Dactylogyrids (Monogenoidea) parasitizing the gills of spinefoots (Teleostei, Siganidae): revision of Tetrancistrum Goto and Kikuchi, 1917, with descriptions of two new species from Siganus spp. of the Red Sea and Celebes

Nine species of Siganus (Siganidae) and Naso brevirostris (Acanthuridae) were examined for Tetrancistrum spp. (Monogenoidea, Dactylogyridae). Tetrancistrum Goto and Kikuchi, 1917 (Monogenoidea: Dactylogyridae) is redefined, and the following species are reported and/or described from Australia, Egypt, and/or China: T. sigani Goto and Kikuchi, 1917 from S. fuscescens in Australia and China, T. fusiforme (Yamaguti, 1953) Young, 1967 from S. lineatus in Australia, T. polymorphum (Paperna, 1972) comb. n. from S. luridus in Egypt (new locality record), T. strophosolenus sp. n. and T. suezicum (Paperna, 1972) comb. n. from S. rivulatus in Egypt (new locality records), and T. makau nom. n. and T. longispicularis (Yamaguti, 1968) comb. n. from N. brevirostris in Australia (new locality records). Under the present revision, Tetrancistrum comprises 16 species: T. sigani (type species), T. fusiforme, T. indicum (Paperna, 1972) comb. n., T. kala (Yamaguti, 1968) comb. n., T. lebedevi Gupta and Sharma, 1982 (species inquirenda), T. longicirrus (Yamaguti, 1968) comb. n., T. longispicularis comb. n., T. lutiani Tubangui, 1931, T. makau nom. n., T. nasonis Young, 1967, T. oraminii Young, 1967, T. polymorphum comb. n., T. strophosolenus sp. n., T. suezicum comb. n., T. waltairense nom. n. (species inquirenda), and T. yamagutii sp. n. Pseudohaliotrematoides Yamaguti, 1953 and Pseudancyrocephalus Yamaguti, 1968 are placed in junior subjective synonymy with Tetrancistrum. Pseudohaliotrematoides granulosum Yao, Wang, Xia, and Chen, 1998 is a junior subjective synonym of T. sigani; P. polymorphus eilaticus Paperna, 1972, P. polymorphus indicus Paperna, 1972, and P. polymorphus suezicus Paperna, 1972 are elevated to specific rank and transferred to Tetrancistrum as T. polymorphum comb. n., T. indicum comb. n., and T. suezicum comb. n., respectively; T. strophosolenus sp. n. is described from S. rivulatus in Egypt; T. yamagutii sp. n. is described from specimens collected from Siganus sp. from Macassar, Celebes, by Yamaguti (1953); T. indicum Raju and Rao, 1978 is renamed T. waltairense nom. n. to remove it from homonomy with T. indicum (Paperna, 1972) comb. n.; Pseudancyrocephalus duplicatus Yamaguti, 1968 is a junior subjective synonym of T. nasonis Young, 1967; T. makau nom. n. is proposed for P. nasonis Yamaguti, 1968 to avoid homonymy with transfer of the species to Tetrancistrum; P. longicirrus Yamaguti, 1968, P. longispicularis Yamaguti, 1968, and P. kala Yamaguti, 1968 are transferred to Tetrancistrum. T. obesum Caballero, Bravo‐Hollis, and Grocott, 1955 and Pseudohaliotrematoides aurigae Yamaguti, 1968 are retained in Haliotrema Johnston and Tiegs, 1922; T. plectocirra (Paperna, 1972) Lim, 2002 is retained in Glyphidohaptor Kritsky, Galli, and Yang, 2007; P. bengalensis Gupta and Khanna, 1974, P. lutjanusi Gupta and Sharma, 1982, and P. rohdei Gupta and Sharma, 1982 are considered species inquirendae; P. chaetodipteri (Caballero and Bravo‐Hollis, 1961) Yamaguti, 1963 is returned to its original status of Parancylodiscoides chaetodipteri Caballero and Bravo‐Hollis, 1961; Pseudohaliotrematoides falcatus Yamaguti, 1968, P. recurvatus Yamaguti, 1968, P. zancli Yamaguti, 1968 and T. longiphallus (MacCallum, 1915) Price, 1937 are considered incertae sedis; and P. microphallus Yamaguti, 1968 and P. triangulovagina Yamaguti, 1968 are retained in Euryhaliotrematoides Plaisance and Kritsky, 2004.


