Four polyopisthocotyleans (Platyhelminthes: Monogenea) from carangid fishes in the Mediterranean, off the Algerian coasts

Four polyopisthocotyleans were collected from the gill filaments of carangids from off the Algerian coast, southern Mediterranean. Specimens of Gastrocotyle trachuri van Beneden & Hesse, 1863 (Gastrocotylidae) and Cemocotyle cf. trachuri Dillon & Hargis, 1965 (Heteraxinidae) from the Mediterranean horse mackerel Trachurus mediterraneus (Steindachner), Zeuxapta seriolae (Meserve, 1938) (Heteraxinidae) from the greater amberjack Seriola dumerili (Risso) and Pyragraphorus hollisae Euzet & Ktari, 1970 (Pyragraphoridae) from the pompano Trachinotus ovatus (Linnaeus) are redescribed based on newly collected specimens. Their taxonomically important morphological features (male copulatory organ and clamp sclerites) are described and illustrated, and the morphometric variation between Mediterranean and oceanic specimens is highlighted. Careful examination of the specimens of Cemocotyle Sproston, 1946 from the Mediterranean revealed that they exhibited unusual features compared with Cemocotyle trachuri Dillon & Hargis, 1965 from the Pacific, mainly the absence of the terminal lappet, thus questioning previous records of this species in the Mediterranean. New geographical locality records are provided for Z. seriolae and P. hollisae. The presence of C. cf. trachuri and Z. seriolae in the Mediterranean is noteworthy as these monogeneans were initially described in the Pacific Ocean. This study extends the geographical range of Z. seriolae to the southern Mediterranean.

In Algeria, as of the year 1995, early efforts of Dr Faiza Amine contributed significantly to the systematics of some monogeneans from off the Algerian coast (Kouider El Ouahed-Amine, 1998;Amine & Euzet, 2005;Amine et al., 2006aAmine et al., , b, 2007a. Nonetheless, regardless of the growing number of possible hosts, surveys of monogenean parasites of marine fishes from off the Algerian coast remain relatively sparse and targeted mostly and largely ecology (Kaouachi et al., 2010;Marzoug, 2012;Ramdane et al., 2013;Brahim Tazi et al., 2016;Benhamou et al., 2017;Hadjou et al., 2017;Ichalal et al., 2017;Ider et al., 2018). Moreover, these surveys focused on economically important host species, mostly sparids (Kouider El Ouahed-Amine, 1998;Kaouachi et al., 2010;Marzoug, 2012;Ramdane et al., 2013;Benhamou et al., 2017;Ider et al., 2018). In addition, most of these data were published in local scientific journals or remain unpublished in MSc and PhD theses, making them difficult to access. It is also likely that some of the monogeneans included within these studies have been misidentified.
Amongst teleost fishes frequently encountered in Algerian marine waters, carangid fishes represent suitable candidates for taxonomic surveys of parasites. They include 146 species of 32 genera (Froese & Pauly, 2016), are largely distributed worldwide, and occur in both tropical and temperate waters (Fischer et al., 1987;Smith-Vaniz, 1999). Estimation of parasite biodiversity supported by these fishes is far from being completed, as most of the implicated fish hosts are with a cosmopolitan distribution and a migratory life-style. Hence, both morphological and molecular data on their parasite fauna, along with type-and voucher specimens in recognised museum collections must be provided for each locality to assure unbiased descriptions of new species.
During a survey of helminth parasites of marine fishes from the South-Western (SW) Mediterranean, six monogenean species were found on the gills of carangids. The Mediterranean horse mackerel Trachurus mediterraneus (Steindachner) distributed in the North-East (NE) Atlantic and the Mediterranean was found parasitized by four species: Pseudaxine trachuri Parona & Perugia, 1890, Allogastrocotyle trachuri Nasir & Fuentes Zambrano, 1983, Cemocotyle cf. trachuri Dillon & Hargis, 1965 and Gastrocotyle trachuri van Beneden & Hesse, 1863; the pompano Trachinotus ovatus (L.) which occurs in the NE Atlantic from Africa to the North Sea, was found parasitized by the rare species Pyragraphorus hollisae Euzet & Ktari, 1970; and the cosmopolitan species Seriola dumerili (Risso) which is found in the Pacific, Atlantic and Indian oceans, and the Mediterranean, harboured a single species, Zeuxapta seriolae (Meserve, 1938). The first two monogenean species were redescribed recently (Bouguerche et al., 2019b, c). Herein, we provide detailed illustrated descriptions of the remaining four polyopisthocotylean monogeneans and discuss their taxonomic status, hosts, and distribution.
Monogeneans were removed alive from gills using fine dissection needles, then preserved in 70% ethanol, stained with acetic carmine, dehydrated in ethanol series (70, 96 and 100%), cleared in clove oil, and mounted in Canada balsam. Some specimens were mounted in Berleseʼs fluid to study the morphology of the clamps and the genital atrium. Drawings were made with the aid of a Leitz microscope equipped with a drawing tube (Leitz, Wetzlar, Germany). Drawings were scanned and redrawn on a computer with Adobe Illustrator (CS5). Measurements are in micrometres and are indicated as the range followed by the mean and the number of measurements in parentheses.  -12, 15-22, 24, 25, 27, 29-31, 32, 34, 36-38, 40, 41, 43, 44, 47, 48; present study].
Site on host: Gills. Voucher material: 38 voucher specimens are deposited in the collections of the Mus eum National d'Histoire Naturelle, Paris, France (MNHN HEL1500-HEL1544).

