Neohexostoma gymnosardae n. sp., 2020

Zhu, Pei-Wei, Li, You-Zhi, Liu, Lin, Ding, Xue-Juan & Yuan, Kai, 2020, Neohexostoma gymnosardae n. sp. (Monogenea, Hexostomatidae), a gill parasite of Gymnosarda unicolor (Valenciennes) (Teleostei, Scombridae) in the South China Sea, Parasite (Paris, France) 27 (71), pp. 1-9 : 2-7

Neohexostoma gymnosardae n. sp.

urn:lsid:zoobank.org:act:E20B680B-EA84-4F93-96CC-1D2393A79C51

Type-host: Gymnosarda unicolor Rüppell, 1836 ( Perciformes : Scombridae , dogtooth tuna).

Site of infection: Gills.

Type-locality: Yongshu Reef (9°33’N, 113°E), South China Sea, Western Pacific Ocean .

Prevalence: 3 of 3 hosts infected (100%) with a total of 10 worms.

Type material: Holotype ( LFP. 2017050801 ), four paratypes (LFP. 2017050802–05), Laboratory of Fish Parasite, College of Life Science, South China Normal University, Guangzhou, China. One paratype ( NHMUK No. 2019.10.30.3 ), Natural History Museum, London ( NHMUK).

Etymology: The species is named after its host.

Description ( Figs. 2 and 3 )

Based on six whole-mounted worms. Body elongate, divided into anterior body proper, peduncle and haptor; body proper tapering anteriorly, followed by a strongly constricted peduncle, and then a broadening haptor ( Figs. 2A , 3A ). Body total length 9660–18,800 (13,827; n = 6), greatest width 2875–5375 (3847; n = 6) at level of ovary. Peduncle 929–1690 (1324; n = 5) long, 566–1021 (824, n = 5) wide, from the termination of testes to the anterior of haptor. Haptor somewhat irregular, clearly marked from body proper by a peduncle and divided by a split in its posterior margin ( Fig. 3D ), 1172–1810 (1446, n = 6) long from the first pair of clamps to the fourth pair, 1369–2582 (1827, n = 6) wide at level of first pair of clamps. Haptor with four pairs of clamps arranged in two longitudinal rows, most anterior pair 505–697 (594, n = 6) long, 275–506 (415, n = 6) wide, second pair 577–713 (620, n = 6) long, 327–477 (395, n = 6) wide, third pair 475–624 (587, n = 6) long, 302–455 (376, n = 6) wide, and fourth pair 389–496 (447, n = 6) long, 233–358 (316, n = 6) wide. Each clamp consisting of one middle sclerite and two pairs of lateral sclerites longitudinally imbedded in two muscular pads; middle sclerite X-shaped in front view ( Fig. 2G ) or C-shaped in lateral view ( Fig. 3G ); two lateral sclerites inverted Y-shaped. Haptor with two pairs of anchors ( Fig. 2F ), large anchors 43–57 (50, n = 4) long, with a long root and well curved blade; inner anchors sickle shaped, 39–42 (41, n = 2) long. One pair of prohaptoral suckers elliptical, 62–99 (83, n = 5) long, 51–102 (70, n = 5) wide. Pharynx small, 63–93 (82, n = 3) long, 49–58 (55, n = 3) wide. Esophagus 700–2691 (1352, n = 5) long, with several lateral branches, bifurcating immediately posterior to genital atrium into two intestinal ceca (Fisg. 2A, 3A). Intestinal ceca extending into the haptor, not united posteriorly, with numerous digitiform lateral diverticula and several medial diverticula in ovarian and testicular region, but without branches in peduncle and haptor region ( Figs. 2A , 3A ). Testes rounded, numerous, 185–246 (218, n = 4), packed together in posterior part of ovary, not extending into peduncle ( Fig. 2B ). Vas deferens arising from testes and passing medially from ovary, then winding strongly forward dorsal to uterus ( Figs. 2A, 2B ). Genital atrium forming a globular sucker, with muscular wall enwrapping male copulatory organ ( MCO), at a distance 706–2510 (1699, n = 5) from head end. MCO, cup-shaped with a bulbous muscular cirrus covered with ring-like corrugated edge ( Figs. 2C , 3B ), 92–321 (225, n = 5) long, 112–348 (228, n = 5) wide. Ovary sinuous, roughly inverted U-shape, with limbs greatly convoluted ( Fig. 2B ), measuring 1150–1650 (1444, n = 4) long. Vagina opening mediodorsally, behind genital pore, 244–359 (301.5, n = 2) long, 191–250 (220.5, n = 2) wide, armed with sparsely small spines ( Figs. 2D , 3C ). Two parallel vaginal ducts, conspicuous, running back along sides of uterus. Vitellaria follicles distributed along intestinal ceca. Vitelline reservoir Y-shape. Uterus arising along left side of ovary, then midventral distended with numerous eggs ( Fig. 2B ). Genitointestinal canal enters right intestinal cecum. Oviduct and oötype not observed; precise junctions between vitelline reservoir, ovary, uterus and genitointestinal canal not elucidated. Eggs oval, 125–193 (156, n = 5) long, 91–137 (114, n = 5) wide, joined in a chain by their filaments, filaments 411–719 (518, n = 3) long ( Figs. 2E , 3E, 3F ).