Results
Four of nine species of Siganus examined during the present study were positive for species of Tetrancistrum. Five species of Tetrancistrum were found on these hosts: one species on S. luridus, two species (one new to science) on S. rivulatus, one species on S. fuscescens from both China and Australia, and one species on S. lineatus. Available specimens of S. argenteus, S. doliatus, S. punctatus, S. corallinus, and S. vulpinus were negative for Tetrancistrum spp.
Diagnosis. Body foliiform, infrequently fusiform, comprising body proper (cephalic region, trunk, and peduncle) and haptor; haptor a simple extension of peduncle. Tegument smooth. Two terminal and two bilateral subterminal cephalic lobes; three bilateral pairs of head organs; cephalic glands lateral or posterolateral to pharynx. Eyespots usually absent; chromatic granules minute, subovate, scattered throughout cephalic region; randomly distributed accumulations of granules infrequent in cephalic region. Mouth midventral, subterminal at level of head organs, opens into buccal tube; buccal tube extends posteriorly along body midline to pharynx to form buccal cavity; pharynx a muscular, glandular bulb; oesophagus short to moderately elongate; intestinal caeca two, with or without diverticula, apparently terminating blindly posterior to gonads. Common genital pore midventral, posterior to intestinal bifurcation. Gonads intercaecal, tandem; germarium pretesticular, forming a cap over anterior end of testis. Testis subspherical to subovate; vas deferens not observed; seminal vesicle fusiform; two prostatic reservoirs dorsal to copulatory complex, each emptying into base of male copulatory organ (MCO) via single duct. Copulatory complex comprises accessory piece and MCO. Oviduct short; ootype receives ducts of vitellarium and vagina; uterus extends anteriorly along body midline to common genital pore; seminal receptacle not observed. Vaginal aperture dextromarginal in anterior portion of trunk; vagina with distal vestibule and proximal meandering duct. Vitellarium coextensive with gut. Haptor with dorsal and ventral anchor-bar complexes; seven pairs of similar hooks with ancyrocephaline distribution (Mizelle 1936;Mizelle and Price 1963) present or absent in adults; ventral anchor with large, flat, grooved, subequal basal roots, delicate shaft and short recurved point; dorsal anchor with elongate flat grooved deep root, comparatively short rod-like superficial root, delicate shaft, short recurved point; bars simple. On gills of marine perciform fishes of the Siganidae, Lutjanidae, and Acanthuridae.
Remarks. Goto and Kikuchi (1917) did not provide a formal generic diagnosis when they proposed Tetrancistrum and described T. sigani from Japan. In a short paragraph at the end of their paper, these authors stated that the genus was characterized by lacking eyes and ''marginal hooks'' and by having a lateral vagina and a ''caudal disc'' bearing two pairs of hooks (anchors) with each pair provided with a connecting ''transverse piece'' (bar). These authors also noted that the intestinal caeca were united posteriorly and were provided with ''lateral secondary coeca (sic)'' (diverticula). This characterization was apparently deemed unsatisfactory by Johnston and Tiegs (1922), Price (1937), and Yamaguti (1963), all of whom provided comparatively non-specific diagnoses for the genus. The non-specificity of these diagnoses allowed the assignments of unrelated dactylogyrid species to the genus, which resulted in an apparent unnatural taxon. Young (1967) provided a more detailed diagnosis and recognized that morphological features of the haptoral sclerites as well as internal anatomy were important to define the genus. He limited the taxon to five species parasitizing fishes of the Siganidae, Acanthuridae, and Lutjanidae. The diagnosis provided herein characterizes Tetrancistrum as dactylogyrids having a foliiform (infrequently fusiform) body, tandem gonads with the germarium forming a conical cap over the anterior end of the testis, two prostatic reservoirs, a dextromarginal vaginal aperture, a distal vaginal vestibule, and each ventral anchor with large, grooved, subequal, basal roots and each dorsal anchor with a large, grooved, deep root and a short, rod-like, superficial root. Members of the genus usually lack eyespots and apparently a seminal receptacle, and haptoral hooks are often absent in adult worms. The morphology of the intestine of Tetrancistrum spp. is problematical. Goto and Kikuchi (1917) described the intestine of T. sigani as running ''backwards on either side of the body and unite directly behind (posterior to) the testis but again separate a little further (sic) backwards and terminate blindly shortly afterwards''. In their respective descriptions, Young (1967) and Tubangui (1931) showed similar configurations of the intestinal caeca of T. nebulosi (5 T. sigani), T. oraminii, and T. lutiani, while Young (1967) described a complex of connections between the two caeca posterior to the testis in T. nasonis. Paperna (1972b) and Raju and Rao (1978) reported simple intestinal caeca uniting posterior to the testis in the three subspecies of Pseudohaliotrematoides polymorphus (now T. polymorphum, T. suezicum, and