Taxonomic Status of Diplectanum robustitubum Wu & Li, 2003 (Monogenoidea: Diplectanidae) from the Purple-Spotted Bigeye Priacanthus tayenus (Priacanthidae) and the Description of Platycephalotrema parile n. sp. (Monogenoidea: Dactylogyridae) from the Bartail Flathead Platycephalus indicus (Platycephalidae), in the Arabian Gulf

Abstract Purple-spotted bigeyes Priacanthus tayenus Richardson (Priacanthidae) and bartail flathead Platycephalus indicus (Linnaeus) (Platycephalidae) were collected from the Arabian Gulf and examined for species of Monogenoidea (Polyonchoinea) from February to December 2020. Diplectanum robustitubum Wu & Li, 2003 and an undescribed species of Platycephalotrema Kritsky & Nitta, 2019 were recovered from the gill lamellae of these hosts, respectively. Diplectanum robustitubum from Iraq was redescribed and transferred to Oliveriplectanum Domingues & Boeger, 2008 (Diplectanidae) as Oliveriplectanum robustitubum (Wu & Li, 2003) n. comb. Platycephalotrema parile n. sp. (Dactylogyridae) from Iraq and Kuwait was described and differentiated from the similar species, Haliotrema indicum Tripathi, 1959, Platycephalotrema ogawai Kritsky & Nitta, 2019, and Platycephalotrema platycephali (Yin & Sproston, 1948) Kritsky & Nitta, 2019, based primarily on the comparative morphologies of the vaginal sclerites. Haliotrema indicum was transferred to Platycephalotrema as Platycephalotrema indicum (Tripathi, 1959) n. comb. and Haliotrema swatowense Yao, Wang, Xia, & Chen, 1998 was considered a junior subjective synonym of P. indicum. The finding of O. robustitubum in the Arabian Gulf represents a new geographic record for the species. Background The present paper represents the third installment concerning the monogenoids collected during surveys to explore their diversity on the marine and freshwater fishes of Iraq. Previous installments on the monogenoids emanating from the surveys included the dactylogyrid and gyrodactylid species parasitizing mugilid fishes. Purpose The purpose of this paper is to further document the diversity of monogenoids infecting the fishes of Iraq. Methods Marine fishes were necropsied for parasites, and standard procedures for collecting, mounting, drawing, and measuring of monogenoids were employed. Results Oliveriplectanum robustitubum n. comb. (Diplectanidae) and Platycephalotrema parile n. sp. (Dactylogyridae) were collected. The occurrence of O. robustitubum in the Arabian Gulf represented a new locality record for the species. Conclusion The recorded presence of O. robustitubum and P. parile n. sp. suggests that the diversity of monogenoids in Iraq is under estimated in the literature.

In the present paper, the taxonomic status of the diplectanid Diplectanum robustitubum Wu & Li, 2003 and that of the dactylogyrid Platycephalotrema parile n. sp. are evaluated.Both helminths occur on the gill lamellae of marine fishes of Iraq: D. robustitubum on the purple-spotted bigeye Priacanthus tayenus Richardson (Priacanthidae) and Pt.parile n. sp. on the bartail hardhead Platycephalus indicus (Linnaeus) (Platycephalidae).The respective hosts occur naturally throughout much of the Indo-West Pacific Ocean, where both are commercially important.Although less so in other regions, bigeyes comprise a significant component of the trawl fishery of southeast Asia, particularly in the areas if the Andaman Sea and the southern region of the South China Sea [23].Within the middle eastern region of Asia, the bartail flathead is the most common flathead appearing in markets [9], where it is sold as a very palatable food fish [5]; the flathead is also a component of Chinese medicine [24].