Differential diagnosis

Monogeneans found on the gills of G. unicolor are allocated to Neohexostoma by the following morphological features: body elongate, widest in ovarian region; haptor with four pairs of clamps arranged as two more or less vertical rows, decreasing in size anteroposteriorly; vitellarium not extending posteriorly beyond the distal portion of the testes [ 26]. Neohexostoma as presently constituted includes seven species: N. thunninae (Parona & Perugia, 1889) Price, 1961 , N. euthynni (Meserve, 1938) Price, 1961 , N. extensicaudum (Dawes, 1940) Price, 1961 , N. pricei (Koratha, 1955) Price, 1961 , N. robustum Price, 1961 , N. kawakawa Yamaguti, 1968 , and N. mochimae Fuentes-Zambrano, 1997 [ 32]. The new species can be distinguished from all Neohexostoma spp. by the shape of the haptor, the armature of the vagina, and the assembly of eggs. The haptor is divided into symmetrical lobes by a slender split in its posterior margin, the vagina is armed with scattered spines, and the eggs are tied by their long polar filaments.

Apart from the above, the new species can also be distinguished from its congeners by its body shape, intestine, and clamp disposition. Neohexostoma gymnosardae n. sp. is characterized by a well visible peduncle: of the seven species of Neohexostoma mentioned above, only N. thunninae and N. mochimae have a constriction at the beginning of the haptor, separating the latter from the rest of the body [ 24, 25, 35]. The new species most closely resembles N. thunninae by its body shape and the possession of a peduncle. Neohexostoma gymnosardae n. sp. differs from N. thunninae by having an esophagus with lateral diverticula, ceca extending more posteriorly into the haptor and the post-atrial intestines bifurcating. Moreover, in N. gymnosardae n. sp. the anchors are placed between clamps of the most posterior pair, whereas they are borne on a short lappet in N. thunninae [ 24, 25].

Neohexostoma gymnosardae n. sp can be distinguished from N. kawakawa , N. mochimae and N. euthynni by having testes tightly packed into a post-ovarian pile [ 20, 34, 35]. Furthermore, in N. gymnosardae n. sp. , intestinal bifurcation is post-atrial and the unfused intestinal ceca extend into the haptor; in N. kawakawa and N. euthynni , the intestinal bifurcation is pre-atrial and ceca are united posteriorly [ 20, 34]. Neohexostoma gymnosardae n. sp. differs from N. pricei by the extension of intestinal ceca into the haptor (up to a shorter distance in N. pricei ) and the shape of lateral sclerites of clamps (straight in N. pricei vs. double-pronging in N. gymnosardae n. sp. ) [ 17]. Neohexostoma gymnosardae n. sp. can be further distinguished from N. extensicaudum and N. robustum by body shape and clamp disposition. Both N. extensicaudum and N. robustum , like most of Neohexostoma spp. , have an elongated waist-like constriction in the testicular region, which is absent in the new species [ 7, 26]. Moreover, clamps of the right row in N. extensicaudum lie very close to those of the left row so that the posterior end of the body is much more attenuated than is the case in other species [ 7]; and in N. robustum , clamps are arranged in oblique transverse rows on a roughly triangular haptor [ 26].

The comparison of morphometrics of Neohexostoma gymnosardae n. sp. and Neohexostoma spp. is presented in Table 3 . Neohexostoma gymnosardae n. sp. most closely resembles its congeners by having clamps decreasing in size anteroposteriorly. It differs from N. euthynni , N. kawakawa , and N. mochimae by the number of testes, host and locality for the latest species. In addition, N. gymnosardae n. sp. can be distinguished from N. kawakawa , N. euthynni and N. extensicaudum by having a longer polar filament. We note that body length in N. gymnosardae n. sp. is greater than in N. euthynni , N. pricei , N. mochimae , and N. kawakawa . However, such a measurement is generally not a reliable characteristic for species differentiation [ 4]. We present herein a key to species of Neohexostoma as follows, which is modified from Millemann [ 21] and Zambrano [ 35].

Figures

Figure 2. Neohexostoma gymnosardae n. sp. from gills of Gymnosarda unicolor. (A) Whole worm, ventral view (p, peduncle); (B) Reproductive system (MCO, male copulatory organ; vg, vagina; vgd, vaginal ducts; vdf, vas deferens; ov, ovary; vr, vitelline reservoir; gi, genitointestinal canal; u, uterus; te, testes); (C) Male copulatory organ; (D) Vagina; (E) Eggs; (F) Anchors; (G) Clamp with sclerites in front view.

Figure 3. Photographs of Neohexostoma gymnosardae n. sp. (A) Holotype, whole worm (ventral view); (B) Male copulatory organ; (C) Vaginal spines; (D) Haptor; (E)–(F) Eggs; (G) Clamp with sclerites in lateral view. (B)–(G) are paratypes.