Description
[Based on 26 specimens; Figs. 1-3; Table 1] Body elongated, narrow, symmetrical in first anterior third and considerably wider and asymmetrical posteriorly. Haptor parallel to longitudinal body axis, occupying 2/3 of total body length. Haptor with single row of clamps (Fig. 1A) and terminal lappet (Fig. 1I, J). Terminal lappet armed with 6 uncini; lateral pairs large and stout, medial pairs small (Fig. 1H). Clamps of Gastrocotyle type (Fig. 1G). Ventral arm of median spring a1 Y-shaped, long, distal part of a1 Y-shaped, with short branches of equal size, each limb abutting on short oblique sclerites. Dorsal arm of median spring a3 shorter than a1, distally broad, with 3-4 pairs of apertures arranged in 2 longitudinal symmetrical parallel rows, distal end of a1 with 2 superposed accessory skeletal pieces at its distal end: a' represented by V-shaped process; a" (Fig. 3A). Ventral arm of ventral jaw sclerites b1, dorsal arm b2 short and curved inwards, b2 not reaching accessory skeletal piece of dorsal arm of median spring (Fig. 3B). Dorsal jaw sclerites c shorter than ventral, c reaching midline on distal side. Oblique sclerites d long, with inner ends folded inwards. Muscle connecting a2 and b2 present on proximal side (Fig. 3C).
Testes c.13 in number, small, follicular, in intercaecal field of posterior body third, often obscured by vitellarium. Vas deferens conspicuous, dorsal to uterus, running forward along body midline, expanding in its terminal part into ejaculating bulb (Fig. 1D). Male copulatory organ composed of genital atrium and stylet (Fig. 1C). Genital atrium muscular, mid-ventral, opening at short distance from anterior extremity; central conical stylet surrounded by 12 hooks arranged in circle; each hook with pointed base and curved end (Fig. 1E).
Ovary pre-testicular, longitudinally elongated, inverted U-shaped (Fig. 2). Oviduct arising from distal end of ovary and connecting common vitelline duct reservoir and o€ otype. Mehlis' gland at base of o€ otype. Uterus originating from o€ otype and extending to genital atrium. Vagina not observed. Vitellarium follicular, well developed, co-extensive with caeca, extending from level of genital atrium to posterior extremity of body. Vitelloducts Y-shaped, with noticeably short branches; dorsal transverse vitelloducts fused at ovary; common vitelline duct median, fairly long. Eggs fusiform with two short polar filaments (Fig. 1F).

Hosts and distribution
Gastrocotyle trachuri was originally described off Brest, France (North-East Atlantic) (van Beneden & Hesse, 1863) and subsequently reported from the Atlantic, from northern and southern localities. Currently, this gastrocotylid is widely distributed in tropical and temperate waters (see Table 2) being recorded from the North-Western Mediterranean and rarely from the South-Western Mediterranean. Records from the Central and North-Western Mediterranean are scarce and include off Italy, France, Montenegro, Spain, and Turkey. In the South-Western Mediterranean this species is known only from Tunisia and Algeria. It was frequently reported from the North Atlantic (13 records, see Table 2) and rarely from the South Atlantic (off Namibia and Angola). It occurs furthermore in North Pacific (Japan, South China Sea, East China Sea, Yellow Sea and Hawaii) and the South Pacific (Australia and Tasman Sea). In the Indian Ocean, the only records of G. trachuri are those of Lebedev and Parukhin (see Table 2). It is likely that the carangid hosts, being vastly migratory and widely distributed in Mediterranean and oceanic waters, and the monogenean being monoxenous triggered its wide dispersal.

