Acanthocephalans from fishes and amphibians in Vietnam, with descriptions of five new species

Eight species of acanthocephalans are reported, and five are new. Specimens of Neoechinorhynchus (Hebesoma) manubrianus Amin, Ha & Ha, 2011 were similar to the original description. Neoechinorhynchus (Hebesoma) spiramuscularis n. sp. (Neoechinorhynchidae), from Xenocypris davidi, has a unique proboscis receptacle wrapped in a spiral muscular layer, and an undulating flask-shaped lemnisci, as well as double para-receptacle structures. Heterosentis mongcai n. sp. (Arhythmacanthidae), from Acreichthys sp., has a small fusiform trunk with an unarmed cone and anterior trunk spines, and a proboscis with two circles of rooted apical hooks and 3–4 circles of rooted posterior spines as well as a para-receptacle-like structure at the posterior end. The poorly known Filisoma indicum Van Cleave, 1928 is fully described and illustrated for the first time. Acanthocephalus parallelcementglandatus n. sp. (Echinorhynchidae), from Clarias batrachus, is distinguished from other species of Acanthocephalus by its small fusiform trunk and parallel tubular cement glands. Pseudoacanthocephalus coniformis n. sp. (Echinorhynchidae), from Hylarana sp., is distinguished from other species by having an anterior trunk collar and staggered prominent filiform cement glands, among other features. Cathayacanthus spinitruncatus n. sp. (Rhadinorhynchidae), from Leiognathus equulus, is distinguished from the only two known species of the genus by having a very long and slender proboscis with more than 50 hooks per row and a totally spined trunk. The generic diagnosis of Cathayacanthus Golvan, 1969 is emended. Rhadinorhynchus johnstoni Golvan, 1969 (Rhadinorhynchidae) perfectly fits the only complete description of that species from the Fiji Islands.


Introduction
A number of acanthocephalan species from freshwater fishes and other vertebrates were previously described in Vietnam by Amin and Ha [3,4] and Amin et al. [6,[9][10][11]32]. Eleven species of acanthocephalans were collected from marine fishes off the eastern seaboard of Vietnam at Halong Bay in 2008 and 2009. Of these, 6 new species belonging to Neoechinorhynchus Stiles and Hassall, 1905, one new species of Heterosentis Van Cleave, 1931, and three new species of Acanthocephalus Koelrouther, 1771, Gorgorhynchus Chandler, 1934, and Neorhadinorhynchus Yamaguti, 1939 were recently described by Amin and Ha [4]. Two new species of Rhadinorhynchus Lühe, 1911 were described from marine fishes in the same bay [7]. Three other species of Rhadinorhynchus were previously reported from marine fishes in Vietnam [14]. The species reported in this presentation have not been previously encountered by us or by any other observer in Vietnam. The above contributions by Amin and collaborators, among others, as well as those listed in Arthur and Te [14] should be consulted for information about the current state of knowledge on the Acanthocephala of Vietnamese vertebrates. The present contribution will add significant information to this database. Live specimens were kept in tap water for a few hours until proboscides were everted then fixed in 70% ethanol. Specimens were then shipped to Parasitology Center, Inc., Arizona. These specimens were stained in Mayer's acid carmine, destained in 4% hydrochloric acid in 70% ethanol, dehydrated in ascending concentrations of ethanol (70%, 80%, 90% twice, 100%), and cleared in 100% xylene, then in 50% Canada balsam and 50% xylene; each step for 24 hr. Whole worms were then mounted in Canada balsam. Measurements are in micrometers, unless otherwise noted; the range is followed by the mean values between parentheses. Width measurements represent maximum width. Trunk length does not include the proboscis, neck or bursa.

Materials and methods
For scanning electron microscopy (SEM) studies, two specimens of Cathayacanthus spinitruncatus n. sp. previously fixed in 70% ethanol were placed in critical-point drying baskets and dehydrated using ethanol series of 95% and 100% for at least 10 min per soak, followed by critical-point drying [23]. Samples were mounted on SEM sample mounts, gold/palladiumcoated, and observed with a scanning electron microscope (XL30 ESEMFEG; FEI, Hillsboro, Oregon). The small numbers of the other species reported did not allow for additional SEM studies.

