) infecting the blotched fantail ray, Taeniurops meyeni, in Taiwan

The monocotylid monogenean Dendromonocotyle pipinna infects the dorsal skin of the blotched fantail ray, Taeniurops meyeni, and is problematic for Hualien Farglory Ocean Park, an aquarium in Taiwan. Over the last 2 years, eight rays have died due to heavy infections with this parasite. In this study, we found that the epidermis of T. meyeni with attached D. pipinna was not markedly thinner but contained decreased numbers of mucous cells and numerous vacuoles. We examined the effects of temperature (from 16 to 30 degrees C in 2 degrees C increments), salinity (from 10 to 50 per thousand in 5 per thousand increments), desiccation (from 1 to 10 min) and sodium hypochlorite (from 5 to 20 ppm in 5 ppm increments) treatment on the embryonation period and hatching success of D. pipinna eggs, with the goal of disinfecting equipment used in aquaria. Temperature strongly influenced embryonation period: eggs first hatched 4 days after being laid at 30 degrees C and 16 days after being laid at 16 degrees C. However, hatching rate was not significantly influenced by incubation temperature, since the final hatching rates under the incubation temperatures tested herein were not significantly different from one another. Hyposalinity had a lethal effect on D. pipinna eggs, completely preventing the hatching of eggs cultured at 10 and 15 per thousand salinity. Hypersalinity was only partially effective, with a hatching rate close to 7% at 50 per thousand salinity. Desiccation was effective at preventing hatching, and the effectiveness increased with increasing treatment duration. The hatching rate of D. pipinna eggs was significantly decreased when incubated under desiccating conditions for even 1 min. Furthermore, a complete inhibition of hatching was achieved by desiccating eggs for 10 min. Sodium hypochlorite treatment completely prevented hatching at concentrations higher than 10 ppm after 18 h of exposure, but a concentration of 5 ppm was ineffective at preventing hatching even after 24 h of treatment. We therefore propose effective combinations of sodium hypochlorite and exposure time as a means to sterilize tanks and equipment containing D. pipinna eggs.

Comparative phylogeography reveals host generalists, specialists and cryptic diversity: Hexabothriid, microbothriid and monocotylid monogeneans from rhinobatid rays in southern Australia

The phylogeography and host specificity of three monogenean species infecting different sites on the southern fiddler ray, Trygonorrhina fasciata (Rhinobatidae) in South Australia (SA) were studied: Branchotenthes octohamatus (Hexabothriidae: gills), Calicotyle australis (Monocotylidae: cloaca) and Pseudoleptobothrium aptychotremae (Microbothriidae: skin). Five rhinobatid species ( Aptychotrema vincentiana, T. fasciata, Trygonorrhina sp. A, Aptychotrema rostrata and Rhinobatos typus) with distributions spanning west, south and east Australian coastal waters, were surveyed for monogeneans resembling the three species documented from T. fasciata in SA. The identities of hosts and parasites collected were investigated using the mitochondrial genes ND4 and Cytochrome b (cyt b), respectively, in addition to the nuclear marker, Elongation factor 1-alpha (EF1a) for Pseudoleptobothrium. Genetic analyses confirmed that B. octohamatus is geographically widespread and displays little genetic structure, suggesting high levels of gene flow. It was collected from four rhinobatid species throughout its distribution and is not, therefore, host specific. For C. australis, genetic analyses revealed two discrete populations with a genetic divergence of ∼4%, one population occurring west of Bass Strait on two sympatric host species and the other population on the east coast, also occurring on sympatric host species. Similarly, for Pseudoleptobothrium, specimens collected west of Bass Strait were genetically distinct (∼3.5%) from those collected to the east. However, on the east coast, a third Pseudoleptobothrium population was revealed, separated by a genetic distance of >11%, indicating a morphologically cryptic species. Host preferences were indicated for each Pseudoleptobothrium lineage. These genetic discoveries are discussed in relation to life history characteristics of each monogenean species, highlighting the value of phylogeographic analyses to understand the parasite–host relationship.

Monocotylids (Monogenea: Monopisthocotylea) from Aetobatus cf. narinari off New Caledonia, with a description of Decacotyle elpora n. sp.

