Ultrastructure of the surface structures of Allodiscocotyla diacanthi (Polyopisthocotylea: Monogenea) from the gills of the marine teleost fish, Scomberoides tol

Internationa/

Pergamon OOZO-7519(94)EOO39-P

Journalfor Para.vifology, Vol. 25. No. I, pp. 43-54. 1995 Copyright 8 1995 Australian Society for Parasitology Elswier Science Ltd Printed in Great Britain. All rights reserved 002&7519/95 $9.50 + 0.00

Ultrastructure of the Surface Structures of Allodiscocotyla diacanthi (Polyopisthocotylea: Monogenea) from the Gills of the Marine Teleost Fish, Scomberoides tol P. RAMASAMY,*t

G.P. BRENNANS

and D.W. HALTONS

of Zoology, Life Science Building, University of Madras, Madras 600025, India fSchoo1 of Biology and Biochemistry, Medical Biology Centre, The Queen’s University of Belfast, Belfast BT9 7BL, U.K. *Department

(Received

13 August

1993; accepted

14 March

1994)

Abstract-Ramasamy, P., Brenoan G. P. and Halton D. W. 1995. Ultrastructure of the surface structures of Allodiscocoryla diacunflu’ (Polyopistbocotyla: Monogenea) from the gills of the marine teleost fish, Scomberoides tol. International Journal for Parasitology 25: 43-54. Scanning and transmission electron microscopic studies have been made of the surface architecture of the tisb-gill parasite, Allodiscocotyla diacanthi. Observations of the baptor region have revealed the presence of cushion-like supports at the base of each clamp, together with a pair of anchor-books. Other topographical features observed included a midventrally positioned genital atrium and a ventrolateral vaginal pore. Tegumental serrations, composed of electrondense bars, partially cover the bindbody, and their presence may serve in the adhesive attitude of the worm, in response to the flow of displacement water currents. Tbe tegumental syncytium contains numerous dense granules and lucent vesicles, the contents of which are released into the cytosol of the syncytium. The clamp sclerites are invested with tegument which, in some specimens, contained inclusions resembling bacteria. Groups of neurons containing characteristic dense-cored vesicles were observed frequently in the clamp region. Each jaw of the clamp is distinguished by the presence of approximately 175 non-ciliated putative sensory endings, and a pair of cone-like sensillae occurs on the ventral surface of the lappet. Additionally, tbe entire body surface of the worm is covered with some 3000-40&l unicilated structures that are presumed to be sensory in nature. Key words:

Allodiscocotyla

diacanthi;

monogenean; electron microscopy; tegument; attachment apparatus;

sense organs.

body cavity, blood stream, urinary system and gut of these and other vertebrates. Within their natural hosts, monogeneans generally occur in relatively low numbers and present few, if any, host problems (Rohde, 1982; Ramasamy, Ramalingam, Hanna & Halton, 1985); however, in conditions where hosts are in culture, monogeneans have been shown to be pathogenic, resulting in economic loss (Paperna, Diamant & Overstreet 1984; Schmidt & Roberts,

INTRODUCTION

Monogeneans are the most common and abundant ectoparasitic flukes of fish, with a greater diversity of species occurring in the tropics than in the temperate regions of the world (Rohde, 1982). Most monogeneans are parasitic on the gills or skin of fish, while a few genera may be found as endoparasites in the tTo

whom all correspondence should be addressed. 43

44

P. Ramasamy et aI.

1989). They exhibit both host and site specificity, and their evolution is considered to parallel that of their hosts (see Bychowsky, 1957; Rohde, 1982; Ramasamyet al., 1985;Ramasamy& Ramalingam,

1989). Moreover, they possesshighly specialized organsof attachmentwhich readily distinguishthem from other groups of parasitic helminths. These consist of clampsand anchor hooks which vary in size, shape and number, as well as in the types of scleritespresent within the haptor. These features, together

with

the disposition

of presumed

sensory

osmium tetroxide (0~0,) for 1 h, dehydrated to 70% ethanol and stored until required for further processing. For SEM, the flukes were dehydrated through graded acetone and dried in a CPD 750 critical-point-dryer using liquid CO,, mounted on aluminium stubs, and sputtercoated with gold-palladium in a E5000 sputter-coating unit and examined in a JEOL SEM 35CF or JEOL SEM 6400 electron microscope, operating at 10 kV. For TEM, the flukes were dehydrated through an ascending series of ethanols and infiltrated and embedded in Epon resin at 60°C for 48 h. Ultrathin sections (60-70 nm) were cut on a Reichert Ultracut E ultramicrotome with a diamond knife, collected on bare copper grids, double-stained with uranyl acetate (5 min) and lead citrate (8 min) (Reynolds, 1963), and examined in a JEOL 1OOCX transmission electron microscope operating at 100 kV.