Species found
Eight species in two classes, two orders, and five families of acanthocephalans were collected from six species of Vietnamese fishes and one species of amphibian between 2010 and 2013 as follows: Etymology: The species is named after its unique spiral muscular coat enveloping the proboscis receptacle.
Twenty-two specimens of this species (11 males and 11 females) were collected from 1 of 10 specimens of Xenocypris davidi (Cyprinidae) from a tributary of the Ma River in Ben En National Park in Thanh Hóa Province (19°37 0 N; 105°31 0 E) on April 25, 2010. This subtropical freshwater fish is found in China where it is also reported in brackish waters [25]. The collected worms included underdeveloped immature males (5) and females (4).

Description
General: With characters of the genus Neoechinorhynchus and the subgenus Hebesoma (Neoechinorhynchidae) as described by Amin (2002). Small fusiform worms, wider at middle, with arched trunk usually posteriorly, gradually tapering at both ends. Body wall with five (rarely four) dorsal and two (rarely three) ventral giant hypodermal nuclei (Figs. 1,5). Secondary branches of lacunar system with reticular anastomoses (Fig. 5). Proboscis small, rectangular, slightly wider than long, with two small giant nuclei near apex on same surface. Lateral hooks in anterior ring slightly more posterior than dorsal and ventral hooks (Fig. 3). Middle hooks slightly smaller than and closer to anterior than to posterior hooks. Posterior hooks smallest. All hooks rooted; roots simple, slender, bladelike, shorter than hooks, directed posteriorly (Figs. 3, 4). Neck prominent, markedly wider posteriorly. Proboscis receptacle single-walled, much longer than proboscis, wrapped in prominent spiral muscular layer fanning from insertion area at anterior ventral side of receptacle. Dorsal and ventral parareceptacle structures (PRS) well developed (Fig. 2, arrow). Dorsal PRS often masked by lemnisci often positioned dorsal to receptacle. Lemnisci sub-equal, much longer than receptacle, narrow anteriorly but becoming heavily undulating and larger posteriorly, each with two prominent giant nuclei (Fig. 2).
Male    Figure 1 showing the spiral muscles wrapping around the proboscis receptacle, the undulating lemnisci, and the para-receptacle structure (arrow). 3. The proboscis of a paratype female. Note the two small giant nuclei near the apex. 4. A row of proboscis hooks. 5. Allotype female. Note a small part of the reticular secondary lacunar branches. 6. The reproductive system of a paratype female. Note the prominent dorso-ventral girdle with dorsal branching characteristic of this species. 7. Egg. Additionally, the new species is distinguished by having a PRS which has not been described in any of the other species of Hebesoma listed above. The PRS is a primitive contractile structure that reportedly uses hydrostatic pressure to effect the eversion of the proboscis in eoacanthocephalans that have only a weak single-walled proboscis receptacle (Amin et al., 2007). The PRS was first reported in Neoechinorhynchus (N.) qatarensis Amin, Saoud, and Alkuwari, 2002 and its structural-functional relationship described in the same species [8]. To date, it is known in a very few species of Neoechinorhynchus Stiles and Hassall, 1905 and of the subgenus Acanthosentis Verma and Datta, 1929  Two female specimens of this species were collected from two species of marine fishes: one of two specimens of the filefish Acreichthys sp.  10-15 (14) in length (Fig. 10). All hooks and spines with slender but prominent roots. Roots of anterior hooks simple, directed posteriorly, about half as long as blades and curved in same direction. Roots of posterior spines somewhat shorter than blades and not curved (Fig. 9). Neck unremarkable. Proboscis receptacle double-walled, about twice as long as proboscis, 447-478 (462) long by 161-177 (169) wide, with large cephalic ganglion at its base. Posterior end of receptacle wall nucleated. Lemnisci digitiform, equal, slightly longer than receptacle (Fig. 8), 624-629 (626) long by 62-65 (63) wide. Reproductive system 1.09-1.15 (1.12) mm long (41% of trunk length), with prominent long uterus, short uterine bell, welldefined vagina, and terminal gonopore (Fig. 11). Eggs not available.