Four species of monocotylid monogeneans were found on a spotted eagle ray Aetobatus cf. narinari caught off Noumea, New Caledonia, South Pacific. The host is illustrated for future possible identification, because it belongs to a currently unresolved species complex. Decacotyle octona (Young, 1967) Chisholm & Whittington, 1998, from the secondary gill lamellae, is figured and previously unreported details of the reproductive system are described. D. elpora n. sp. is described from numerous specimens found between the secondary gill lamellae. The new species, 780-1650 microm in body length, is smaller than D. octona, but has a longer sclerotised male copulatory organ. It is characterised by two dorsal haptoral accessory structures, each with three bumps, and a 9-shaped sclerotised male copulatory organ 240-290 microm in length. D. octona and Clemacotyle australis Young, 1967, both from the branchial cavity, and Thaumatocotyle pseudodasybatis Hargis, 1955, from the nasal tissue, are new geographical records for New Caledonia. It is suggested that differences in the monocotylid fauna between the Atlantic and Indo-Pacific forms of Aetobatus cf. narinari could be of interest for the systematic status of the ray, but the sporadic nature of the sampling probably casts doubt on the usefulness of the currently available data.

Invasion of the shovelnose ray (Rhinobatos typus) by Neoheterocotyle rhinobatidis and Merizocotyle icopae (Monogenea: Monocotylidae).

This study examined the route of infection by free-swimming larvae of 2 monocotylid monogeneans that inhabit the gills (Neoheterocotyle rhinobatidis) and the nasal tissue (Merizocotyle icopae) of the shovelnose ray, Rhinobatos typus, from Heron Island on the Great Barrier Reef, Australia. Larvae of N. rhinobatidis and M. icopae attached directly to the gills and the nasal tissue of the ray, respectively, and did not first settle on the skin. Initial development of the post-oncomiracidium of N. rhinobatidis was rapid and hamuli formed between 6 and 24 h p.i. at a mean temperature of 26 degrees C. However, growth then slowed markedly and was variable; only 2 fully mature individuals were found 20 days p.i. at a mean temperature of 24.5 degrees C. Development of M. icopae was slow and variable throughout; hamuli did not appear until 10 days p.i. and no mature individuals were obtained even 22 days p.i. at a mean temperature of 24.5 degrees C. No character could be found as an indicator of parasite age for N. rhinobatidis or M. icopae due to the high variability in development in both species.

Metazoan parasites and food composition of juvenile Etmopterus spinax (L., 1758) (Dalatiidae, Squaliformes) from the Norwegian Deep.

A total of 37 juvenile Etmopterus spinax from the Norwegian Deep were examined for stomach contents and metazoan ecto- and endoparasites. These squaliform elasmobranchs were caught by benthopelagic net in May 2001 at a depth of 194-214 m. The euphausiid Meganyctiphanes norvegica and the teleost Maurolicus muelleri were the principal prey organisms. With increasing total length of E. spinax, the frequency of Meganyctiphanes norvegica prey decreased and that of M. muelleri increased. Seven different metazoan parasite species were found: adult Monogenea (2), larval and adult Cestoda (3), and larval Nematoda (2). The predominant parasites were an unidentified monocotylid monogenean and the cestode Aporhynchus norvegicus, with respective prevalences of infestation of 83.8% and 81.1%. The sites of infestation were the gills ( Squalonchocotyle spinacis, Monogenea), nasal cavities (Monocotylidae indet.), body cavity ( Lacistorhynchus tenuis, Cestoda), stomach wall and organs of the body cavity ( Anisakis simplex, Nematoda), and stomach and spiral valve ( A. norvegicus and Pseudophyllidea indet., Cestoda; Hysterothylacium aduncum, Nematoda). No other metazoan parasite taxa were found, and the musculature was free of parasites. Five new host and three new locality records were established. M. muelleri seems to be an important intermediate host for the endoparasitic nematodes which were found, with E. spinax serving as a paratenic host. E. spinax also serves as an intermediate host for the trypanorhynch cestode L. tenuis, and as the definitive host for the two monogeneans and the trypanorhynch A. norvegicus. The latter uses Meganyctiphanes norvegica as the second intermediate host in the Norwegian Deep. The relationship between the feeding ecology, habitat, and vagility of E. spinax and the resulting parasite fauna is discussed.

Efficacy of Praziquantel Bath Treatments for Monogenean Infections of theRhinobatos typus

Abstract The efficacy of the anthelmintic Praziquantel in removing monocotylid monogeneans (Platyhelminthes) from the branchial tissue (Neoheterocotyle rhinobatidis, N. rhynchobatis, and Troglocephalus rhinobatidis) and nasal tissue (Merizocotyle icopae) of juvenile giant shovelnose rays Rhinobatos typus from Heron Island, Australia, was studied. Although effective in removing monogeneans in vivo from the skin and gills of the teleost yellow stripey Lutjanus carponotatus from the same locality, two 2-h treatments using 20 mg/L Praziquantel was ineffective for the giant shovelnose ray. Adult monocotylids were removed, but many post-oncomiracidia and some juveniles remained attached. Juvenile and adult N. rhinobatidis and T. rhinobatidis reacted similarly when exposed to 20 mg/L Praziquantel during in vitro experiments, exhibiting muscle contractions and dying after 12 and 14 h, respectively. We attribute the failure to remove small monogenean stages in vivo to the structure of the branchial and nasal tissu...