Materials and Methods
Specimens of the purple-spotted bigeye and the bartail hardhead were collected off the southern marine coast of Iraq near the mouth of the Shatt Al-Arab River (29°53′-29°85′N, 48°13'-48°40′E) by fishermen using trawl nets during February through December, 2020, where salinity levels varied between 24.5-32.5PSU.Fish hosts were identified using Carpenter et al. [9],the higher classification for the fishes was that presented by Betancur-R et al. [6],and common and scientific names of fishes were verified using Froese & Pauly [12] and Fricke et al. [11], respectively.The fishes were transported in an icebox filled with crushed ice to the laboratory located in Basrah, Iraq, and examined for monogenoids within 48 h of capture.Gill baskets were removed and place in vials containing a hot (~ 60 C) 5% formalin (2% formaldehyde) solution for relaxation and fixation of monogenoids.The vials were labeled and then shaken to ensure suitable fixation and removal of helminths from the gill tissues.Helminths were subsequently picked from the gills or usually from the sediment using a fine probe, placed in small labeled vials, and shipped to Idaho State University for study.Methods for preparation, illustration, and measurement of monogenoidean specimens were those of Kritsky [13], see Sey & Nahhas [22] for methods for collection and preservation of specimens from Kuwait.Illustrations were prepared using a microprojector or a camera lucida mounted on a compound phasecontrast microscope.Measurements, all in micrometers, represented straight-line distances between extreme points and were expressed as the mean followed by the range and number (n) of structures measured in parentheses; body length included that of the haptor.Numbering (distribution) of haptoral hooks followed the convention proposed by Mizelle ( [18], see Mizelle & Price [19]; direction of the coil of the shaft of the MCO, clockwise vs. counterclockwise, was determined using the method proposed by Kritsky et al. [15].Definitions of ecological terms were those of Bush et al. [7], except that prevalence and intensity were reported as "minimum prevalence" and "minimum intensity" sensu Kritsky et al. [14] In the various sections of this paper, the following abbreviations for host and parasite genera beginning with the letter "P" are utilized: Pr. for Priacanthus, Pl. for Platycephalus, and Pt. for Platycephalotrema.These abbreviations were only used after the respective generic and appended specific names were first presented within a given passage or paragraph.
Class Monogenoidea Bychowsky, 1937   Previous record: There have been no other records of the species since its original description by Wu & Li [26].
Infection site: Gill lamellae.Redescription: Body proper fusiform, slightly flattened dorsoventrally; greatest width (excluding haptor) at level of gonads.Cephalic region with rounded medial and two moderately developed bilateral cephalic lobes; three bilateral pairs of head organs; cephalic glands composed of two bilateral groups of glandular cells lying posterolateral to pharynx.Tegument smooth.Eyespots absent; chromatic granules small, ovate, numerous, scattered throughout cephalic region, often forming small clumps anterior to pharynx.Mouth midventral at level of anterior margin of pharynx; pharynx subspherical; esophagus short to non-existent; intestinal ceca two, lacking diverticula, terminating blindly in posterior trunk.Peduncle short, tapered posteriorly; haptor with dorsal and ventral squamodiscs and two bilateral lobes containing hook pairs 2-4, 6, and 7. Dorsal and ventral squamodiscs usually with 11 rows of rodlets each (varying from 9 to 12); deep (anterior-most) one to two rows forming closed circles; rodlets of anterior seven rows of squamodisc dumb-bell shaped, those of rows 8-11 with posterior shield-like processes.Ventral anchor with deep and superficial roots of base subequal in length, gently arced shaft, recurved point.Dorsal anchor with subtriangular base, slightly arced to straight shaft, recurved point.Ventral bar with medial constriction, tapered ends, longitudinal ventral groove.Paired dorsal bars; each with bilobed medial end and anteriorly directed lateral end.Hook with protruding terminally blunt thumb, slender shank comprised of single subunit; filamentous hook loop about 3/4 shank length.Common genital pore ventral near level of intestinal bifurcation.MCO a U-or C-shaped tube with internal (nested sensu [10]) tubular canal and distal thorn-like processes.Accessory piece absent.Gonads tandem, testis postgermarial.Testis ovate.Proximal portion of vas deferens not observed; seminal vesicle inconspicuous, a slightly dilated segment of distal vas deferens, lying to left of body midline, looping anterior to MCO before forming posteriorly directed ejaculatory duct.Prostates, prostatic reservoir not observed.Germarium pyriform, dorsoventrally looping right intestinal cecum.Oötype, Mehlis' glands not observed.Uterus dilated, extending from region anterior to germarium along body midline to common genital pore.Vaginal pore sinistroventral, submarginal; elongate vaginal vestibule giving rise to delicate vaginal canal.Seminal receptacle overlying anterior end of germarium and oviduct.
Vitellarium dense, coextensive with intestinal ceca; transverse vitelline ducts at level of seminal receptacle.Egg not observed.
