New species of Paracolpenteron n. gen. and Ancyrocephalus (Monogenea, Dactylogyridae) inhabiting the urinary bladder and gills of the Maya needlefish Strongylura hubbsi (Beloniformes, Belonidae) from Chiapas, Mexico

Parasitological examination of the maya needlefish Strongylura hubbsi Collette (Belonidae) from the Rio Lacantún basin in the Montes Azules Biosphere Reserve, Chiapas, Mexico showed that specimens were parasitized by two monogenean species in two different sites: Paracolpenteron hubbsii n. gen., n. sp in the urinary bladder and Ancyrocephalus chiapanensis n. sp in the gill lamellae. Paracolpenteron hubbsii differs from other dactylogyrid species without a haptoral anchor/bar complex infecting the urinary systems, gills and nasal cavities by the general morphology of hooks, a dextral vaginal opening, a tubular male copulatory organ comprising a base from which a coiled shaft arises in counterclockwise direction, and an unarticulated Y-shaped accessory piece. Ancyrocephalus chiapanensis n. sp. resembles Ancyrocephalus cornutus William & Rogers, 1972 from the gills of Strongylura marina from Florida from which it differs in possessing a twisted tube of the male copulatory organ (curved in A. cornutus), ventral bar with cavities on the ends (cavities absent in A. cornutus) and by the size of the ventral (length 31–34 μm vs. 24–27 μm in A. cornutus) and dorsal (length 25–28 μm vs. 18–22 μm in A. cornutus) anchors. These new monogeneans are described herein and their biogeography is briefly discussed based on the previous phylogenetic hypotheses concerning the host family.


Introduction
In Mexico, monogeneans from sites other than the gills and skin of fishes have rarely been studied, with a few dactylogyrid species described and/or recorded, i.e., Enterogyrus malmbergi Bilong Bilong, 1988 in the stomach of the introduced tilapia Oreochromis niloticus (L.) and the native cichlid Cichlasoma callolepis (Regan) (now Thorichthys callolepis) from Santa Anita Lagoon in the state of Tabasco [13]; Pavanelliella scaphiocotylus Kritsky & Mendoza-Franco, 2003 in the nasal cavity of the catfish Rhamdia guatemalensis (Günther) (Heptapteridae) from a cenote (= sinkhole) in the Yucatán Peninsula [19]; Pseudempleurosoma carangis Yamaguti, 1965 and Pseudempleurosoma gibsoni Santos, Mourão & Cárdenas, 2001 in the rectum of the puffer fish Sphoeroides testudineus (Linnaeus) (Tetraodontidae) and the pyloric ceca of the cobia Rachycentron canadum (Linnaeus) (Rachycentridae), respectively, from the northern coast of the Yucatan Peninsula [27]; and Cacatuocotyle chajuli Mendoza-Franco, Caspeta-Mandujano & Salgado-Maldonado, 2013 in the external surface of the anal opening of the characid Astyanax aeneus (Günther) from the Rio Lacantún basin in the state of Chiapas [25] (see [28]). During studies carried out between February and August 2015 on the parasites of fishes from the Rio Lacantún basin, two undescribed dactylogyrid species were found at two sites in the Maya needlefish Strongylura hubbsi Collette (Belonidae): Paracolpenteron hubbsii n. gen., n. sp. in the urinary bladder and Ancyrocephalus chiapanensis n. sp. in the gill lamellae. In the present paper, both new species are described.