structuresand oral and genital apertures,are considered to have functional, taxonomic and evolutionary significance. Studies on the ultrastructural characteristics of monogeneansof tropical fish are limited to a few RESULTS species,such as Pricea multae. Vallisia indica and Gotocotyla spp., even though some220 speciesof monogeneanshave been described at the light General topography The body of A. diacunthi is approximately 1500 microscopy level from Indian fish. In the present study on A. diacanthi, scanning and transmission pm long and 300 pm broad when extended,and 400 electron microscopy was used to explore surface pm long and 200 pm broad when contracted. The specializations associated with the tegument and, in forebody bears the buccal and genital apertures, particular, the distribution of presumed sensory while the hindbody possesses the haptor with associreceptors, the locations of oral and genital apertures, ated clampsand anchor hooks (Figs 1 and 2). tegumentalarchitecture, suchas folds, reticular and serrated structures on the body surface, and those Oral and genital apertures structural features of the clamps and anchor hooks The mouth opens subterminally on the ventral in the haptor that have taxonomic significance. This side and is closely associatedwith approx. 10-15 is the first ultrastructural descriptionof A. diucunthi, papillae-like presumed sensory receptors, each which hasbeendescribedonly previously at the light possessing a knob-like cilium. Two genital apertures microscopiclevel by Unnithan (1962). are presenton the ventral surface,a short distance from the mouth; of these,a commongenitalaperture MATERIALS AND METHODS opensmidventrally, while the vaginal pore opensat Adult specimens of A. diacanthi were recovered from the the ventro-lateral margin (Figs 3-5). An excretory gills of locally-caught Scomberoides rol landed at Marina pore could not always be identified clearly in the beach, Madras, India by local fishermen. The flukes were examined. washed thoroughly in natural seawater to remove any fish- specimens gill mucus and fixed for 24 h at 4°C in either 4% (w/v) glutaraldehyde or glutaraldehyde-paraformaldehyde (PFA) buffered to pH 7.2 with 0.1 M-sodium cacodylate-HCl containing sucrose (3%) and NaCl (O.S%), post-fixed in 1%

Surface topography

Tegumentalridges are present on the dorsal and ventro-lateral

surfaces of the hindbody

(Figs &8),

Fig. 1. Scanning electron micrograph of the ventral surface and haptor of Allodiscocotyla diacanthi. Note the tegumental ridges on the ventrolateral hindbody and their absence in the midventral hindbody and on the forebody. Anchor hooks (h), pad-like clamp cushions (co), genital atrium (Ga), vaginal pore (VP) and preoral pit (po). Scale bar, 100 pm. Fig. 2. Scanning electron micrograph of the dorsal view of A. diacanthi. Note the tegumental ridges on the hindbody and their absence in the haptoral region and forebody and the distribution pattern of presumed sensory receptors (SR) on the dorsolateral margins. Scale bar, 100 pm. Fig. 3. A. diacanthi: Forebody showing locations of the preoral pit (PO). the buccal cavity (bc), genital atrium (Ga) and vaginal pore (VP). Scale bar, 10 pm. Fig. 4. A. diacanthi: Vaginal pore (VP). Presumed uniciliated sensory ending (us). Note the pit-like depressions on the surface (arrows). Scale bar, 10 pm.

Ultrastructure c

Figs 1

46

P. Ramasamy et al.

surfaceand in the haptor region, particularly around the clamps and hooks where the tegument displays clusters of plate-like structures(Figs 1 and 9). However, in the absenceof suchtegumentalridgeson the dorsal and ventral surfacesof the forebody, transverseannulabut are absent on the midventral

shaped, putative sensory structures occur on the clamps and in the haptoral

region (Figs 13 and 14).