T. indicum) and T. indicum of Raju and Rao (1978) (now T. waltairense), respectively, while Gupta and Sharma (1982) and Yao et al. (1998) did not mention the morphology of the gut in their respective descriptions of T. lutiani and Pseudohaliotrematoides granulosus (now T. sigani). In the species of Pseudancyrocephalus described from species of Naso by Yamaguti (1968) (all herein transferred to Tetrancistrum), the gut is described as simple, with each caecum ending blindly in the posterior trunk, including that of P. dupicatus, herein considered a junior synonym of T. nasonis. In none of the specimens available to us, including the types of previously described species, were intestinal caeca seen to have terminal or subterminal connections. Available specimens suggest that the intestinal caeca end blindly in the posterior trunk. Young (1967) also noted that the intestine of species of Tetrancistum was highly variable and did not consider the presence of lateral diverticula sufficient to differentiate Pseudohaliotrematoides from Tetrancistrum. As a result, he placed Pseudohaliotrematoides in junior subjective synonymy with Tetrancistrum and transferred its type species, P. fusiforme, to the latter genus. We concur with Young's actions concerning Pseudohaliotrematoides and also consider Pseudancyrocephalus with species from acanthurid fishes a junior subjective synonym of Tetrancistrum. Machida (1979) proposed Nasoancyrocephalus Machida, 1979 for a dactylogyrid, N. diorchis Machida, 1979, from the gills of Naso unicornis in Japan. The species was stated to resemble those of Pseudancyrocephalus (5 Tetrancistum) by having longitudinally striated anchor roots, two prostatic reservoirs, and a seminal receptacle. Machida (1979) distinguished Nasoancyrocephalus from Pseudancyrocephalus by its species possessing two testes (testis single in species of Pseudancyrocephalus), an autapomorphic character that clearly developed secondarily within the Dactylogyridae (see Kritsky 1993, 2001). Acceptance of Nasoancyrocephalus may therefore result in Tetrancistrum becoming paraphyletic because a synapomorphy for the latter is not apparent if both genera are recognized. Should it be shown that N. diorchis has a common ancestor with one or more (but not all) species of Tetrancistrum, Machida's genus should be rejected and placed in synonymy with Tetrancistrum. This synonomy is not proposed at this time, however, pending phylogenetic analyses of species comprising the two genera.
Redescription. Based on 15 voucher specimens from S. fuscescens from the Great Barrier Reef. Body foliiform; trunk broad; cephalic region and peduncle narrow, tapered; greatest body width at level of germarium. Cephalic lobes moderately developed; each head organ comprises several groupings of terminations of cephalic-gland ducts; large bilateral groups of cephalic glands posterolateral to pharynx. Eyespots absent; accumulations of minute subovate chromatic granules uncommon; isolated granules scattered throughout cephalic region. Pharynx elongate ovate to pyriform. Testis subspherical; vas deferens not observed; seminal vesicle forming inverted ''J'' to left of copulatory complex; two small prostatic reservoirs. Copulatory complex comprised of anterior and posterior basal flanges, MCO, and accessory piece. MCO tubular, sigmoid to loosely coiled, appearing J-shaped in slightly to moderately compressed specimens; rod-shaped accessory piece dextroventral to MCO, with flattened proximal end and club-like distal end. Germarium comparatively large; ootype receives vaginal duct and bilateral common vitelline ducts; uterus expanded distally; vaginal pore at level of copulatory complex; vaginal vestibule with posterior bulge near midlength, variable, lightly sclerotized; vaginal duct meandering; vitellarium dense, empties via three bilateral pairs of vitelline ducts, each group of three ducts forming short common vitelline duct just anterior to germarium. Haptoral hooks absent in adult. Ventral and dorsal anchors typical; superficial root of ventral anchor slightly longer than deep root. Ventral and dorsal bars straight, with terminal expansions. Similarly, Ko and Chan (2002) reported seeing three pairs of haptoral hooks in their specimens from Hong Kong, while Yao et al. (1998) did not mention these structures in the English version of their description of Pseudohaliotrematoides granulosum (5 T. sigani). In current specimens from S. fuscescens in China and Australia, hooks were not observed, suggesting that occurrence of hooks is variable and may depend on differing age or development of individual specimens of T. sigani. Although Young (1967) differentiated T. nebulosi from T. sigani by ''constant disparity between the recorded sizes of the penis stylet (MCO), accessory piece and hamuli (anchors), and because of the different shape of the copulation canal (vagina)'', these characters do not justify specific separation of the two forms. While the respective dimensions reported by Goto and Kikuchi (1917) for the copulatory complex and anchors are greater than those reported by Young (1967), ranges of measurements obtained from specimens collected from Australia and China do not suggest significant differences since considerable