Description
[Based on 16 specimens; Figs. 4-6; Table 3] Body stocky, haptor considerably smaller (Fig. 4A), asymmetrical, triangular, armed with numerous clamps distributed in 2 unequal lateral rows (Fig. 4F); clamps of "muzzle" type ( Fig. 4E). Ventral arm of median spring a long, enlarged in its proximal part and Y-shaped, with very short, barely visible branches. Sclerites of ventral jaw b asymmetrical; right sclerite longer than left sclerite, resulting in slight clamp asymmetry (Fig. 6A). Dorsal arm of median spring short, ending by slightly prominent T. Sclerotised piece f articulated at dorsal distal base of a. Sclerites of dorsal jaw c with the same asymmetry as anterior jaw (Fig. 6B); asymmetry induced by right sclerite longer than left sclerite c (Fig. 6C).
Prohaptoral suckers oval, smooth-edged, opening ventrally in anterior part of body, widely separated from each other. Pharynx muscular, subspherical. Oesophagus short. Intestinal bifurcation immediately in front of genital atrium. Caeca extend to posterior extremity of body, not confluent, do not extend into haptor.
Testes c.11 in number, follicular, post-ovarian, intercaecal; few testes obscured by vitellarium and difficult to observe and count. Vas deferens runs upwards to join ejaculatory bulb (Fig. 4D). Genital atrium armed with numerous hooks (Fig. 4C) of different sizes and shapes arranged in anterolateral part of atrium (Fig. 4B).
Ovary elongated, pretesticular, originates in mid-posterior third of body (Fig. 5), extends forward, forms a loop, and then descends to lead into oviduct. Oviduct passing dorsally to seminal receptacle and joining genito-intestinal canal. Genito-intestinal canal abutting into right intestinal branch. Uterus dorsal to ovary, extending anteriorly to genital atrium. Seminal receptacle visible, ventrally to oviduct. Vitellarium follicular, well developed, extending in 2 lateral fields from posterior level of genital atrium to haptor. Posterior extremities of vitelline fields asymmetrical; left field slightly longer. Vitelline reservoir Y-shaped; ventral transverse vitelloducts thick, fused on midline; common vitelline duct short and thick.

Hosts and distribution
First described off South Island, New Zealand (South-West Pacific) (Dillon & Hargis, 1965), C. trachuri was subsequently reported only from South-East Pacific off Chile and Peru (Oliva, 1999). This species has also been reported from Atlantic waters: a few records from the North-East Atlantic and a single record from the South-East Atlantic (off Namibia). In the Mediterranean, C. trachuri was reported from off France, Tunisia and Algeria (see Table 4). Note that there are no records of this species in the Indian Ocean, suggesting the presence of separated stocks of fish hosts.
Vitellarium surrounding caeca, extending into 2 lateral fields from level of vagina to haptor. Posterior extremities of vitelline fields symmetrical, partially extending into haptor peduncle. Transverse vitelloducts not observed. Common vitelline duct exceptionally long. Vagina middorsal, at level of anterior constriction of body (Fig. 8), divided into 2 openings. Eggs subspherical, each with a long polar filament (Fig. 7D).