Remarks
Amin et al. [13] described Heterosentis holospinus Amin, Heckmann and Ha, 2011 from the striped eel catfish, Plotosus lineatus (Thunberg, 1787), from the Tonkin Gulf, Halong Bay, Vietnam, and provided a key to all 15 known species [2]. Another species, Heterosentis mongcai n. sp., was found in two other fish species from the same waters and reported herein; an interesting correlation. There are now four known species of Heterosentis with an anterior trunk cone: Heterosentyis holospinus and Heterosentis mongcai from Vietnam, Heterosentis plotosi Yamaguti, 1935 from Japan, and Heterosentis septacanthus (Sita, 1949) Golvan, 1969 from India. The three other species differ from the new species as follows. The trunk spines of H. holospinus cover the whole trunk except the anterior cone. In the two other species, the anterior trunk cone is covered with spines and the posterior wall of the proboscis receptacle is not nucleated. Additionally, Heterosentis plotosi has four giant nucleated muscle cells in the trunk and Heterosentis septacantus has a cylindrical trunk; the trunk of Heterosentis mongcai is fusiform. Voucher specimens: HWML collection no. 49916 (one slide).
Six specimens of this species (three males and three females including one juvenile) were collected from 1 of 5 specimens of the spotted scat, Scatophagus argus (Scatophagidae), in the Pacific Ocean at Kiên Giang Province in the Mekong Delta region of southern Vietnam (10°21 0 N; 104°26 0 E) in October, 2012. This tropical marine fish is also found in brackish reef-associated and fresh waters (Reide, 2004) of the Indo-Pacific region [24]; it feeds on worms, crustaceans, insects, and plant matter [26].

Description of our Vietnamese specimens
General: With characters of the genus Filisoma (Cavisomatidae). Trunk unarmed, long, uniformly cylindrical (Fig. 12). Body wall with many scattered nucleated cells. Shared structures larger in females than in males. Proboscis long and cylindrical with short neck and 14-15 longitudinal rows of 24-28 (26) hooks each. Sub-apical 5-11 hooks in two ventral rows stout. Largest hooks near middle, gradually decrease in size anteriorly, and posteriorly. Posterior hooks smallest. All hooks rooted. Hook roots simple, directed posteriorly, shorter than blades, almost plug-like in spine-like posteriormost hooks. Basal spines not arranged in a ring. Double-walled proboscis receptacle about twice as long as proboscis with cephalic ganglion at its posterior end. Lemnisci digitiform, barely equal, about as long as proboscis receptacle (Fig. 12).

Remarks
Amin [2] listed 14 species of Filisoma Van Cleave, 1928 and acknowledged the synonymy of Filisoma hoogliensis Datta and Soota, 1962 with F. indicum first proposed by Gupta and Jain [20] and accepted by Amin and Nahhas [5]. Gupta and Jain's [20] own ''F. indicum'' must be another species as it has 17-18 proboscis hook rows with hooks reaching as large as 40-51 by 10-19. We concur with Yamaguti [37], Golvan [19] and Gupta and Jain [20] in accepting Yamaguti's [36] F. indicum as genuine despite his counting of six cement glands instead of the usual four. The orifice of the female gonopore in our and Yamaguti's [36] specimens lacked papillae, in contrast to Van Cleave's [33] description.
Yamaguti's [36] detailed description included illustration of the posterior end of a male and female showing a barely discernible female reproductive system and the posterior part of a male reproductive system. These structures and the wellillustrated proboscis in Van Cleave (1928, Fig. 3) are complemented by our illustrations of a whole male, showing the relationships between the proboscis, receptacle, lemnisci and reproductive system (Fig. 12), a recognizable female reproductive system (Fig. 13), and an egg (Fig. 14).
All records of F. indicum were reported from the same fish species, S. argus, in diverse habitats. The spotted scat is adaptable to a wide range of salinity and can be found in marine, brackish reef-associated or fresh waters [30] of the Indo-Pacific region [24], which explains the wide geographical distribution of F. indicum. It was found in fresh waters of Chilka Lake, India [33], River Hooghly, Calcutta, India [16], and the marine waters around Celebes (Sulawesi Island, Indonesia) [36] and Mekong Delta, Vietnam (this paper). The parasite is apparently able to extend its range of distribution along the geographical range of its host, which is also found in Kuwait to Fiji, north to southern Japan, south to New Caledonia, Samoa, Tonga, and the Society Islands [24]. It should not be surprising to discover F. indicum in the same host species from these other locations too.
Etymology: The new species is named for its characteristic parallel tubular cement glands.
One mature male specimen was collected from 1 of 15 specimens of the walking catfish Clarias batrachus (Clariidae) in the Ma River, Ben En National Park, Thanh Hóa Province (19°37 0 N; 105°31 0 E) on April 25, 2010. This omnivorous Asian freshwater fish feeds voraciously and rapidly on smaller fish, mollusks, and other invertebrates as well as on detritus and aquatic weeds [18]. Yet, only 1 of 15 examined fish had only 1 acanthocephalan.