Preliminary characterisation and extraction of anterior adhesive secretion in monogenean (platyhelminth) parasites.

Secreted anterior adhesives, used for temporary attachment to epithelial surfaces of fishes (skin and gills) by some monogenean (platyhelminth) parasites have been partially characterised. Adhesive is composed of protein. Amino acid composition has been determined for seven monopisthocotylean monogeneans. Six of these belong to the Monocotylidae and one species, Entobdella soleae (van Beneden et Hesse, 1864) Johnston, 1929, is a member of the Capsalidae. Histochemistry shows that the adhesive does not contain polysaccharides, including acid mucins, or lipids. The adhesive before secretion and in its secreted form contains no dihydroxyphenylalanine (dopa). Secreted adhesive is highly insoluble, but has a soft consistency and is mechanically removable from glass surfaces. Generally there are high levels of glycine and alanine, low levels of tyrosine and methionine, and histidine is often absent. However, amino acid content varies between species, the biggest differences evident when the monocotylid monogeneans were compared with E. soleae. Monogenean adhesive shows similarity in amino acid profile with adhesives from starfish, limpets and barnacles. However, there are some differences in individual amino acids in the temporary adhesive secretions of, on the one hand, the monogeneans and, on the other hand, the starfish and limpets. These differences may reflect the fact that monogeneans, unlike starfish and barnacles, attach to living tissue (tissue adhesion). A method of extracting unsecreted adhesive was investigated for use in further characterisation studies on monogenean glues.

New Species of Spinuris (Monogenea: Monocotylidae) from Zapteryx exasperata (Elasmobranchii: Rhinobatidae) from Baja California Sur, Mexico

Monocotylid monogeneans were collected from the gills of 2 Rhinobatos productus and 5 Zapteryx exasperata in Bahia Almeja, Baja California Sur, Mexico. All are parasitized by monocotylids of the genus Spinuris, 9 specimens of Spinuris lophosoma Doran on Rhinobatos productus (new record) and 27 specimens of Spinuris zapterygis n. sp. on Z. exasperata. This new species differs from the other species in the genus Spinuris by the number of dorsal haptoral sclerites, morphology of hooks, and male copulatory apparatus. The generic diagnosis of the genus Spinuris is revised.

Ciliated cells and chaetotaxy of the larvae of seven species of monocotylid monogeneans (Platyhelminthes) from Heron Island, Great Barrier Reef, Australia.

The distribution and the number of ciliated cells and sensilla of the oncomiracidia of seven species representing five of the six subfamilies of monocotylid monogeneans are described. The species examined include Troglocephalus rhinobatidis Young, 1967 from the Dasybatotreminae; Decacotyle floridana (Pratt, 1910) Chisholm and Whittington, 1998 from the Decacotylinae; Heterocotyle capricornesis Chisholm and Whittington, 1996 and Neoheterocotyle rhinobatidis (Young, 1967) Chisholm, 1994 from the Heterocotylinae; Merizocotyle icopae Beverley-Burton and Williams, 1989 from the Merizocotylinae; and Clemacotyle australis Young, 1967 and Monocotyle spiremae Measures, Beverley-Burton and Williams, 1990 from the Monocotylinae. The number of ciliated cells in each zone (anterior, median and haptoral) is similar between the species, but the distribution of these cells varies and generally supports the subfamilial divisions proposed for the Monocotylidae on the basis of a phylogenetic analysis of morphological characters of the adults. A silver-staining technique modified from the method of Lambert is described.

Observations on the ultrastructure of the anterior adhesive areas and other anterior gland cells in the monogenean Merizocotyle australensis (Monocotylidae) from the nasal fossae of Himantura fai (Dasyatididae).

