Two new species of Aristocleidus (Monogenea) from the gills of the Mexican mojarra Eugerres mexicanus (Perciformes, Gerreidae) from southwestern Mexico

Aristocleidus mexicanus n. sp. and Aristocleidus lacantuni n. sp. are described from the gills of the Mexican mojarra Eugerres mexicanus (Gerreidae, Perciformes) from the Rio Lacantún basin, Chiapas State, Mexico. These new species differ from previously described congeneric species in the characteristics of several structures, including: (a) ventral anchors, with differences in length (i.e. 46–50 µm in A. mexicanus vs. 38–43 µm, 34–37 µm, and 26–33 µm in Aristocleidus hastatus Mueller, 1936, Aristocleidus sp. of Mendoza-Franco, Violante-González & Roche 2009, and Aristocleidus lamothei Kritsky & Mendoza-Franco, 2008, respectively) and shape (i.e. slightly angular union of elongate arcing shaft and point in A. mexicanus vs. point and shaft united at a conspicuous angular bend in A. hastatus and Aristocleidus sp., and evenly curved shaft and point in A. lamothei); (b) male copulatory organ, i.e. a coiled tube with less than one ring in A. mexicanus and A. lacantuni (vs. a coiled tube of about 1½ in Aristocleidus sp.); (c) distal end of the accessory piece (ornate in A. mexicanus vs. distally flattened and trifid in A. hastatus and A. lamothei, respectively); (d) vaginal tube (moderately long in A. mexicanus vs. short in A. lamothei and looping in Aristocleidus sp.); and (e) ventral bar (anteromedial process with terminal horn-like ornamentation in A. lacantuni vs. ornamentation absent in the other species). This study reports for the first time species of Aristocleidus from freshwater environments in Mexico.


Résumé -
. During investigations into the helminth fauna of fishes from the Rio Lacantún basin in the state of Chiapas (Mexican Pacific), two undescribed species of Aristocleidus were found on the gills of the Mexican mojarra E. mexicanus in January and August 2014. These new species are described in this article.

Materials and methods
Host specimens of E. mexicanus were captured by hookand-line and throw nets in January and August 2014 in the Rio Lacantún basin in the state of Chiapas, Mexico (16°09 0 96.6 00 N, 90°95 0 56.9 00 W). Live fish were sacrificed bloodlessly by puncturing the brain region (a needle is introduced dorsally via the eye socket and moved about to destroy the spinal cord) [13]. 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. Subsequently, parasites were isolated and stained with Gomori's trichrome and mounted in Canada balsam. In addition, some specimens were mounted with a mixture of acid-lactic (AL) and glycerin-ammonium picrate [12] 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. Drawings were made with the aid of a drawing tube using a Leica microscope DM50 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; measurements of the copulatory complex, anchor/base are represented in Figures  2 and 3 as the perpendicular distance between parallel lines.
Direction of the coil of the copulatory organ, i.e., clockwise, was determined using the procedure suggested by Kritsky et al. [7]. Haptoral terminology for species of Aristocleidus is that provided by Kritsky & Mendoza-Franco [8]. Type specimens are deposited in the National Helminthological Collection of Mexico (CNHE), Institute of Biology, National Autonomous University of Mexico, Mexico.
Etymology: named after the location (i.e., Rio Lacantún) from which this species was found.

Differential diagnosis
The morphology of the haptoral and copulatory sclerites of this species clearly allows its formal generic assignment within Aristocleidus. A. lacantuni n. sp. resembles A. hastatus and A. lamothei in the length of the copulatory complex, i.e., 23-30 vs. 27-36 and 22-26, respectively. It differs from these latter species as well as A. mexicanus n. sp. and Aristocleidus sp. in having shorter anteromedial process of the ventral bar (comparatively larger in A. hastatus, A. mexicanus n. sp., and Aristocleidus sp. and indistinct in A. lamothei) and a ventral anchor with straight shaft and elongate arcing point (point and shaft united at a conspicuous angular bend in A. hastatus and Aristocleidus sp., evenly curved shaft and point in A. lamothei, and elongate arcing shaft and straight point [slightly angular union] in A. mexicanus n. sp.) [8,11].