The non-ciliated structureson the clamp are smaller in size than thoseon the dorsal and ventral surfaces of the lappet (Figs 12, 1416), which itself supports two pairs of non-ciliated structures on the dorsal tions were present on the surface which is folded into surface.Additionally, severalnon-ciliated structures a complex anastomosing,lamellae-like reticulum occur on the dorsal surface of the haptor in the containing pit-like depressions (Fig. 10). Uniciliated, region of the lappet (Figs 12 and 16), while on the papillae-like, presumed sensory structures were ventral surfacethere is a pair of cone-like, presumed presentover the entire body surface(Fig. 11). sensillaeand severalnon-ciliateddome-shapedstructures (Figs 15 and 16). Haptor The haptor

is asymmetrical

in shape, consisting

of

two rows with four pincer-like gastrocotylid-type clamps, supported on a cushion-like base on the ventral surfaceof the haptor (Figs 1 and 12). In the specimensexamined, each row of clampsappeared similarin size.The clampsmay be extendedor withdrawn into this muscularstructure. Additionally, the haptor

itself may be infolded

so that the clamps are

no longer visible in the SEM. Each clamp consistsof a pair of opposable hinged jaws supported by median-and edge-sclerites for attachment to the gill lamellaeof the host (Fig. 13). The outer surface of each jaw of the clamp displays characteristic tegumental serrations,and about 175non-ciliated domeshapedpresumedsensorystructures occur. A short terminal lappet bears a pair of anchor hooks (Figs 14-16). Sensory structures

Four types of presumed sensory receptors are present

on

the

body

surface

of

A.

diacanthi.

Approximately 3000-4000 unciliated structures occur over the body surface, including the haptor, the serratedhindbody and the forebody (Figs 2, 3, 6, 8, 10 and 11). About 28OtK3000non-ciliated dome-

of the tegument

Ultrastructure

In the regionsof the tegumentalsyncytium lacking surfaceridges,the apical plasmamembraneis irregular and the underlying syncytium contains numerous electron-densegranules, electron-lucid vesicles and occasionalmitochondria (Figs 17 and 18). A uniformly fibrous electron-dense terminal web, 0.03-0.05 urn in thickness,is apparent beneath the apical plasmamembraneof the syncytium. The basal membrane is infolded and supported by a basal lamina. Situated beneath the basal membrane are both circular and longitudinal muscle bundles. In contrast,

the surface syncytium

of the tegumental

ridges,presentin the region of the hindbody, is characterized by deep infoldings of the apical plasma membrane (Figs 18 and 19). The surface ridges contain characteristically fibrous, electron-dense bars beneath the outer plasma membrane. These dense bars occur in all tegumental serrations and appearto consistof aggregationsof the densematerial forming

the cell web with which they seem to

be in continuity. The surfacesyncytium of the clamp measures0.34.05 urn in depth (Fig. 20). The syncytium of the serratedclamp region alsocontains fibrous electron-densebars similar to those of the

Fig. 5. A. diacnnthi: Egg-shell filaments (eg) emerging from the genital atrium (Ga). Note the ciliated structures (probably spermatozoa) (s) in the vaginal aperture (VP). Scale bar, 10 pm. Fig. 6. A. diucunthi: View of tegumental ridges (ts) in lateral view. Note that the tegumental ridges are discontinuous at the lateral margin. Uniciliated putative sensory ending (us). Scale bar, 10 unr. Fig. 7. A. diacanthi: View of tegumental ridges (ts) in the ventro-lateral region of the hindbody. Scale bar, 10 un~. Fig. 8. A. diucanthi: Transition of pit-like depressions and annulations of forebody to ventro-lateral tegumental ridges of the hindbody. Note the uniciliated presumed sensory endings (us). Scale bar, 10 ).tm. Fig. 9. A. diacanthi: Mid-ventral surface of the hindbody. Note the plate-like specialization of the mid-ventral tegumental surface. Scale bar, 10 pm. Fig. 10. A. diacanthi: Note the numerous surface pits (p) and uniciliated sensory endings (us) on the transverse annulations of the ventral surface of the forebody. Scale bar, 10 urn.

Ultrastructure of A. diucanthi

Figs S-10.