Remarks
Amin [2] listed 53 species in the genus Acanthocephalus Koelreuther, 1771, of which 26 species are known in Asia and associated regions. Of these 26 species, 16 species are found in mainland Asia, 8 in Japan, and 2 in Australia. Acanthocephalus parallelcementglandatus n. sp. is separated from all 26 species by having (1) a small fusiform body, and (2) tubular parallel cement glands in 2 close clusters of 3 glands each. Each of the other 25 species has a cylindrical body (except Acanthocephalus criniae Snow, 1971as per Lesley  Warner, pers. comm., who examined type material at the Tasmania Museum) and round, ovoid, or pyriform cement glands in various arrangements. In A. criniae, the trunk is flattened dorsoventrally, the proboscis has 11-16 longitudinal hook rows, the testes are equatorial, the cement glands are pyriform in a rosette pattern with long common ducts, and the genital pore is subterminal in both sexes. The new species is also distinguished by having two sets of three very short cement gland ducts joining into two prominent common sperm gland ducts (reservoirs). A list of the 26 Asian, Japanese, and Australian species of Acanthocephalus referred to above follows: Of the 26 species listed above, and in addition to A. criniae, only A. curtus has a near fusiform trunk, the posterior part of A. nickoli appears sharply pointed, and the trunk of A. minor is sub-cylindrical. All three species, however, have ovoid, round, or pyriform cement glands. Only A. sinensis has a proboscis armature that overlaps that of A. parallelcementglandatus n. sp., 15 Type specimen: HWML collection no. 49918 (holotype male).
Etymology: The new species is named for its characteristic cone at the anterior end of the trunk.
One adult male was collected from 1 of 2 frogs, Hylarana sp. (Ranidae), in the Bidoup Nui Ba National Park, Lâm Dông Province in southeast Vietnam (12°18 0 N; 108°40 0 E) on June 6, 2013. The genus Hylarana Tschudi, 1838 contains around 86 species found from Sri Lanka to the western Ghats of India, through Nepal and southern China and Taiwan, down to southeast Asia to the Philippines and Papua New Guinea, and in Northern Australia as well as in tropical Africa [31].

Description
Male (based on one mature male with sperm): with characters of the genus Pseudoacanthocephalus (Echinorhynchidae). Trunk small, unarmed, cylindrical, 7.40 mm long by 0.95 mm wide. Anterior 1/2 of trunk wider with anterior cone 198 long by 270 wide posteriorly (Fig. 19). Proboscis cylindrical, slightly swollen posteriorly, truncated anteriorly, 988 long by 260 wide posteriorly, with 13 rows of 7 rooted hooks each (Figs. 20, 21). Anterior hooks largest, gradually decrease in size posteriorly. Posterior dorsal hooks relatively longer than ventral. Hook roots prominent, slender blade-like, slightly curved (Fig. 20). Length of dorsal hooks (and roots) from  (32). Posteriormost hooks most slender, anteriormost hooks next most slender. Proboscis receptacle double-walled but outer wall not continuous posteriorly where retractor muscles pass (Fig. 19), 988 long by 260 wide, with cephalic ganglion at its posterior end. Lemnisci sub-equal, somewhat shorter and longer than receptacle. Longer lemniscus 1.02 long by 0.13 mm wide posteriorly; shorter lemniscus 0.83 long by 0.15 mm wide. Reproductive system in posterior 3/5 of trunk. Testes oblong, large. Anterior testis 0.97 mm long by 0.56 mm wide. Posterior testis 1.05 mm long by 0.48 mm wide. Cement glands 8, filiform, staggered, 1.56-1.72 (1.64) mm long by 0.11-0.16 mm wide, with long ducts. Each of four cement gland ducts join into one duct; both common ducts join other reproductive ducts into genital orifice. Saefftigen's pouch 520 long by 125 wide. Saefftigen's pouch, common sperm duct and common cement gland ducts overlap and join into spheroidal non-ornate bursa, 759 long by 728 wide (Fig. 22).