Measurements (corresponding measurements provided by Wu & Li [26] [10] were apparently unaware of the description of Diplectanum robustitubum by Wu & Li [26], as the present authors initially were, as indicated by the species being unlisted as a member of the Diplectanidae in their paper. In addition to characters termed "traditional" by Dominques & Boeger [10], i.e., apparently characters not included in their analysis and apparently ubiquitous among diplectanid species, Oliveriplectanum was characterized by species having three features that they identified as plesiomorphies and autapomorphies: (1) the genital pore opening anterior to the MCO, (2) the anterior rows of rodlets of the squamodiscs being closed, and (3) the superficial root of the ventral anchor "developed" (i.e., the superficial and deep roots of the ventral anchor subequal in length).The latter three characters represented either autapomorphic reversals to the respective plesiomorphic states or parallel developments of features occurring in other ingroup taxa.Autapomorphic features unique to members of Oliveriplec tanum, i.e., features not present in any other ingroup taxon in the analysis, were not identified.However, the absence of a unique autapomorphy for Oliveridiplectanum does not necessarily mean that the genus is invalid.Whereas a character state present in a common ancestor or other ingroup taxa may be impossible or difficult to differentiate from the respective feature found in species of Oliveriplectanum, these characters in the respective taxa are not homologs as each apparently has a unique evolutionary history.Thus, assuming that the hypothesis presented by Domingues & Boeger [10] actually represents the evolutionary history of the Diplectanidae, Oliveriplectanum is here accepted as valid.
The proposed transfer of D. robustitubum to Oliveriplec tanum, as Oliveriplectanum robustitubum (Wu & Li, 2003) n. comb., is based on the species having of all of the socalled plesiomorphic and autapomorphic characters defining the genus (see [10]).The species resembles O. priacanthi and O. curvivagina by having a U-or C-shaped MCO, an uncommon feature among members of the Diplectanidae including their remaining congener O. opakapaka that possesses a coiled MCO with several apparently counterclockwise rings.Oliveriplectanum robustitubum, with one to three rows of the squamodiscs forming closed circles, differs from O. priacanthi with apparently all rows of the squamodiscs being closed (see Fig. 93b  Description: Based on the specimens from Iraq.Greatest width of body proper usually near midlength of trunk.Cephalic region with well-developed medial and bilateral cephalic lobes.Eyespots four, apparently lacking lenses; individual eyespots occasionally dissociated; members of respective pairs equidistant, those of posterior pair larger than those of anterior pair.Accessory chromatic granules absent or few in cephalic region.Pharynx ovate; esophagus short to absent.Peduncle moderately long, slightly tapered posteriorly.Haptor subhexagonal, with lateral lobes containing hook pairs 3, 4, 6, 7; bilateral vesicles filled with granular material, lying lateral to bases of dorsal anchors.Ventral and dorsal anchors similar; each with elongate superficial root, short knob-like deep root, slightly arced to straight shaft, elongate point.Ventral bar with long spatulate ends directed laterally in haptor.Dorsal bar variable in width along its length, with unequal terminal limbs directed anterolaterally.Hook with uniform shank, conspicuous protruding blunt thumb; FH loop nearly shank length.MCO with inverted-bowl-shaped base, slightly tapering tubular shaft; distal end of shaft with clockwise terminal loop and weakly sclerotized flap.Testis ovate; proximal vas deferens dorsoventrally looping left intestinal cecum, distal vas deferens expanding to form fusiform seminal vesicle lying to left of MCO.Prostates not observed; two large prostatic reservoirs; anterior (ventral) reservoir usually containing two poorly differentiated zones of granules; posterior (dorsal) reservoir smaller than and often overlain by anterior reservoir.Germarium ovate; oviduct, oötype, Mehlis' glands not observed; uterus delicate, diverted to left of large vaginal sclerite and prostatic reservoirs.Vaginal pore dextroventral, submarginal, surrounded by delicate sphincter-like muscle; vaginal ampulla lying posterolateral to vaginal sclerite and with comparatively thin wall; vaginal canal delicate, meandering from ampulla to small seminal receptacle.Vaginal sclerite appearing morphologically variable depending on degree of rotation within body, with