41

48

P. Ramasamy et al.

tegumental ridges of the hindbody, but there are differences in the size and shape of the electrondense regions which vary from one serration to another. The tegumental syncytium of the oral sucker region contains many membrane-boundelectron-densegranulesand electron-lucentvesiclesand mitochondria (Fig. 21). The electron-densegranules appear more prevalent in the oral suckersyncytium while the electron-lucent vesicles are apparently more prevalent in the rest of the body. The tegumental perikarya lie beneath the basal lamina and tegumentalmusculature,and contain electron-dense granulesand electron-lucentvesicleswhich are identical to those in the surfacesyncytium. Immediately beneath the basallamina in the region of the tegumental ridgesof the hindbody, numerousmitochondria are presentwhich are apparently absentin the region of the tegumentalridges of the clamp (Figs 18, 19and 25). A fibrous zone containing well-developed longitudinal and circular musclesoccursimmediately beneaththe basallamina of the syncytium of the forebody (Fig. 18). The musclesare non-striated and contain both thick and thin myofilaments.

basal lamina. The myofibrils, containing thick and thin filaments, are tightly arranged and there is no interstitial spacebetweenthe sarcolemmaof adjacent muscle fibres; tight junctions and desmosomes are presentbetweenthe musclefibres. The cytoplasm of the muscle cell contains many mitochondria, together with sarcoplasmicreticulum and numerous nerve fibres. Neuronesare particularly abundant in the vicinity of the clamps (Fig. 25), and consist of nerve tracts that extend into the clamps.Individual nerve axons lie adjacent to the clampsand presumably enter the musculatureof each clamp; synapses were not observed.The axons contained numerous large, dense-coredvesicles and smaller less-dense neurovesicles. Ultrastructure

of the presumed sensory endings

Each uniciliated papilla consistsof a nerve bulb attached to the apical tegumental membraneby a ring-like desmosome(Fig. 26). The finely granular cytoplasmof the nerve bulb containsspherical,granular or electron-densevesiclesand microtubules.A singlecilium is anchoredin the nerve bulb by a dense basalbody, but is lacking a rootlet. The non-ciliated Ultrastructure of the haptor papillae are similar in structure to the uniciliate The anchor hooks consistof electron-lucentfibres receptor-like structure, but are without the cilium arranged in an inner and outer core (Fig. 22). In and the ring desmosome. contrast, the clamp sclerites are electron-dense, containing fibres embeddedwithin a densematrix DISCUSSION and invested with a tegumental syncytium containThe positionsof the oral and genital apertures,the ing mitochondria, electron-densegranules,electron- occurrenceof asymmetricalrows of clamps and a lucent vesicles and presumed sensory-nervefibres pair of anchor hooks on the lappet of the haptor (Fig. 23). Bacteria were observedoccasionallywithin correspond with the description of A. diacanthi by the syncytium, but occurred more commonly in the Unnithan (1962). However, a pair of oral pouches haptoral region outside the syncytium (Figs 23 and and an excretory pore, as describedby Unnithan, 24). Clamp scleritesare interconnectedby radially- were not found in any of the specimensexamined. oriented musclefibres that are insertedinto a fibrous Although Unnithan (1962) reported that a cushion-

Fig. 11. A. diacanthi: Ventral view of anterior region of the forebody showing the transverse annulations and unicilliated sensory receptors (us), buccal cavity (bc), genital atrium (Ga), vaginal pore (VP). Scale bar, 10 urn. Fig. 12. A. diacanthi: Haptor showing the four partially withdrawn clamps (c) in a single row, a partially extended clamp in the second row (cc) and a pair of hooks (h). Note the pattern of tegumental ridges (ts) on the ventral surface of the haptor and non-ciliated (ns) and cone-like sensory endings (cs). Scale bar, 10 urn. Fig. 13. A. diacanthi: Clamp (c) showing non-ciliated sensory endings (cs). Upper-jaw (uj), lower jaw (lj), edge sclerite (es), median sclerite (ms) and tegumental ridges (ts). Scale bar, 10 pm. Fig. 14. A. diucanthi: Dorsal view of the lappet. Note the hooks (h) and non-ciliated presumed nerve endings on the lappet. Clamp exhibits non-ciliated sensory endings (ns) and tegumental ridges (ts). Scale bar, 10 pm. Fig. 15. A. diacanthi: Ventral view of lappet showing a pair of cone-like sensilla (s), anchor hooks (h) and non-ciliated sensory endings (ns). Scale bar, 10 pm. Fig. 16. A. diucanrhi: Cone-like sensillium (s) on the ventral surface of the lappet. Note the non-ciliated endings (ns). Scale bar, 1 urn.

sensory

Ultrastructure

of A. diucunfhi

Figs 11-16

49

50

P.Ramasamy et al.