Discussion
The diagnostic characters used previously to differentiate species of Aristocleidus involved the morphology of the haptoral armament (i.e., shape of the connection between shaft and point of anchors) and copulatory complex (i.e., shape of the distal end of the accessory) [8,11]. This study showed that these characters are reliable for separation of A. mexicanus n. sp. and A. lacantuni n. sp. The occurrence of species of Aristocleidus on primarily marine fish (i.e., species of Diapterus) has generated the hypothesis that these parasite species originated in the marine environment [8]. For example, A. hastatus and A. lamothei have been reported from Eugerres plumieri, Diapterus auratus, and Diapterus rhombeus from rivers (i.e., Rio Maquinas in Veracruz State) and/or estuarine systems (i.e., Ria Celestún in Yucatán State) draining to the Gulf of Mexico. Similarly, on the Pacific coast of Mexico and Panama, these monogeneans as well as Aristocleidus sp. have been reported from Diapterus peruvianus and Gerres cinereus from the Chautengo and Tres Palos Lagoons in Guerrero State and Eugerres brasilianus from Gatun Lake in Panama [8,11].
However, this study demonstrates that A. mexicanus n. sp. and A. lacantuni n. sp. in Eugerres mexicanus are clearly secondary invaders of freshwater. E. mexicanus inhabit freshwater habitats along the rivers Grijalva-Usumacinta and Coatzacoalcos basins in southeastern Mexico (Chiapas, Tabasco, and Veracruz) and northern Guatemala [14,15], with a noteworthy distribution in highlands with elevations of 100-300 m [4] (i.e., Río Lacantún basin) and the northern part of Guatemala (upper Usumacinta). Thus, a marine ancestor of these monogeneans along with their hosts could have colonized this freshwater environment from which they speciated through adaptive   [6,9]. Examination of other freshwater gerreids in the Rio Lacantún basin (i.e., Eugerres castroaguirrei González-Acosta & Rodiles-Hernández 2013) would help to clarify the rule of the freshwater vs. saltwater on the presence or absence of species of Aristocleidus. For example, salinity has been reported to be a determining factor for the occurrence of species of Rhabdosynochus (Diplectanidae) on snooks, Centropomus spp. (Centropomidae) in Florida [5].
Knowledge about the phylogenetic affinities between gerreids and the moronid type host (i.e., M. saxatilis) of the first described species of Aristocleidus (i.e., A. hastatus) is unknown. Chen et al. [2] provided a molecular phylogenetic hypothesis using four genera (D. auratus plus E. plumieri and Eucinostomus gula [Quoy & Gaimard] plus G. cinereus) of the Gerreidae including other percomorph fishes and stated that this latter host family is monophyletic. In that hypothesis, gerreids are placed at an intermediate position in the percomorph tree between early branching lineages such as scombrids (i.e., Scomberomorus commerson [Lacépède]), serranids (i.e., Holanthias chrysostictus [Günther]), and a terminal clade containing tetraodontiforms (i.e., Takifugu rubripes [Temminck & Schlegel]), Lutjanids (i.e., Lutjanus analis [Cuvier]), and other percoid relatives such as north American moronids, i.e., Dicentrarchus labrax (Linnaeus). These gerreids within the phylogenetic hypothesis of Chen et al. [2] suggest that this gerreid group is comparatively old and predates the origins of the Moronidae. If this were true, then, the occurrence of A. hastatus on M. saxatilis apparently took place comparatively recently and probably corresponds to the concomitant colonization of an anadromous common ancestor to this host family that gave rise to the north American estuarine and freshwater species [17]. It is clear that further surveys of species of Aristocleidus infesting gerreids (and probably moronids) from the western Atlantic and the eastern Pacific will be necessary to understand the diversification of Aristocleidus spp. in the tropical areas.