overlap exists among them. In addition, the morphology of the vagina and vaginal vestibule is variable among all specimens of T. sigani examined during this study. Thus, T. nebulosi Young, 1967 is considered a junior subjective synonym of T. sigani Goto andKikuchi, 1917. Yao et al. (1998) described Pseudohaliotrematoides granulosum from S. canaliculatus from China. While these authors did not compare their species to T. sigani, their drawing of the copulatory complex and its reported dimensions and those of the anchors are consistent with present specimens. Thus, P. granulosum is placed in synonymy with T. sigani as a junior subjective synonym. Woodland (1990) and Froese and Pauly (2006) considered S. nebulosus (host to T. nebulosi) and S. fuscensens (host to T. sigani) to be synonyms, with S. fuscescens having priority. In addition, Woodland (1990), who considered S. canaliculatus and its sibling S. fuscescens to be distinct, indicated that the two species have frequently been confused during identification or considered to be conspecific by previous workers. Because the latter two fishes are sympatric and morphologically similar, it is uncertain whether or not the reported records of T. sigani on S. canaliculatus are valid. The synonymy of S. nebulosus and S. fuscescens, along with the potential for confusion during identification of S. canaliculatus and S. fuscescens, provides additional support for the proposed synonymies of T. sigani, T. nebulosi, and P. granulosum.
Remarks. This species was originally described by Yamaguti (1953) as Pseudohaliotrema (Pseudohaliotrematoides) fusiforme and serves by monotypy as the type species of the subgenus. Yamaguti (1963) subsequently elevated the subgenus to generic rank, but Young (1967) transferred the species to Tetrancistrum based primarily on the comparative morphology of internal body features and haptoral sclerites. Young's (1967) action resulted in Pseudohaliotrematoides becoming a junior subjective synonym of Tetrancistrum. The species was originally described from an undetermined species of Siganus from Macassar. However, examination of the slide containing the type specimens, and comparing the parasite mix with those found in Australia, suggests that S. lineatus may be its natural host (see . The original description of this species is adequate for diagnosis. The species is distinguished from all other species of Tetrancistrum by having a copulatory complex comprising a dilated, curved, strongly sclerotized MCO and an accessory piece consisting of two distinct parts, with the sinistral part composed of two portions diagonally articulated to each other ( Figure 9). Seven pairs of haptoral hooks with an ancyrocephaline distribution (Mizelle 1936;Mizelle and Price 1963) appear to occur consistently in adults, although the full complement of hooks is often difficult to observe in fixed and mounted specimens.
Slide labels of the three available specimens of this species from the BMNH indicated that they were deposited in 1992 by Geets and Martens, who identified the parasites as T. sigani collected from S. sutor from the Indian Ocean, Mombasa, Kenya. The parasite specimens were insufficient for redescription of the species, but Geets et al. (1997) stated that the monogenoids they reported as T. sigani are smaller and the copulatory organ differs morphologically from that reported by Paperna (1972b) for T. indicum. Paperna (1972b)  provided a measurement of 1690 mm without a range for body size of T. indicum. However, maximum body sizes for present specimens of T. sigani (from S. fuscescens), 1576 mm, and those of specimens from S. sutor, 1485 mm, do not appear to vary significantly from that originally reported by Paperna (1972b). Further, the primary difference in the morphology of the copulatory complex of that depicted by Paperna (1972b) and that shown by Geets and Martens' specimens involves the terminal flare of the MCO, which in the original description of T. indicum is larger than that of the specimens from S. sutor (Figure 15). This difference is likely a result of different methods used to mount the worms by respective investigators. In his many investigations of African monogenoids, Paperna generally used compressed specimens to describe the sclerotized parts, which could have resulted in distortion of the MCO, while the specimens deposited by Geets and Martens are mounted in resin under less compression. Thus, we tentatively consider the specimens from S. sutor to represent T. indicum. Confirmation of this identification, however, will depend on collection and description of new parasite material from the coast of eastern Africa.
Remarks. The original description of this species was developed by Paperna (1972b) for general delineation of three subspecies assigned to the specific taxon: Pseudohaliotrematoides polymorphus eilaticus, P. p. suezicus, and P. p. indicus. Of the three subspecies, Paperna (1972b) designated P. p. eilaticus as ''syntype'' for P. polymorphus. Because Pseudohaliotrematoides is considered a junior subjective synonym of Tetrancistrum and the three subspecies are elevated to specific rank, the epithet Tetrancistrum polymorphum is assigned to the form designated as syntype by Paperna (1972b).