Hosts and distribution
Zeuxapta seriolae has a wide geographical distribution in both farmed and wild fish, and was reported from the Pacific and Atlantic oceans, and the Mediterranean. First described off Japan (NW Pacific), Z. seriolae was  Table 6). This the first report of Z. seriolae off Algeria. We note that this polyopisthocotylean had never been reported in the Indian Ocean. The wide spatial distribution of this monogenean is likely linked to the host mobility (the hosts species being highly migratory and sympatric). The host range of Z. seriolae is limited to two carangid genera: Seriola Cuvier and Caranx Lac ep ede. First described from Seriola quinqueradiata (Ishii & Sawada), this species was reported from Seriola lalandi Valenciennes (syn. S. dorsalis (Gill)), S. grandis Castelnau and S. dumerili Castelnau (see Table 6) and was also recorded on Caranx hippos (L.) (Boada et al., 2012). In the present study, this species was solely observed on its host S. dumerili and no other monogeneans were collected from this host.   (1933); Jones (1933); Sproston (1946); Llewellyn (1956Llewellyn ( , 1959Llewellyn ( , 1962Llewellyn ( , 1983; Shaw (1979); Rahemo (2012)
("muzzle"-type) (Fig. 11A), elevated on stalks and occupying oblique part of the haptor; unstalked clamps small (Microcotyle-type) (Fig. 11A), extending to apex of haptor. Clamps of anterior series characteristic of the genus Pyragraphorus (Fig. 11D). Each clamp formed by 2 jaws of unequal size: ventral jaw (Fig. 15A) and larger posterior jaw (Fig. 15B). Ventral arm of median spring a long, T-shaped on proximal side. Lateral sclerites b of ventral jaw asymmetrical. On proximal side, sclerites b drawing semicircle giving front half of clamps circular appearance. Sclerites b straight and parallel to each other in posterior half of clamps. Circular anterior part of clamps marked by epidermal expansions. Dorsal arm of median spring a prolonged by median sclerification f; the latter ending in a spearhead in circular part of clamps. Lateral sclerites of dorsal jaw c arranged as sclerites b.
Clamps of posterior series relatively small, of Microcotyle-type (Fig. 11C). Ventral arm of median spring long, thin, ends distally in slightly prominent T (Fig. 16A). Lateral sclerites of ventral jaw b approaching midline distally. Dorsal arm of median spring inverted Tshaped (Fig. 16B). Median sclerite in "spearhead" s.m. articulated on dorsal arm of median spring. Lateral sclerites of dorsal jaw c arched and longer than b.
Testes c.12 in number, subcircular to oval, few and confined to postovarian intercaecal field, do not reach posterior junction of vitellarium. Vas deferens sinuous, arising dorsally to right of midline, extending along body midline and abutting in cirrus-pouch. Cirrus-pouch with thick wall of muscle fibres (Fig. 12), containing internal seminal vesicle, prostatic reservoir and cirrus proper. Internal seminal vesicle subspherical, connected to channel projecting forward, reaching the base of elongated and pleated chamber, corresponding to prostate reservoir. Numerous prostatic glands distributed on either side of basal third of cirrus pocket. Two ducts originating from 2 vitellarium strips, cross cirrus-pouch wall, and open at base of prostate reservoir. Prostate reservoir thins anteriorly into narrow duct, which ends in cirrus. Cirrus lateral to uterus, long, cylindrical, armed with numerous thorns ("cils" in the French nomenclature used by Euzet & Ktari (1970)).
Ovary folded, dorsal, pretesticular (Fig. 13), originates on right side, passes to left, goes up, and describes a loop to return to right where its descending branch crosses transverse branch of immature part, before throwing itself into oviduct. Genito-intestinal canal detaches from oviduct and abuts in right intestinal branch. Oviduct first receives vitelline reservoir, then forms posterior loop, which follows a swollen o€ otype. Mehlisʼ gland not observed. Uterus extending along body midline, dorsally, leading into genital pore. Vagina complex (Fig. 14), with smooth anterior part and posterior part forming funnel marked at its base by cilia grouped in brushes. Posteriorly, funnel continuing through narrow, short channel ending in anterior part at point of junction of transverse vitelloducts. Vitelloducts unite again at mid-length of body, at level of ovary to form ventral vitelline reservoir opening into oviduct. Vitelline follicles globular, surrounding intestinal branches.

Hosts and distribution
Pyragraphorus hollisae is a poorly known species. Described for the first time from off the southern Mediterranean coasts of Tunisia by Euzet & Ktari (1970), it was recorded in the Atlantic, off Morocco; and allegedly from the Pacific off Mexico and from the Indian Ocean (off Indonesia) (Table 8). In the Mediterranean, P. hollisae was also reported only off France and Spain. Algeria is a new geographical record for this monogenean.