like support occurred at the baseof eachclamp on are involved with maintenance of the parasite’s only one side of the haptor, the present study has adhesive attitude between the gills. The electronrevealedthat cushion-likesupportsare presentat the densebars may provide a rigid mechanicalsupport, baseof each clamp and on both sidesof the haptor. while the well-developed musculature could be Moreover, the clamps, the anchor hooks and the involved in the extensionor withdrawal of the teguentire haptor can be withdrawn or extendedinto the mental serrationsin responseto changesin the direcmuscularinfolds. The functional significanceof the tion and flow of displacementwater currents, aswell extensionand withdrawal mechanismsof the clamps asin the movementof the parasiteitself. and hooks is unknown. Unnithan (1962) reported Tegumentalridgeshave been describedfor many that Allodiscocotyla chorinemi differed significantly monogeneans (Unnithan, 1962) and those on from A. diacanthi, displaying additional structural Vallisia indica have been previously described features absent in A. diacanthi, namely: possessing (Ramasamyet al., 1987).The electron-densefibrous three pairs of anchor hooks in the lappet, a dorsally- web found beneaththe apical plasmamembraneof positionedvaginal pore, and a greater number of rib the tegumentof A. diacanthi is apparently similar to scleritesin the clamp capsule.The presenceof non- that present in Rajonchocotyle emarginata (see ciliated presumedsensoryendingsand serrationson Lyons, 1972)and Vallisia indica (seeRamasamyet eachjaw of the clamps,a pair of cone-likesensillaon al., 1987), and may have a skeletal function. The the ventral surface of the lappet, and two pairs of presenceof a lamellate-reticulumbetweenthe tegunon-ciliated putative sensoryendingson the dorsal mental serrationssimilar to those of the forebody, surfaceof the lappet of A. diacanthi may form addi- suggeststhat the tegumentalserrationsof the hindtional diagnosticand taxonomical characteristics. body have originated as a modified plate-like teguTegumentalridgeson the dorsal and ventrolateral ment in responseto the parasite’snecessityto attach surfacesof the hindbody, and also on the clamps, itself to its host. may help in attaching the worms to the secondary The tegumentalsyncytium of A. diacanthi is similar gill lamellaeof the host, so that the fluke is posi- in structure to that describedfor many other monotioned in its preferred site of attachment similar to geneansand digeneans(Smyth & Halton, 1983; that of its co-habitant, Vallisia indica, found at the Ramasamyet al., 1987;Ramasamy& Bhuvaneswari, baseof the secondarygill filaments(seeRamasamy 1993). Exocytosis of the contents of electron-dense et al., 1985;Ramasamy,Hanna & Threadgold 1987). granules and electron-lucent vesiclesat the apical Fibrous electron-densebars within the syncytium surfacesuggests a possibleinvolvement in glycocalyx of the tegumental ridges may contain actin-like maintenance,or in the provision of a protective layer contractile elementsthat probably provide a rigid of mucus over the apical plasma membrane to mechanical support. Additionally, the presence minimise mechanical, osmotic and immunological beneath the basal lamina of the tegumental serra- damageto the tegumentsurface(Ramasamy,Hanna tions of well-developedmusclesand numerousmito- & Threadgold, 1986;Ramasamyet al., 1987;Ramachondria support the suggestionthat the serrations samy& Bhuvaneswari,1993).

Fig. 17. A. diacanthi: Sectionthroughthe tegumentof the forebodyshowingthe syncytium(s), basallamina(bl), muscles (ms),tegumental cell(ts).Scalebar, 1 w. Fig. 18.A.

diacanthi: Sectionthrougha tegumental ridgeof hindbodyshowingthe synctium (s), electron-dense web (w), electron-dense secretory granules (ed) and electron-lucent vesicles (ev), mitochondria (m), basal plasma membrane infoldings (unlabelled arrows), basal lamina (bl). Note the fibrous muscular layer (ml). Scale bar, 1 km.

Fig.

Fig.

19. A. diacanthi:

20. A. diacanthi:

Section through a tegumental Syncytium (s), basal lamina Surface

syncytium

of clamp

ridge of the hindbody showing the electron-dense (bl), muscles (ms). Scale bar, 1 urn.

showing electron-dense (ev). Scale bar, 1 pm.

bars (eb) in the ridges,

Fig. 21. Transmission electron micrograph of A. diacanthi: Section through the tegument the oral apertures, showing the electron-dense secretory granules (ed) and electron-lucent Pit-like depression (p). Scale bar, 1 urn. Fig.