Remarks
Amin et al. [9] and Tkach et al. [31] provided keys to the species of Pseudoacanthocephalus that emphasized proboscis armature. Later, Amin (2013) listed 18 species, most of which are from Asian or associated geographies. The new species differs from all 18 species listed below in 2 traits, proboscis armature aside. (1) The anterior trunk of P. coniformis n. sp. features a prominent cone. A cone at the anterior trunk was not described in any of the 18 listed species. However, Figure 4A, E in Tkach et al. [31] suggest that Pseudoacanthocephalus smalesi may have an anterior trunk cone. Pseudoacanthocephalus smalesi is distinguished from P. coniformis n. sp. by having a small fusiform trunk, 12 proboscis hook rows each with 4-5 hooks, only 4 cement glands, and much larger hooks reaching 108 long, except for the smaller posteriormost hook.
(2) The cement gland pattern of P. coniformis is uniquely different. All species of Pseudoacanthocephalus have six cement glands, as per the generic diagnosis, with three exceptions, P. coniformis and P. nguyenthileae with eight glands, and P. smalesi with four. In P. coniformis, the eight cement glands are long, filiform, and staggered longitudinally in a sequence one after the other. The cement gland ducts are also long and hardly marked off the glands. The cement glands of all the other species are usually claviform, with two qualified exceptions. The eight cement glands of P. nguyenthileae are filiform to claviform arranged in two overlapping anterior and posterior tiers each with four glands [9]. The six cement glands of P. reesei are in one tightly packed cluster at the same level; no staggering [15].
The 18 known species of Pseudoacanthocephalus discussed above are listed below: The cement gland pattern could not be discerned in P. elongates and P. paratiensis as their descriptions were based on females only; no trunk cones were described or illustrated. Not much information is available on the contracted 15-25-mm-long specimens of P. xenopeltidis except for having 8-12 proboscis hook rows in a dissected invaginated proboscis [19].
Etymology: The new species is named for its fully spined trunk. Six adult females were collected from 4 of 5 common pony fish, Leiognathus equulus, south of Tonkin Gulf at the Hue City area, Thua Thien Hue Province, Central Vietnam (16°43 0 N; 107°45 0 E) in August, 2013. Two specimens were used for SEM studies. The common pony fish is a widely distributed Indo-Pacific species found in East Africa, the Red Sea, the coast of India, throughout southeast Asia to southern Japan and northern Australia, and eastwards to Samoa [21]. It occurs in inshore muddy-bottomed coastal waters, river mouths and estuaries, sometimes entering lower reaches of freshwater streams [1]. It feeds on polychaetes, small fishes, and crustaceans [35]. Golvan's [19] relegation of Rhadinorhynchus exilis Van Cleave, 1928 from the Crucian carp, Carassius carassius (Linn.) (Cyprinidae), in China to his new genus Cathayacanthus based on the absence of large specialized basal proboscis hooks was perfectly justified. Furthermore, the cephalic ganglion of C. exilis appears to be at the anteriormost end of the proboscis receptacle, the same as in our specimens. Van Cleave's [33] very brief and incomplete description made no reference to the cephalic ganglion but his Figure 9 clearly showed the ganglion's anterior position. The cephalic ganglion in Rhadinorhynchus is near the middle of the proboscis receptacle. Van Cleave [33] also stated ''hooks on the ventral surface conspicuously larger than those on dorsal surface and more strongly recurved'' as shown in his Figure 9. Our specimens show the same dorso-ventral differentiation of proboscis hooks.
The only other species of Cathayacanthus is C. bagarri Moravec and Sey, 1989, which was collected from another freshwater fish, Bagarius bagarius (Hamilton) (Sisoridae), in the Red River near Hanoi, Vietnam. The anterior end of all described specimens of C. bagarri was withdrawn with the proboscis totally inverted within the receptacle. Moravec and Sey [27] dismissed the importance of ''alleged dorsoventral asymmetry of proboscis hooks'' advanced by Golvan [19] and included in Van Cleave's [33] (1928) treatment because they could not demonstrate it in their distorted specimens. They also indicated and showed (Fig. 6a, h) the position of the cephalic ganglion to be in the ''posterior half'' of the receptacle. Those two characters alone conflict with their assignment of their specimens to Cathayacanthus, which was based on the absence of specialized basal proboscis hooks. We assume that the cephalic ganglion was displaced posteriorly by the invaginated proboscis in the withdrawn receptacle, which also obscured the delineation of dorsoventral hook asymmetry. That species is provisionally retained in Cathayacanthus as per Amin [2] until more informative specimens are examined.
Cathayacanthus spinitruncatus n. sp. is distinguished from the above two species by having a very long and slender proboscis with more than 50 hooks per row and a totally spined trunk. The proboscis of C. exilis and C. bagarri has 12 longitudinal rows of 32 proboscis hooks each and 14 rows of 33-35 hooks each, respectively; both species have only small anterior trunk spines. The anterior trunk spines of C. spinitruncatus are large and the proboscis has 14 longitudinal rows of 53-61 hooks each. The proboscis of C. spinitruncatus is sharply curved dorsally, similar to that of C. exilis (Fig. 9 of Van Cleave) [33].
Rhadinorhynchus plagioscionis Thatcher, 1980 collected from Plagioscionis squamosissimus (Heckel) in the Brazilian Amazon poses a different kind of taxonomic problem. The proboscis of that species also possesses no basal specialized hooks and would qualify as another species of Cathayacthus. Proboscis hooks are also dorsoventrally asymmetrical and become considerably smaller and more crowded posteriorly, as is the case in C. spinitruncatus. Its trunk is almost totally armed with small spines except posteriorly. However, its cephalic ganglion was clearly described and shown (Fig. 6) to be in the posterior half of the receptacle and not at its anteriormost end, as is characteristic of Cathayacanthus. It is also found in the ''wrong'' continent.