Discussion
To date, 42 polyopisthocotylean species have been reported from teleosts from off Algeria (Bouguerche, 2019). Unfortunately, most of these records were included in unpublished MSc and PhD theses, making them difficult to access. In addition, the monogeneans included within are often poorly and insufficiently described and these descriptions generally lack morphometric data. In the present paper, we redescribe four polyopisthocotyleans collected from the gill filaments of three carangids from off the Algerian coast, South-Western Mediterranean: Gastrocotyle trachuri (Gastrocotylidae) and Cemocotyle cf. trachuri (Heteraxinidae) from the Mediterranean horse mackerel T. mediterraneus; Zeuxapta seriolae (Heteraxinidae) from the greater amberjack S. dumerili and Pyragraphorus hollisae (Pyragraphoridae) from the pompano T. ovatus. The following sections provide accurate comparisons the morphometric variations between Mediterranean and oceanic specimens; we also briefly discuss the taxonomic status and host specificity of the four monogeneans.

Gastrocotyle trachuri van Beneden & Hesse, 1863
Among the Gastrocotylidae, the genus Gastrocotyle van Beneden & Hesse, 1863 is unique due to having only one side of the haptor developed as a marginal frill bearing a row of clamps extending along one side of the body to about halfway between the anterior margin of the ovary and the vaginal pore. Currently, Gastrocotyle includes seven species, all parasites of fishes belonging to the Carangidae Rafinesque and Scombridae Rafinesque ( Table 9). The most recently described species in this genus is Gastrocotyle buckleyi Gupta & Krishna, 1980 from the malabar trevally Carangoides malabaricus (Bloch & Schneider) (syn. Caranx malabaricus) off India (Gupta & Tandon, 1980). Gastrocotyle buckleyi exhibits marked differences compared to the congeners: an elongated triangular body separated from the haptor, oblique haptor located very far from the ovary unlike in Gastrocotyle spp. in which the haptor only occupies only one side of the body and extends over more than half of its length. Gastrocotyle buckleyi should probably be removed from Gastrocotyle; however, details of the male copulatory organ of this species (unavailable in the original description) are warranted to determine its taxonomic status and we propose to consider this taxon incertae sedis pending further studies.
Overall, the present specimens agree with the diagnosis of G. trachuri, the type-species of the genus Gastrocotyle, originally described on the Atlantic horse mackerel T. trachurus off Brest (van Beneden & Hesse, 1863). These authors provided a brief description lacking morphometric data and a poor illustration. Gastrocotyle trachuri was redescribed by Parona & Perugia (1889) and Jones (1933). The latter author pointed out that the number of hooks reported by van Beneden & Hesse (1863) is linked to the stage of development as they probably described immature specimens (Jones, 1933). We fully agree with this suggestion based on the examination of the present specimens (see Fig. 1H-J). Examination of 32 specimens of G. trachuri (Table 1) revealed that the material from off the Algerian coast agrees morphologically well with the descriptions of Jones (1933) and Radujkovic & Euzet (1989), particularly in having a terminal lappet armed with three pairs of hooks (Fig. 1H-J; also see figure 2C in Jones, 1933).
The description provided here adds several details, contributing to the diagnosis (clamp sclerites and male copulatory organ) and extends the known range for the morphometric data of G. trachuri. We provide herein for the first time, illustrated drawings of the male copulatory organ and descriptions and organisation clamp sclerites. The present specimens differ from the Atlantic specimens of G. trachuri ex T. capensis (see Piasecki, 1982) in having smaller clamps (60-75 Â 38-58 vs 70-100 Â 55-77 μm), smaller postero-lateral hooks (16-20 vs 25 μm), slightly reduced hamuli (35-52 vs 58 μm), and an apparently larger genital atrium (25-30 vs 16 μm).
Unfortunately, only two measurements are available for comparison with the Atlantic specimens reported from off Portugal byÂngelo (2011) since the author provided only body length (3,000-4,000 μm) and clamp number (24-43).  Abbreviations: SE, South-Eastern; SW, South-Western; NW, North-Western. a Diameter.
We note that only the specimens from the Indian ocean (ex T. novaezelandiae off Australia) are reported with a greater number (16) of atrial hooks (see Lebedev, 1968) whereas 12 hooks were described in all other studies (Jones, 1933;Pillai, 1968;Piasecki, 1982;Radujkovic & Euzet, 1989;Ângelo, 2011). Additionally, our careful comparison of the present morphometric data with the Mediterranean specimens described by Radujkovic & Euzet (1989) revealed a general overlap whereas both sets of Mediterranean specimens differ considerably from the oceanic specimens, from the Atlantic and Indian oceans. It is therefore possible that G. trachuri does not have a wide specificity nor geographical distribution but rather represents several G. trachuri-like species, each specific to a single host, which could not be distinguished morphologically. It is likely that future study will show that the gastrocotylid from the Mediterranean is a distinct species; this would require a detailed morphological and a molecular study of specimens from both the Mediterranean and oceanic waters.