The anterior adhesive areas of a monocotylid monogenean, Merizocotyle australensis (Merizocotylinae), were investigated. They comprise 6 ventral apertures in 2 groups of 3 arranged at the anterolateral margins of the head. These regions are also well supplied with groups of cilia. Each aperture is 13.8 to 15.8 microm wide and contains multiple tubular projections that are covered with microvilli through which open 2 types of secretory cell ducts that carry either rod-shaped or spherical secretory bodies. The gland cell bodies that produce these 2 types of secretions co-occur at the anterior end. The 2 types of secretory bodies occur adjacent to one another and both are present in the extruded adhesive. The membranes of rod-shaped bodies are retained in the extruded glue. Rod-shaped bodies are 390 +/- 18 nm wide, at least 10.9 microm long, and show 2 types of internal periodic banding: 10.6 nm and 143 +/- 3 nm. The spherical vesicles are 130 +/- 6 nm in diameter and are electron-dense. A third secretion is present in separate ducts that also open anteriorly but emerge through the tegument between the ventral apertures. This secretion does not appear to be part of the adhesive secretion. The bodies of the third secretion are elongate, electron-dense, and 374 +/- 23 nm long. Inside the "lip" of the aperture, general body tegument abuts tegument specific to the aperture. The general body tegument is thicker, contains electron-dense vesicles, and has a ridged surface devoid of microvilli. Where the 2 kinds of tegument meet, they are connected by septate desmosomes.

Morphology and development of the haptors among the Monocotylidae (Monogenea)

Monocotylid monogeneans inhabit a wide diversity of sites on their chondrichthyan hosts including the skin, gills, nasal fossae, urogenital system and coelom. The large variation in the morphology of the haptor appears to reflect this diversity in attachment sites. We demonstrate that the complexity of the haptor can be related to the habitat of the parasite. Generally, those parasites which live in habitats subject to strong water currents such as the gills and dorsal skin surface have more complex haptors than those in environments exposed to weaker or no water currents including the nasal fossae, urogenital system and body cavity. However, there can be considerable variation in haptoral components, even among congeners, living on the ‘gills’ of their hosts. The microhabitat was determined for Monocotyle helicophallus and M. spiremae, both from the gills of the pink whipray, Himantura fai, and M. corali from the gills of the cowtail ray, Pastinachus sephen. We demonstrate that differences in the morphology of the hamuli and the number and morphology of septal sclerites and marginal papillae among these species of Monocotyle can be related directly to their microhabitat. It also appears that different haptoral structures are important for attachment to the host at different stages in the development of the parasite, based on studies on the development and distribution of Neoheterocotyle rhinobatidis from the gills of the common shovelnose ray Rhinobatos typus.

Spermiogenesis and sperm ultrastructure in Troglocephalus rhinobatidis, Neoheterocotyle rhinobatidis and Merizocotyle australensis (Platyhelminthes, Monogenea, Monopisthocotylea, Monocotylidae)

The ultrastructural events of spermiogenesis and the ultrastructure of mature sperm of 3 species of monocotylid monogeneans are described. Two of these species, Troglocephalus rhinobatidis and Neoheterocotyle rhinobatidis are extremely similar in all the aspects studied, and their placement in separate subfamilies is questioned. Evidence is presented in both species for extensive distalward movement during spermiogenesis of an ornamented region associated with cortical microtubules, originally located in the zone of differentiation. Spermatids of Merizocotyle australis lack this ornamentation and the mature sperm also lacks cortical microtubules. Troglocephalus rhinobatidis exhibits the highest number (128) of spermatids in an isogenic group recorded for a flatworm to date. Sperm of all 3 species have 2 normal axonemes, shifted slightly relative to one another. Comparative data are presented on sperm and spermiogenesis of all monocotylids examined to date and the phylogenetic implications are discussed.

Descriptions of the larvae of six species of monocotylid monogeneans from Himantura fai (Dasyatididae) and Rhinobatos typus (Rhinobatidae) from Heron Island, Great Barrier Reef, Australia

The oncomiracidia of the following six species of monocotylids are described: Monocotyle helicophallus and M. spiremae from the gills and Merizocotyle australensis from the nasal fossae of Himantura fai; Neoheterocotyle rhinobatidis and Troglocephalus rhinobatidis from the gills and Merizocotyle icopae from the nasal fossae of Rhinobatos typus. With these descriptions, larvae from 17 described monocotylids representing 11 genera from five subfamilies are now known. The distribution, types of secretions, and possible functions of the anteromedian, lateral and posterior gland-cells are discussed. The distribution of flame-cells and other characters which may help distinguish between oncomiracidia of monocotylids are also tabulated and discussed.

Empruthotrema dasyatidis n. sp. (Monogenea: Monocotylidae) from the olfactory sacs of Dasyatis fluviorum (Rajiformes: Dasyatidae) from Moreton Bay, Queensland

The monocotylid monogenean Empruthotrema dasyatidis n. sp. is reported from the olfactory sacs of the brown stingray, Dasyatis fluviorum Ogilby, 1908, from Moreton Bay, Queensland, Australia. This is the first record of Empruthotrema from the family Dasyatidae. E. dasyatidis n. sp. differs from other species of Empruthotrema by possessing eye pigment, which may be scattered, and by its small size. The generic diagnosis for Empruthotrema is amended.