Etymology. The specific name is from Greek (strophos5twisted+solenos5a pipe) and refers to the morphology of the copulatory complex.
Remarks. Tetrancistrum strophosolenus sp. n. differs from all congeners by possessing a vaselike base of the MCO from which a narrow distal coiled tube arises. The species was originally described in an unpublished portion of a thesis (Diamant 1985) as Pseudohaliotrematoides nagati (a nomen nudum). References to the species have been frequently made in the literature under different combinations of names by Diamant and co-workers, but in none of these accounts were the names suggested to be available sensu the International Code of Zoological Nomenclature. We have confirmed that specimens described in the thesis are conspecific with T. strophosolenus through copies of the original drawings from the thesis provided by Dr A. Diamant, who graciously allowed us to describe the new species.
Remarks. This species was originally described as a subspecies of Pseudohaliotrematoides polymorphus by Paperna (1972b). The subspecies was elevated to full specific rank by Diamant (1989), but Diamant et al. (1999) reversed this taxonomic revision without comment when the authors referred to the species as P. polymorphus suezicus. The species is now re-elevated to specific rank and transferred to Tetrancistrum based on the morphology of its haptoral armament and internal organ systems.
Etymology. The new species is named in recognition of Dr S. Yamaguti's massive contributions to our present knowledge and understanding of helminth diversity and taxonomy.
Remarks. The holotype and paratype of T. yamagutii were found on an MPM slide labelled (22838, 22839) and containing the type specimens of Pseudohaliotrema (Pseudohaliotrematoides) fusiforme and Pseudohaliotrema (Pseudohaliotrema) sphincteroporus, both of which were described by Yamaguti (1953). The new species most closely resembles T. sigani and T. indicum. It is differentiated from these species by possessing a doubly
Remarks. The original description by Young (1967) of T. oraminii is adequate. Present respective measurements correspond to those in the original description, but Young (1967) did not provide the dimensions of the gonads, haptor, and pharynx. The morphology of the intestinal caeca, described by Young (1967) as lacking lateral diverticula and connected by a transverse ''crus'' posterior to the testis, could not be verified in the type specimens; the stain (haemalum) apparently had faded through time. Young (1967) recorded the host of T. oraminii as S. oramin, but this species is currently considered a junior synonym of S. canaliculatus by Woodland (1990) and Froese and Pauly (2006).

Site of infestation. Gills.
Previous record. No other records except for the original description by Raju and Rao (1978).
Etymology. The specific name refers to the region of India from which the species was originally described.
Remarks. Specimens of this species were not available for study and the type specimens have apparently not survived. This species was originally described by Raju and Rao (1978) as T. indicum from the gills of Siganus oramin (now S. canaliculatus) from the Waltair Coast of India. While it clearly belongs to Tetrancistrum, the validity of this species is problematical, and the drawings provided by the original authors, particularly those of the copulatory complex and haptoral structures, are insufficient for specific diagnosis. Meaurements of the anchors reported by Raju and Rao (1978) and their Figure 4b of the MCO and accessory piece suggest that this species may be a junior synonym of T. oraminii reported from the same host in Australia. In their original figure, the MCO is depicted as a sigmoid tube with a long proximal flange, and the accessory piece, while comparatively smaller, has the general shape of that of T. oraminii. However, Raju and Rao (1978) describe the dorsal bar as being ''slender and recuved (sic)'', which does not correspond to that of T. oraminii. Therefore, we do not at this time consider the species a synonym of T. oraminii but consider it a species inquirenda.
With the transfer of Pseudohaliotrematoides polymorphus indica Paperna, 1972, to Tetrancistrum as T. indicum comb. n., Raju and Rao's specific name for the species becomes a junior homonym and must be replaced. Thus, T. waltairense nom. n. is proposed for the species.