Cemocotyle cf. trachuri Dillon & Hargis, 1965
The genus Cemocotyle Sproston, 1946 includes four species considered valid (Table 10). The type-species Cemocotyle carangis (MacCallum, 1913) was described based on material from the blue runner Caranx chrisoms (Mitchill) as Microcotyle carangis MacCallum, 1913(see MacCallum, 1913. Based on the presence of modified clamps on one side of the haptor, Sproston (1946) erected the genus Cemocotyle to accommodate M. carangis. In the diagnosis of the genus, Sproston (1946) mentioned the presence of a terminal lappet and noted that the haptor did not contain vitellarium nor intestinal diverticula. However, the illustration of the general morphology by MacCallum (1913) clearly shows the presence of intestinal branches, vitellarium and testes penetrating the haptor. Price (1962) indicated the presence of a terminal lappet in his description of Cemocotyle borinquensis Price, 1962 from the green jack Caranx caballus Günther (syn. Paratractus caballus (Risso)) off Puerto Rico, USA. After re-examination of specimens of C. carangis from the collection of MacCallum, Price (1962) suggested Cemocotyle noveboracensis (MacCallum, 1919) Price, 1962to group specimens collected on Caranx hippos (L.) and Caranx ruber (Bloch) (see Price, 1962); this author also considered that the presence of C. carangis in Trachinotus carolinus (L.) is unusual and due to an accidental parasitism. However, we remark that a Cemocotyle sp. has already been reported on Trachinotus goodei Jordan & Evermann, another carangid host of the same genus (Luque & C ezar, 2004).
In addition, the presence of a terminal lappet in Cemocotyle spp. has been indicated in three species: C. carangis, C. borinquensis (see Price, 1962;Kritsky et al., 2011) and C. noveboracensis (see Price, 1962). However, Kritsky et al. (2011) after examining the type-specimens and 52 other specimens of C. noveboracensis from the USA, confirmed the lack of a terminal lappet. Similarly, these authors studied several specimens of C. carangis from Caranx hippos off Puerto Rico from Museum collections and confirmed the lack of a terminal lappet and attributed these specimens to C. noveboracensis (see Kritsky et al., 2011). However, these authors emphasized that C. noveboracensis differs from all congeners in the lack of a terminal lappet and the organization of the male copulatory organ. Table 6 Hosts and localities of Zeuxapta seriolae (Meserve, 1938) Sharp et al. (2003Sharp et al. ( , 2004 Cemocotyle trachuri was reported on various Trachurus spp. (see Table 4). None of the previous records included descriptions, illustrations or morphometric data. These host records are probably based on insufficient evidence and unjustifiable. Although Piasecki (1982) provided a very brief description, he included no details of internal anatomy and morphology and listed only a few measurements. Hence, an illustrated redescription along with morphometric data seemed necessary considering the dubious reports of this species and the uncertainty of its occurrence on the hosts in previous records.
The Mediterranean specimens of Cemocotyle can be distinguished from C. trachuri (reported on the same host, T. trachurus) off Namibia (Piasecki, 1982) in having slightly reduced clamp width (25-34 vs 30 μm). However, only a few measurements for the Atlantic specimens are available for comparison (Table 3).
Details of morphology and anatomy of the specimens collected from T. trachurus in the present study were carefully considered and we conclude that these specimens are similar to C. trachuri in general morphology and internal anatomy, except with regard to the terminal lappet, which is lacking in the present material. Our specimens are clearly smaller than specimens of C. trachuri from T. novaezelandiae, but differences were not obvious; they have different hosts (T. trachurus vs T. novaezelandiae) and the localities are very distant (Mediterranean vs Pacific Ocean). However, the proposal of a new species based on the absence of terminal lappet is not desirable as it would cause additional instability to the already confused composition of the genus (Kritsky et al., 2011). Hence, pending comparison of molecular sequences from both hosts and localities, we use Cemocotyle cf. trachuri to designate the heteraxinid collected from T. trachurus from the South-Western Mediterranean.
We emphasize the simultaneous occurrence of four polyopisthocotylean species, i.e. Gastrocotyle trachuri, Cemocotyle cf. trachuri, Pseudaxine trachuri and Allogastrocotyle trachuri in the same fish host specimen. The latter two species were recently described and illustrated (Bouguerche et al., 2019b, c). (Meserve, 1938) At present, the genus Zeuxapta includes three species considered valid (Table 11): Z. kahala, Z. seriolae and Z. taylori. The first two species parasitize carangids of the genus Seriola Cuvier and the third parasitizes the scombrid Thunnus albacares (Bonnaterre). Only Z. seriolae occurs in the Mediterranean.