22. A. diacanthi:

Section

through

an anchor electron-dense

hook: outer

Note the moderately electron-dense core (dot). Scale bar, 1 urn.

electron

bars

lucent

(eb).

vesicle

of anterior region surrounding vesicles (ev) in the syncytium.

fibrous

inner

core (fit)

and

Ultrastructure of A. diacanthi

Figs 17-22.

51

Fig. 23. A. diacanthi: Section through a fibrous clamp sclerite (cs), a non-ciliated presumed sensory (ns) ending consisting of mitochondria and nervous elements. Syncytium (s) and bacteria (b). Scale bar, 1 um. Fig. 24. A. diucunthi:

Surface syncytium (s) of the flap-like clamp cushions showing possible endocytosis of bacteria (b) and fish (f) host components. Scale bar, 1 pm.

Fig. 25. A. diucanthi:

Section through a neurone (Ne). Note the presumed neurosecretory materials (nv) within the neurone. Also endoplasmic reticulum (er) and mitochondria (m). Scale bar, 1 pm.

Fig. 26. A. diucunrhi:

Section through a ciliated presumed sensory ending. Cilium (c), muscles (ms), mitochondria syncytium (s), electron dense rings (dr), ring desmosomes (d). Scale bar, 0.5 unr.

(m),

Ultrastructure of A. diucanthi

53

The lamellate-reticulum/plate-likesurfacefolds of Ramasamy& Hanna, 1985, 1986a,b, 1989;Ramaclearly increasethe surface area of the samyet al., 1986,1987;Ramasamy& Bhuvaneswari, tegument, and likely facilitate respiratory gaseous 1993).The morphology of the cone-like sensillaon exchange and possibly uptake of micromolecular the ventral surface of the lappet of A. diacanthi nutrients from the surrounding medium, as suggestsa possiblealternative function, in providing suggestedfor I/ indica (seeRamasamyet al., 1987; gyroscopicinformation about its spatial orientation. Halton, 1978). Microvillous structures have been A pair of cone-like sensillaehas been describedon demonstratedalso in someother monogeneans,such the ventral surface of the lappet of Vullisia indica as Diclidophora merlangi (see Halton, 1979), Plec(seeRamasamyet al., 1987); the cone like sensillae tanocotyle gurnardi (seeLyons, 1972)and Heterapta of A. diacanthi differ from thoseof V. indica by the chorinemi (see Ramasamy& Hanna, 1986b),but are absenceof reticulum-like microvilli. According to apparently absentin others, notably Rajonchocotyle Rohde et al. (1986), there is little experimental emarginata (see Lyons, 1972), Pseudothoracocotyle evidenceto suggestthe function of the various types indicu (see Ramasamy& Hanna, 1985)and Bicotyle of receptorsfound in platyhehninths and, as in the veiluvoli (see Ramasamy& Hanna, 1986a). present study, all functional interpretation of The occurrence of neuronescontaining electron- presumedsensorystructures is basedsolely on the densevesiclesin the clamp area is consistent with ultrastructural anatomy and is therefore speculative. neuromuscularactivity in this area. Recent imunoUltrastructurally, the clamp scleritesof A. diacanchemicalstudiesof flatworm parasitesindicate that thi are similar to those describedfor other polyregulatory peptides form a substantial, if not a opisthocotyleans(Shaw, 1979a,b, c; Ramasamyet predominant, component of their nervous systems al., 1986, 1987;Ramasamy& Bhuvaneswari,1993), (Halton, Shaw, Maule, Johnston & Fairweather consistingof an electron-densematrix within which 1992). Brennan, Halton, Maule, Shaw, Johnston, are embeddednumerousdensefibrils. Ramalingam Moore & Fair-weather(1992)have demonstratedthe (1973)reported that the protein of the clamp sclerites presence of immunoreactivities for a number of is stabilized by dityrosine linkages, while Lyons neuropeptides, principally neuropeptide F [M. (1966)showedthat scleriteswere composedof nonexpansa (NPF)] in dense-coredvesiclesof the nerve sulphur containing scleroproteinsand not chitin. In axons supplying the pedunclesand clamps of the A. diacanthi, the scleriteswere embeddedwithin a haptor of the fish-gill monogenean,Diclidophora matrix invested by the tegumental syncytium and, merlangi. Similar studieswill be required to identify therefore, it is possiblethat they arise through the processof protein synthesisin specializedareasof the possibleneuropeptidespresentin A. diacanthi. A number of distinctly different, presumedsensory tegument.The clamp scleritesof A. diacanthi resemendingsweremappedon the surfaceof A. diacanthi, ble thoseof other monogeneans(Shaw, 1979a,b, c; namely: unciliated sensorystructures over the body Ramasamy et al., 1986, 1987; Ramasamy & surface;non-ciliated sensorystructuresoccurring on Bhuvaneswari, 1993), comprising of an electronthe clamps,and on the lappet/haptor; and cone-like densematrix with denserfibrils embeddedin it. This sensillaon the ventral surface of the lappet. The finding is consistentwith the view that the clampsclelength and flexibility of the cilia in each of the rites of the monogeneansmay have a similar origin, unicilated forms suggest they may be rheorecep- function and phylogenetic relationship. The anchor tors/tangoreceptorsinvolved in the orientation of the hooksof A. diacanthi consistof electron-lucentfibrils parasite’sbody in relation to the direction and flow embeddedin its matrix. Lyons (1966)has shownthat of ventilation water currents. Non-ciliated sensory the anchor hooks of monogeneans are composedof endings on the clamp may respond to strain or sulphur-containingproteins. The origin and evolutensionand aid in positioning the body for effective tion of anchor hooks in monogeneanswould thus attachment. The dimensions of the non-ciliated appearto differ from thoseof clampsclerites. sensory structures on the lappet and haptor are In the presentstudy, bacteria wereobservedin the different to thoseon the clamps,suggestingperhaps syncytium of the haptoral region. However, it is functional differences.Alternatively, they may have unclear if they are accidentalinvaders as a result of combined functions in chemo-mechanoreception. tegumental damage causedby natural mechanical Uniciliated, non-ciliated and various other types of forces, as reported by Rohde (1980) and by presumedsensoryreceptorshave beendescribedon Ramasamy & Bhuvaneswari (1993). Numerous the body surface and haptor of monogeneansand bacteria are present in the fish mucus and may be other platyhelminths (Smyth & Halton, 1983;Rohde sequesteredby the parasite’s surface syncytium & Garlick, 1985; Rohde, Watson & Garlick, 1986; through endocytosis, along with other fish-host A. diucunthi