Materials and methods
Following approval from the Ethics Committee of the Autonomous University of Nuevo Leon (UANL), and after obtaining a permit from the Secretaría del Medio Ambiente y Recursos Naturales (SEMARNAT), Mexico (permit numbers: FAUT-017 and SGPA/DGVS/03492), specimens of S. hubbsi were captured by hook-and-line and throw nets between February and August 2015 in the Rio Lacantún basin in the state of Chiapas (16°09 0 96.6 00 N, 90°95 0 56.8 00 W). Live fish were sacrificed by puncturing the brain region (a needle is introduced dorsally via the eye socket and moved about to destroy the brain and spinal cord) [29]. The gills of each fish were removed and placed in finger bowls containing 4-5% formalin solution to fix any of the ectoparasites that might be present. The internal cavity of each fish was exposed by an incision made along the middle of abdomen from anus to mouth. The monogenean specimens were removed from the urinary bladder and preserved in 4% formalin. Subsequently, parasites preserved in formalin were isolated and stained with Gomori's trichrome and mounted in Canada balsam. In addition, some specimens were mounted in a mixture of lactic-acid (LA) and glycerineammonium picrate (GAP), and then remounted in Canada balsam [25] to obtain measurements and line drawings of haptoral structures and the copulatory complex. All other measurements were obtained from unflattened specimens stained with Gomori's trichrome stain. Drawings were made with the aid of a drawing tube using a DM2500 Leica microscope with Nomarski interference contrast. Measurements, all in micrometers, represent straight-line distances between extreme points and are expressed as the mean followed by the range and number (n) of structures measured in parentheses; body length includes that of the haptor. The direction of the coil (clockwise vs. counterclockwise) of the copulatory organ was determined using the procedure suggested by Kritsky et al. [16]. Type specimens are deposited in the National Helminthological Collection of Mexico (CNHE).  Etymology: The specific epithet is derived from the specific name of the type host.

Description (Figs. 1-5)
Diagnosis (based on eight specimens fixed in a mixture of LA and GAP and nine specimens stained with Gomori's trichrome): Body 294 (220-343; n = 8) long; greatest width 71 (65-78; n = 9) near midlength or in anterior trunk. Three pairs of bilateral head organs; cephalic glands indistinct. One to four eyespots; accessory chromatic granules that are similar in size and shape are scattered in the cephalic region.  , with that of the dactylogyrid found in the urinary bladder of S. hubbsi (Beloniformes), we concluded that it is a new dactylogyrid that does not fit into any genera listed above (see [2, 17-19, 21-23, 26, 30, 32]). Therefore, Paracolpenteron n. gen. is proposed to accommodate the new species.
P. hubbsii n. gen., n. sp. most resembles species of Acolpenteron infecting the urinary systems of freshwater cypriniform and perciform fishes, in having overlapping gonads, a copulatory complex comprising an unarticulated Y-shaped accessory piece with the MCO base, a dextroventral vaginal pore and haptor cup-like with 14 similar hooks (see [7][8][9] Diagnosis (based on four specimens fixed in a mixture of LA and GAP and 17 specimens stained with Gomori's trichrome). Body fusiform, 347 (290-430; n = 19), greatest width 105 (90-120; n = 16) at midlength. Tegument smooth. Similarly, A. chiapanensis n. sp. lacks such identifying features. Therefore, the generic assignment of A. chiapanensis n. sp. and A. cornutus is provisional and based on agreement with the diagnosis provided by Williams [34]. A. cornutus and A. chiapanensis n. sp., both on their respective host species of Strongylura, share the following characteristics: anchors with a welldeveloped superficial root, short rounded deep root, elongate shaft and point extending just past level of tip of superficial root; MCO inside of a tube acting as an accessory piece (nested tube) (as a club-shaped accessory piece in A. cornutus). Although not originally described in A. cornutus, the copulatory complex depicted in Figure 7 from the original description of this species suggests a bulbous-shaped base of the MCO, similar to that of A. chiapanensis n. sp. (see [35]). While convergence cannot be discounted, these above-mentioned morphological characters indicate that the two monogenean species are morphologically very similar, suggesting that they may have coevolved alongside their respective host species. A. chiapanensis n. sp. differs from A. cornutus in having a twisted MCO and accessory piece (curved in A. cornutus); two intestinal ceca confluent posterior to gonads (ceca not confluent in A. cornutus); a ventral bar with cavities on the ends (absent in A. cornutus); and by the size of the ventral (length 31-34 vs. 24-27 in A. cornutus) and dorsal (length 25-28 vs. 18-22 in A. cornutus) anchors (see [27], present study). While a strict definition of Ancyrocephalus (s.l.) remains wanting, erection of a new genus to accommodate A. chiapanensis n. sp. and A. cornutus will additionally depend on examination of type specimens of A. cornutus (not currently available) or on newly collected specimens from S. marina in Florida. This latter will allow us to clarify whether the ceca are confluent or not as well as the position of other organs, i.e., vagina which was not described in the original description of A. cornutus.
In terms of other features, A. chiapanensis n. sp. exhibits some similarity with Xenentocleidus xenentodoni (Jain) Tripathi, Agrawal & Pandey 2007 from Xenentodon cancila (Hamilton) and Hemirhamphiculus exserocephalus Kritsky 2018 from Tylosurus gavialoides (Castelnau), both belonid hosts from India and Australia, respectively (see [15,31]). These three monogenean species possess a dextroventral and submarginal vaginal aperture (marginal in H. exserocephalus) and the shaft of the MCO in a counterclockwise direction. A. chiapanensis n. sp. differs from X. xenentodoni in having slightly overlapping gonads (tandem in X. xenentodoni); a twisted MCO (coiled tube in X. xenentodoni); an unarticulated accessory piece and MCO (accessory piece articulated with the base of the MCO in X. xenentodoni); by lacking anchor roots connected by a threadlike sclerotized piece (present in X. xenentodoni); hooks with a shank composed of two subunits (present in X. xenentodoni); and tegument transversally striated ventrally at the region of the copulatory complex (present in X. xenentodoni) (see [15,30]).