Diagnosis of the genus Cathayacanthus
Given the above information and that in Moravec and Sey [27] cautiously interpreted, Golvan's [19] diagnosis of the genus Cathayacanthus should be modified (changes in bold) to read: ''Rhadinorhynchinae, parasites of freshwater fish in East Asia. Cuticular trunk spines very small or enlarged anteriorly, in one anterior zone or covering whole trunk. Proboscis long, subcylindrical, armed with hooks showing dorsoventral asymmetry, decrease in size and crowd posteriorly, with no large specialized basal hooks. Proboscis receptacle double-walled, markedly longer than proboscis, with cephalic ganglion at anteriormost end. Cement glands 4, tubular. Female gonopore terminal. Eggs spindle-shaped with polar prolongation of fertilization membrane.  (Fig. 24). Proboscis long, slender, curved dorsally (Fig. 31), widest at posterior end, 2.25-2.60 (2.38) mm long by 0.14-0.17 wide posteriorly, with one lateral pair of sensory pits between 2 and 3 posteriormost hooks. All proboscis hooks with ribbed surface (Fig. 36), rooted, in 14 longitudinal rows with 53-61(in two specimens) hooks each. Hooks dorsoventrally asymmetrical with ventral and lateroventral hooks more robust and strongly recurved than slender and straight dorsal and laterodorsal hooks (Figs. 28,34,35). Shape of ventral and dorsal hooks' transition gradually being intermediate laterally (Figs. 34, 35). Hooks largest in anterior half of proboscis and gradually becoming smaller and more crowded posteriorly; posterior 10 hooks smallest and most crowded. No large specialized basal hooks. Basal hooks, however, slightly larger than pre-basal hooks (Fig. 37). Length and width (at base) of every 5 ventral hooks numbered from anterior: (hook # 1) 40-45 (42) · 12-15 (13) (17) · 7 (7). Hook roots simple, directed posteriorly, proportional in size to length of blades (Fig. 28). Roots of ventral hooks slightly shorter than blades. Roots of dorsal hooks markedly shorter than blades. Roots are intermediate in lateral hooks. Neck 281-312 (291) long by 198-229 (208) wide posteriorly. Proboscis receptacle markedly longer than proboscis, double-walled, with cephalic ganglion at its anteriormost end. Lemnisci multinucleated, subequal, long, but shorter than receptacle, cylindrical, gradually tapering posteriorly (Fig. 24). Reproductive system very short compared with trunk length, 832 long in one specimen (5% of trunk length), with vagina lacking terminal sphincter, constricted uterus, and short uterine bell with few nucleated cells. Posterior end rounded with terminal gonopore in papillated orifice (Figs. 29, 42, 43). Eggs oblong, 62-67 (65) long by 12-17 (15) wide, with polar prolongation of fertilization membrane (Figs. 30, 44). Golvan, 1969 One female specimen in the ovarian ball stage was collected from the marine darkbar flying fish Cypselurus hexazona (Exocoetidae) in the Pacific Ocean at Quang Binh Province along Vietnam's north central coast south of Tonkin Bay (17°31 0 N; 106°39 0 E) on April 24, 2013. The host, C. hexazona, is reported in the Indo-West Pacific, Red Sea to the Philippines, Vietnam, New Guinea and New South Wales, Australia. It is found in near-shore surface waters, never spread to open sea, and feeds on zooplankton and small fish [28,29].