anteriorly directed spine originating from base, planate distal flap having thickened margins (Fig. 15); anterior margin of flap often folded posteroventrally; fold, when present, appearing as a tapered spine (Fig. 16); base of vaginal sclerite resting on muscular pad.Seminal receptacle ovate, lying immediately anterior to germarium.Vitellarium dense, coextensive with intestinal ceca; transverse vitelline duct at level of seminal receptacle.Egg not observed.Five dactylogyrid species are known to parasitize the bartail flathead in the Indo-Pacific Ocean: Platycephalotrema parile n. sp., Pt. macassarense, Pt. platycephali, H. indicum, and H. swatowense.That the bartail flathead serves as host for a comparatively large number of Platycephalotrema spp. is not unexpected, however.Puckridge et al. [21], who utilized an analysis of molecular data in development of their hypothesis on the relationships of populations of Pl. indicus in the Indo-Pacific Ocean, recognized eight lineages and as a result suggested that Pl. indicus may comprised a complex of cryptic species.Whereas dactylogyrids are presumed to have a comparatively high host specificity, specimens parasitizing Pl. indicus in the Arabian Gulf, the localities as part of the Western Indian Region [21], could be expected to represent a species of Platycephalotrema distinct from those occurring on the bartail flathead in other regions of the host's natural range.This is borne out by Pt. macassarense having been described from the Macassar Strait, Indonesia (Indo Region of [21]) and later reported from the South China Sea off China (NW Pacific Region of [21]) [28,29,33], and Pt.platycephali, H. indicum [herein transferred to Platycephalotrema as Platycephalotrema indicum (Tripathi, 1959) [21]) [8,27,[31][32][33]; H. indicum was originally described from the Indo Region of Puckridge et al. [21] by Tripathi [25].
The internal organ systems and the haptoral sclerites of Platycephalotrema spp.are morphologically similar and provide few if any features sufficiently distinct to differentiate species within the genus.Species identifications most often depend on comparisons of the respective vaginal sclerites (when present) and the MCOs.Based on the common morphology of their MCOs, Pt. parile n. sp.most closely resembles Pt. ogawai and the Platycephalotrema and Haliotrema species infecting the bartail flathead.Platycephalotrema parile is differentiated from these species by having a vaginal sclerite with a planate distal flap having thickened margins, which when folded, may form a spinelike structure that extends nearly the entire length of the sclerite (Figs.15,16) (a planate flap is absent in Pt. ogawai, H. indicum, and H. swatowense).The vaginal sclerite of Pt. ogawai (not figured in the original description of the species by [16]) has two semi-acute lobes and a basal notch (Fig. 17), whereas those of H. indicum and H. swatowense were reported to have serrated margins (serrated margins absent in Pt. parile) (see Fig. 18 in Tripathi [25] and Fig. 2C in Yao et al. [31]).The vaginal sclerite is apparently absent or represented by a "wide cavity" with sclerotized folded walls in Pt. macassarense (see [28]), whereas the homologs of Pt. parile, Pt. ogawai, and H. indicum are not cavernous.
Kritsky and Nitta [16], who transferred Haliotrema platycephali Yin & Sproston, 1948 to Platycephalotrema, suggested that H. indicum and Pt.platycephali were synonymous but refrained from formally proposing the synonymy pending new collections of the two species from both India and China.Currently, the synonymy of these species remains in doubt.However, the vaginal sclerite of H. indicum as depicted by Tripathi [25] and that of H. swatowense as shown by Yao et al. [31] are morphologically similar, by having, among other features, serrated or frayed margins.Whereas H. indicum and H. swatowense are apparently parasitic on the same host species and that they possess similar haptoral, copulatory, and vaginal sclerites, the two nominal species are therefore placed in synonymy, with H. indicum having priority.Finally, based on the similarity of the haptoral sclerites as depicted by Tripathi [25] and Yao et al. [31] for H. indicum and H. swatowense, the presence of a sclerite associated with the vagina in both species, and other features defining Platycephalotrema sensu Kritsky & Nitta [16], H. indicum is transferred to Platycephalotrema as Platycephalotrema indicum (Tripathi, 1959) n. comb.
n. comb.sp.(see below)], and H. swatowense [a junior synonym of Pt. indicum (see below)] have been reported from the western Pacific Ocean off China (NW Pacific Region of