Discussion
The Río Lacantún basin of southeastern Mexico and northern Guatemala in the Neotropical Region is characterized by numerous derivatives of marine species adapted to freshwater [10]. The evolutionary radiation of fish in this region indicates old ichthyofauna with a high degree of endemism at the generic and suprageneric levels [3]. An example of this in the area is the endemic freshwater needlefish S. hubbsi and their monogeneans. Most needlefishes are marine, but 12 species are restricted to freshwater and several species of Strongylura move long distances into freshwater. The freshwater fish species mentioned above include species in three genera (Belonion Collette, Potamorrhapis Gunther and Pseudotylosurus Fernández-Yépez, with seven endemic species from South American rivers) plus two species of Strongylura in freshwaters of Central and South America, and one genus (Xenentodon), with two species plus Strongylura krefftii (Günther) in South-east Asian freshwaters [5,24]. As stated in Lovejoy & Collette [24], based on morphological and molecular tools, freshwater needlefishes from the Atlantic basins do not make up a monophyletic group and Strongylura is clearly polyphyletic. Instead, the genus consists of several small monophyletic species groups, such as S. marina (Atlantic) and S. exilis (Girard) (Pacific) as a sister taxon of S. hubbsi. In accordance with this, the present study shows that A. chiapanensis n. sp. infecting S. hubbsi (Pacific) exhibits some morphological similarity (see Differential diagnosis section for A. chiapanensis n. sp.) with a species assigned to Ancyrocephalus (s.l.), A. cornutus, from S. marina. This morphological evidence supports Lovejoy & Collette 0 s clade containing S. marina within a sister taxon of S. hubbsi and suggests that these monogeneans from S. hubbsi and S. marina form a monophyletic group from which they could have coevolved with their hosts. This latter hypothesis is consistent with the sister taxon of S. hubbsi that consists of an Atlantic and Pacific species pair (S. marina and S. exilis, respectively) from which S. hubbsi is thought to have diverged prior to the last connection between these oceans (3 Mya) [5], when freshwater ancestors of needlefishes were originally distributed from the Atlantic drainage of the Rio Usumacinta of Mexico and Guatemala to the Pacific slopes of South America, and the Atlantic drainages of South America [24]. However, a phylogenetic hypothesis for members of all dactylogyrids infecting fishes in the New World (i.e., needlefishes) is necessary to formally address the hypothesis mentioned above.

Conflict of interest
The authors declare that they have no conflict of interest.