Tetrancistrum species from non-siganid hosts Tetrancistrum nasonis Young, 1967 (   211-535; n54) [412 (334-517; n56)] in posterior trunk at level of gonads. Haptor 189 (109-150; n53) [168 (148-190; n55) Previous records. Naso annulatus: Heron Island, Australia (Young 1967;Lester and Sewell, 1989). N. unicornis: Hawaii as Pseudancyrocephalus duplicatus by Yamaguti (1968); Irabu Island, Okinawa Prefecture, Japan as P. duplicatus by Machida (1979). Remarks. Young (1967) described the gut of T. nasonis ''without lateral diverticula, united posterior to testis by two or three transverse crura which are connected medially by longitudinal crura'', while Yamaguti (1968) showed them terminating separately and blindly in the posterior trunk of Pseudancyrocephalus duplicatus. However, the intestinal caeca of the available type specimens of both T. nasonis and P. duplicatus were not visible, and confirmation of their configurations could not be determined. In addition, Yamaguti (1968) confused the dorsoventral positions of the anchors and bars in his description of P. duplicatus, as shown by his references to the ventral anchor with the ''ventral root (superficial root)'' reduced to a ''stumpy rod'' and to the dorsal anchor with the ''ventral root (deep root)'' a little shorter than the ''dorsal root (superficial root)''. Finally, the specimens of P. dupicatus are somewhat larger in most respects than those of T. nasonis, which are heavily compressed. Nonetheless, the two species are clear synonyms based on the comparative morphology of the copulatory complex and haptoral sclerites, with T. nasonis Young, 1967  Source of current specimens. Naso brevirostris: off Heron Island, Great Barrier Reef, Queensland, Australia. Type host and locality. Naso brevirostris: Hawaii.
Etymology. The specific name, a noun, is Hawaiian (makau5fishhook) and refers to the haptoral anchors.
Remarks. This species is renamed Tetrancistrum makau nom. n. to avoid secondary homonymy with T. nasonis Young, 1967 that would result with the proposed transfer of Pseudancyrocephalus nasonis to Tetrancistrum. The characterization of the anchors and bars in the original description of the species by Yamaguti (1968) indicates that the dorsoventral positions of these structures were originally reversed by this author. Yamaguti's (1968) original description is otherwise adequate for diagnosis. A single specimen of T. makau was recovered from a specimen of Naso brevirostris collected off Heron Island, Australia, which represents a new locality record for the species. This specimen was noticeably smaller than those from Hawaii but is clearly conspecific with the latter based on the morphology of the copulatory complex and anchors.
Remarks. The original description of this species, as Pseudancyrocephalus longicirrus, is adequate for diagnosis except that the dorsoventral positions of the anchors and haptoral bars were reversed by Yamaguti (1968). The species is transferred to Tetrancistrum as T. longicirrus (Yamaguti, 1968) comb. n. based on the morphology of the internal body organs and haptoral armament. Tetrancistsrum longispicularis (Yamaguti, 1968)   Remarks. With the exception of the reversal of the dorsoventral axis of the haptor, the original description by Yamaguti (1968) is adequate for the diagnosis of the species. The species is transferred to Tetrancistrum as T. longispicularis comb. n. based on the arrangement and morphology of the internal organs and the morphology of the haptoral armament. Although the body dimensions and soft internal organs of specimens collected from Naso brevirostris in Australia were generally smaller than those of the type series, specimens from Australia and Hawaii are considered conspecific based on the comparative morphology of the copulatory complex. The finding of T. longispicularis on N. brevirostris in Australia is a new locality record for the helminth.
Type host and locality. Naso brevirostris: Hawaii.
Remarks. Unlike all other species of Pseudancyrocephalus that Yamaguti (1968) described, the anchors and bars, and therefore the dorsoventral axis of the haptor, were correctly interpreted in the original description of this species. The original description serves as an adequate diagnosis of the species, which is transferred to Tetrancistrum as T. kala comb. n. based on the comparative morphology of the haptoral armament and internal organs. Tetrancistrum lutiani Tubangui, 1931 Type host and locality. Lutjanus monostigma: aquarium at the Bureau of Science, Manila, Philippines.

Site of infestation. Gills.
Previous record. No other records except for the original description by Tubangui (1931).
Remarks. Specimens of T. lutiani were not available for study, and the type specimens have apparently been destroyed (Hayward 1996). Tubangui (1931) described this species from specimens collected from Lutjanus lioglossus (now L. monostigma) held in an aquarium containing other marine fishes, including Siganus virgata (Valenciennes) (Siganidae) and Anyperodon leucogrammicus (Valenciennes) (Serranidae), at the Bureau of Science in Manila (Philippines). While the original description, particularly the illustrations, lack adequate detail for diagnosis, the species is definitely a member of Tetrancistrum as defined herein. The copulatory complex resembles that of T. fusiforme in that both species possess a relatively thick-walled, expanded, Cshaped MCO. However, they are differentiated by the superficial root of the ventral anchor being longer than the dorsal root and the absence of an accessory piece in the copulatory complex of T. lutiani (roots subequal in length and accessory piece present in T. fusiforme).