P. Rama .samy et al.

54

materials. The functional significance, if any, these associated bacteria remains unknown.

of

authors are grateful to the British Council, U.K. and the University Grants Commission, New Delhi, India for financial assistance. Our thanks are also due to Mr Jim McCrae and his staff, The Queen’s University Central EM Unit, to Allan Moffett and George McCartney for their expert photographic assistance, to Mr Karuppannan for technical assistance, and to Karen Moore for skilful secretarial help. Acknowledgements-The

REFERENCES

Brennan G. P., Halton D. W., Maule A. G., Shaw C., Johnston C. F., Moore S. & Fairweather I. 1992. Immunoelectron microscopical studies of regulatory peptides in the nervous system of the monogenean parasite, Diclidophora merlangi. Parasitology 106: 171-176. Bychowsky B. E. 1957. Monogenetic Trematodes, their Systematics and Phylogeny (Translated by Oustinoff P. C, Edited by Hargis W.J. Jr.). American Institute of Biological Sciences, Washington, D.C. Halton D. W. 1978. Trans-tegumental absorption of oalanine and a-leucine by a monogenean Diclidophora merlangi.

Parasitology

76: 29-37.

Halton D. W. 1979. The surface topography of a monogenean, Diclidophora merlangi, revealed by scanning electron microscopy. Zeitschrtft fiir Parasitenkunde 61: 1-12. Halton D. W., Shaw C., Maule A. G., Johnston C. F. & Fairweather I. 1992. Peptidergic messengers: a new perspective of the nervous system of parasitic platyhelminths. Journal of Parasitology 78: 179-193. Lyons M. 1966. The chemical nature and evolutionary significance of monogenean attachment sclerites. Parasitology 56: 63-100. Lyons M. 1972. Ultrastructural observations on the epidermis of the polyopisthocotylean monogeneans Rajonchocotyle emarginata and Plectanocotyle gurnardi. Zeitschrift

fiir

Parasitenkunde

40: 87-100.