Zeuxapta seriolae
By the possession of an asymmetric haptor, the organization of the genital atrium and the vagina, our specimens collected on S. dumerili are placed within the Heteraxinidae Unnithan, 1957 and are members of the genus Zeuxapta Unnithan, 1957(see Mamaev, 1990Yamaguti, 1963). By their morpho-anatomical characters, our specimens of Z. seriolae are similar to those described in different regions.
The Mediterranean specimens of Z. seriolae differ from those collected from S. dorsalis off the Galapagos (Meserve, 1938) in having a significantly greater body length (13.07-16.2 mm vs 5,110-7,540 μm), more clamps on short and on long side, and an apparently larger prohaptoral sucker (100-180 vs 76-92 μm). Dimensions of clamps, pharynx, and genital atrium are not available for specimens off Galapagos (Table 5).
The Mediterranean specimens of Z. seriolae can be distinguished from the specimens from S. grandis collected in the Pacific (off Australia; see Rohde, 1978) in having an apparently greater body length     54-90 Â 54-152 μm) and larger pharynx (50-75 Â 50-75 vs 39-47 Â 36-43 μm). Specimens from Australia have fewer clamps (Table 5) but the number of clamps (and egg size) of specimens from both localities (Algeria and Australia) show overlapping ranges. Dimensions of clamps, pharynx and genital atrium were not given for the Australian specimens. We note however, that the two populations differ in the distance between the genital atrium and anterior extremity (972-1,400 vs 430-750 μm).
In light of the available data, we could not draw any conclusion regarding differences between the Mediterranean and Pacific specimens, as specimens from off Algeria are smaller than Pacific specimens from off Japan (Ishii & Sawada, 1938) but larger than those from off Galapagos (Meserve, 1938) and off Australia (Rohde, 1978). Probably the size of Z. seriolae depends on host size and some specimens from previous studies were collected from larger host individuals, as shown for another polyopisthocotylean occurring on the same host (Price, 1962;Montero et al., 2003). Similarly, we stress the importance of including data on host size in taxonomic studies of monogeneans (Montero et al., 2003). Large variation in morphometric data of Z. seriolae was already demonstrated by Rohde (1978). In addition, Montero et al. (2003) listed an additional species of the Heteraxininae, Pseudoallencotyla pricey (Kritsky, Noble & Moser, 1978), occurring on more than one host species and from a broad geographical area (Montero et al., 2003). It is likely that the long-distance migrations of the host allowed the establishment of the monogenean Z. seriolae in different waters. Nevertheless, it would be interesting to obtain cox1 sequences of these heteraxinids from the various fish host species; this would possibly lead to detection of the presence of several cryptic species.

Pyragraphorus hollisae
Pyragraphorus spp. are characterized by having a horizontally oriented haptor with a fish-tail appearance and armed with modified clamps; the distal half bears two rows of normal clamps and the proximal half bears two rows of modified clamps (Sproston, 1946).

Funding
The research leading to the results presented in this publication was partly carried out with infrastructure funded by the Direction G en erale de la Recherche Scientifique et du D eveloppement Technologique(DGRSDT) and by the Laboratoire de Biodiversit e et Environnement: Interactions -G enomes (LBEIG), Universit e des Sciences et de la Technologie Houari Boumediene (USTHB), Algiers, Algeria. This study was also supported by the Institut de Syst ematique, Evolution, Biodiversit e (ISYEB), Mus eum national dʼHistoire naturelle (MNHN) Paris, France), and a framework agreement project of the DeepBlue Project: Distance Crossborder Traineeship Programme co-financed by the European Maritime and Fisheries Fund (EMFF) for the analysis, interpretation of data and the writing of the manuscript. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Declaration of competing interests
The authors declare that they have no competing interests.