Although Tubangui's (1931) report of a species of Tetrancistrum on a lutjanid host is not unique (see Gupta and Sharma 1982), L. monostigma may not be the natural host for this species but rather an incidental infestation resulting from association of L. monostigma with siganids present in the aquarium from which the parasite was collected. Examination of L. monostigma for gill parasites from the Philippines will be necessary to verify the natural host for this parasite.

Site of infestation. Gills.
Previous record. No other records except for the original description (Gupta and Sharma 1982).
Remarks. The type specimens of T. lebedevi have not survived (N. Agrawal, personal communication) and specimens of the species were not available for study. The original description is incomplete, and the illustrations of the species, which lack sufficient detail for diagnosis, were clearly mixed with those of another dactylogyrid species described in the paper by Gupta and Sharma (1982); their Plate 7, Figure 6 (identified as the haptor of T. lebedevi) apparently represents the haptor of Pseudohaliotrematoides lutjanusi Gupta and Sharma, 1982, which they described from Lutjanus lutjanus (Lutjanidae), while Plate 6, Figure 2 (identified as the haptor of P. lutjanusi) is that of T. lebedevi. Their depiction of the copulatory complex of T. lebedevi (Plate 7, Figure 3) resembles that of T. yamagutii (Figure 36), from which it differs by lacking the basal recurves of the shaft of the MCO. Because of the absence of type specimens and the inadequate original description, T. lebedevi is considered a species inquirenda.

Other dactylogyrid species of concern
In the above revision, Tetrancistrum is restricted to species having tandem gonads (germarium forming a cap over the anterior margin of the haptor) and haptoral anchors with longitudinally striated basal roots. Two dactylogyrid genera, Pseudohaliotrematoides and Pseudancyrocephalus, are considered junior subjective synonyms of Tetrancistrum. These taxonomic actions have resulted in a number of described species previously placed in these genera without formal taxonomic positions. The following brief remarks pertain to the taxonomic status of these species.
Tetrancistrum obesum Caballero, Bravo-Hollis, and Grocott, 1955 This species was originally described from the gills of Tetraodon hispidus Linnaeus (Tetraondontidae) in Panama by Caballero et al. (1955). Young (1968), who provided drawings of the species from Australian waters, transferred it to Haliotrema as H. obesa Grocott, 1955) Young, 1968 based on the presence of the vas deferens looping the left intestinal caecum and on the general shape of the body. Although Caballero et al. (1955) did not depict the copulatory complex of T. obesum, the general body shape and basic morphology of the haptoral armament suggest that the species belongs in Thylacicleidus Wheeler and Klassen, 1988 (see Ř ehulková andGelnar 2005). However, transfer of T. obesum to the latter genus is not made, pending further study of the parasite from Panama. The species is retained in Haliotrema as originally proposed by Young (1968).
Tetrancistrum plectocirra (Paperna, 1972) Lim, 2002 This species, originally described as a member of Pseudohaliotrema, was transferred to Tetrancistrum by Lim (2002). It has subsequently been transferred to Glyphidohaptor by .
Pseudohaliotrematoides aurigae Yamaguti, 1968Yamaguti (1968 described P. aurigae from the gills of Chaetodon auriga (Forsskål) (Chaetodontidae) in Hawaii. The species was redescribed and transferred to Haliotrema (sensu lato) by  based on specimens collected from several chaetodontid species from coral reefs of the Indo-West Pacific Ocean. Khanna, 1974 Gupta andKhanna (1974) described this species from an unidentified ''marine fish (Teleost)'' from Indian waters. Specimens of P. bengalensis were not available for study and the original description does not allow appropriate generic placement. Thus, the species is considered a species inquirenda.
Pseudohaliotrematoides falcatus Yamaguti, 1968 This species was described by Yamaguti (1968) from the gills of Holocentrus spinifer (Forsskål) [now H. spiniferum (Forsskål)] (Holocentridae) off Hawaii. Although this species would comfortably fit in Haliotrema (sensu lato), formal transfer is not made at this time in order to prevent possible unnecessary synonymy upon completion of revisory work on Haliotrema (s. l.). The species is considered incertae sedis.
Pseudohalitrematoides lutjanusi Sharma, 1982 (5 Pseudohaliotrematoide lutjanusi Gupta andSharma, 1982, lapsus.) Type specimens of this species have not survived (N. Agrawal, personal communication) and the original description is inadequate for generic placement. The species was collected from the gills of Lutjanus lutjanus Bloch (Lutjanidae) and likely belongs to either Haliotrema (sensu lato) or Euryhaliotrema Kritsky and Boeger, 2002 based on the original drawings of the body and haptor. The transfer is not made, however, to avoid possible unnecessary synonyms. The species is considered a species inquirenda.