Description of our Vietnamese specimen
Female (based on one specimen in the ovarian ball stage): with characters of the genus Rhadinorhynchus (Rhadinorhynchidae). Specimen long, cylindrical, with anterior half somewhat wider and gradually merging with posterior half, 13.55 mm long by 0.57 mm wide anteriorly. Anterior cuticular trunk spines in two regions barely separated by spine-free zone. Anterior group of trunk spines completely encircling trunk, in 3-4 loosely arranged rings with 12-14 spines each. Anterior spines 47 long, posterior spines 57 long. Second group of spines in 14 more loosely arranged rings of dorsally incomplete rings of spines; dorso-lateral spines disappear progressively posteriorly, ending with one posteriormost spine. Ventral spines somewhat longer than dorsal spines of second group: dorsal spines 59 long anteriorly and 67 long posteriorly; ventral spines 62 long anteriorly and 82 long posteriorly. Cuticular spines deltoid-shaped, with the largest being about as long as the longest proboscis hook (82 long). Proboscis long, 1.87 mm long by 0.23 mm wide, with 17 rows of 25 hooks each. Length of hooks 45, 75, 82, 75, 47, 80 in apical, sub-apical, middle, posterior, posteriormost and basal ring positions, respectively. Neck 385 long by 260 wide posteriorly. Proboscis receptacle about twice as long as proboscis, 3.57 mm long by 0.27 mm wide. Reproductive system long, about 1/3 as long as trunk: 4.38 mm long; 32% of body length. Length of uterine bell, very long uterus and vagina 0.24 mm, 3.94 mm, and 0.20 mm, respectively. Posterior end of trunk obtuse with subterminal gonopore.

Remarks
One immature female specimen of R. johnstoni was poorly described as R. pristis (Rudolphi, 1802) Lühe, 1911 by Johnston and Edmonds [22]; it was collected from the intestine of a southern bluefin tuna, Thunnus maccoyi (Castelnau, 1872), caught in St. Vincent Gulf, South Australia. The worm was 17.1 mm long with an invaginated proboscis (1.9 mm long), a proboscis receptacle (2.8 mm long ?) and unripe eggs (62 · 12). Johnston and Edmonds [22] made reference to the two groups of trunk spines and illustrated the anterior end of their specimen, two trunk spines and the egg. Golvan [19] repeated Johnston and Edmonds's [22] description and assigned it to the genus Rhadinorhynchus as R. johnstoni. That species was not reported until Amin and Nahhas [5] provided for the first time a full description of a few specimens of the species collected from a related fish, the mackerel tuna, Euthynnus affinis (Cantor, 1849) off the Fiji Islands. The host of our specimen from Vietnam, C. hexazon, belongs to a distant family of flyfishes, Exocoetidae [Actinopterygii (rayfinned fishes): Beloniformes (needle fishes)], while T. maccoyi and E. affinis belong to the Family Scombridae [Actinopterygii (ray-finned fishes): Perciformes], suggesting a wide adaptability to a diverse host taxa.