Papema I., Diamant A & Overstreet R. M. 1984. Monogenean infestations and mortality in wild and cultured Red Sea fishes. Helgolaender Wissenschaftliche Meeresuntersuchungen

37: 345462.

Ramalingam K. 1973. Chemical nature of monogenean sclerites. I. Stabilization of clamp protein by formation of dityrosine. Parasitology 66: l-7. Ramasamy P., Ramalingam K., Hanna R. E. B. & Halton D. W. 1985. Microhabitats of gill parasites (Monogenea and Copepoda) of telests (Scomberoides spp.). International

Journal

for Parasitology

15: 385-397.

Ramasamy P. & Hanna R. E. B. 1985. The surface topography of Pseudothoracocotyla indica (Unnithan, 1956) (Monogenea) from the gills of Scomberomorus commerson. Zeitschrtft ftir Parasitenkunde 71: 575-58 1. Ramasamy P., Hanna R. E. B. & Threadgold L. T. 1986. The surface topography and ultrastructure of the tegument and haptor of Pricea multae (Monogenea). International

Journaffor

Parasitology

16: 581-589.

Ramasamy P. & Hanna R. E. B. 1986a. The surface topography of Bicotyle vellavoli (Monogenea) from the gills of Pampus chinensis. 16: 591-594.

International

Journal

for

Parasitology

Ramasamy P. & Hanna R. E. B. 1986b. The surface topography of a monogenean Heterapta chorinemi from the gills of Scomberoides commersonianus. International Journal

for Parasitology

16: 595-600.

Ramasamy P., Hanna R. E. B. & Threadgold L. T. 1987. Scanning and transmission electron microscopic studies of the surface of Vallisia indica (Monogenea: Polyopisthocotylea). International Journal for Parasitology 17: 1187-l 195. Ramasamy P. & Ramalingam K. 1989. The occurrence, site specificity and frequency distribution of Bicotyle vellavoli on Pampus chinensis and Pampus argenteus. International Journal

for

Parasitology

19: 761-767.

Ramasamy P. & Hanna R. E. B. 1989. The surface topography of Gotocotyla secunda and Gotocotyla bivaginalis (Monogenea: Polyopisthocotylea) from Scomberomorus commerson. 63-69.

International

Journal

for

Parasitology

19:

Ramasamy P. & Bhuvaneswari R. 1993. The ultrastructure of the tegument and clamp attachment organ of Gotocotyla bivaginalis (Monogenea: Polyopisthocotylea). International Journal for Parasitology 23: 2 13-220. Reynolds E. S. 1963. The use of lead citrate at high pH as an electron-opaque stain in electron microscopy. Journal of Cell Biology

17: 208-212.

Rohde K. 1980. Some aspects of the ultrastructure of Gotocotyla secunda and Hexostoma euthynni. Angewandte

Parasitologie

21: 3248.

Rohde, K. 1982. Ecology of Marine Parasites. University of Queensland Press, St Lucia. Rohde K. & Garlick P. R. 1985. Subsurface sense receptors in the larva of Austramphilina elongata Johnston, 1931 (Amphilinidea). Zoomorphology 105: 34-38. Rohde K., Watson N. & Garlick P. K. 1986. Ultrastructure of three types of sense receptors of larval Austramphilina elongata (Amphilinidea). International Journal for Parasitology 16: 245-25 1. Schmidt G. D. & Roberts L. S. 1989. Foundations of Parasitology. The C.V. Mosby Company, St Louis. Shaw M. K. 1979a. The ultrastructure of the clamp sclerites in Gastrocotyle trachuri and other clamp-bearing monogeneans. Zeitschrtft fir Parasitenkunde 59: 43-51. Shaw M. K. 1979b. The development of the clamp attachment organs of the monogenean Gastrocotyle trachuri. Zeitschrift

fiir

Parasitenkunde

59: 277-294.

Shaw M. K. 1979c. The ultrastructure of the clamp wall of the monogenean gill parasite Gastrocotyle tranchuri. Zeitschrift

fiir

Parasitenkunde

58: 243-258.

Smyth J. D. Kc Halton D. W. 1983. The Physiology of Trematodes. Cambridge University Press, Cambridge. Unnithan R. V. 1962. On the functional morphology of a new fauna of Monogenoidea on fishes from Trivandrum and environs. Parasitology 52: 3 15-35 1.