Pseudohaliotrematoides rohdei Gupta and Sharma, 1982 The type specimens of this species have not survived (N. Agrawal, personal communication). The original description and illustration of this species are generalized and inadequate for definite generic placement of the species. Although Gupta and Sharma (1982) indicated that the gonads are tandem (germarium pretesticular) in the description of P. rohdei, their whole-body illustration (ventral view) (Plate 5, Figure 1) shows the testis lying to the right side of the germarium. Should the dorsoventral axis of the body have been erroneously determined by these authors (a not uncommon occurrence) and the fact that the species was described from a siganid, Siganus oramin (now S. canaliculatus), supports the possibility that this species belongs in Glyphidohaptor (see ). Because of the inadequate description provided by Gupta and Sharma (1982), however, the formal transfer of the species to this genus is not made in order to avoid possible unnecessary synonymy; P. rohdei is considered a species inquirenda. Yamaguti, 1968Yamaguti (1968 described this species from three chaetodontids, Forcipiger longirostris (Broussonet), Chaetodon auriga, and C. multicinctus Garrett from Hawaii. Plaisance and Kritsky (2004) recorded the species from numerous chaetodontids collected from the coral reefs of western Pacific islands, considered Haliotrema hainanensis Pan and Zhang, 2000 its junior synonym, and transferred it as a new combination to Euryhaliotrematoides. Yamaguti, 1968 Pseudohaliotrematoides zancli was described by Yamaguti (1968) from the gills of Zanclus canescens (Linnaeus) (Zanclidae) from Hawaii. This species is a likely junior synonym of Haliotrema canescens (Mizelle and Price, 1964) based on the comparative morphology of the copulatory complexes. Formal proposal of synonymy will depend on further study of all species reported from Z. canescens by Mizelle and Price (1964) and Yamaguti (1968). Pseudohaliotrematoides zancli is thus considered incertae sedis.

Discussion
Although two species, Tetrancistrum lutiani and T. lebedevi, infest members of the Lutjanidae, the majority of species in the genus occur on perciform fishes comprising the Acanthuridae and Siganidae. Greenwood et al. (1966) considered the latter two families of fishes to form the suborder Acanthuroidei, while Leis and Richards (1984) assigned the Acanthuridae and Siganidae, along with the Zanclidae and Luvaridae, to the Acanthuroidei based on the comparative morphology of their larvae. The latter systematic arrangement was supported by Tyler et al. (1989), who conducted a phylogenetic analysis based on characters derived from the osteology and larval morphology of members of the four families.
If the Siganidae and Acanthuridae are phylogenetically related as suggested by these authors, it is likely that Tetrancistrum originated on fishes of the Acanthuroidei. Although a phylogenetic analysis of the species comprising Tetrancistrum followed by an objective coevolutionary analysis would be necessary to substantiate this hypothesis, the idea is presently supported by the restricted occurrence on siganid fishes of species comprising two similar genera, Glyphidohaptor and Pseudohaliotrema. Tetrancistrum species differ from those of Glyphidohaptor and Pseudohaliotrema primarily by having tandem gonads with the germarium forming a cap over the anterior margin of the testis (germarium lying dextral to the anterior portion of the testis in Glyphidohaptor and Pseudohaliotrema species) and longitudinal striae on the elongate roots of the anchor bases (anchor roots comparatively short and lacking striae in species of Glyphidohaptor and Pseudohaliotrema). Tandem gonads are clearly a symplesiomorphic feature of the Dactylogyridae (see Boeger and Kritsky 2001), while the relative positions of the gonads displayed by species of Glyphidohaptor and Pseudohaliotrema are apparently synapomorphic and serve to define the sister-group relationships of the two genera. The elongate longitudinally striated roots of the haptoral anchors are an apparent synapomorphy that, in part, defines Tetrancistrum. Other features of the internal anatomy of the members of the three genera are strikingly similar (see ). This all suggests that the three genera form a natural clade, with Tetrancistrum serving as the sister group to the taxon including Pseudohaliotrema and Glyphidohaptor, and that the common ancestor of members of the three genera likely occurred on an early member of the Acanthuroidei. The two species of Tetrancistrum from lutjanid hosts, while clearly members of the genus (as presently defined), apparently resulted from host switching followed by speciation, a common occurrence in the historical development of other taxa comprising the Monogenoidea (see Klassen 1994;Kritsky and Boeger 2002;Boeger et al. 2003, Domingues andBoeger 2005; among others).