New species of Eugerres from the Usumacinta Province , México and Guatemala with a redescription of E . mexicanus ( Steindachner , 1863 ) ( Teleostei : Gerreidae )

Eugerres castroaguirrei, new species is described from the río Grijalva-Usumacinta basin of southeastern Mexico and northern Guatemala. Eugerres castroaguirrei and E. mexicanus are distinguishable from their marine estuarine congeners by the dorsalfin origin posterior to the insertion of the pectoral and pelvic fins, a shorter and broad based supraoccipital crest, and a distinct geographic distribution restricted to freshwater habitats. Eugerres castroaguirrei differs from E. mexicanus by diagnostic characters of the body skeleton: anterior process of supraoccipital convex, infraorbital 1 with foramen, premaxillary ascending process developed with margins curved, pharyngeal plate granular and not serially arranged, and dorsal and anal fin-rays reduced. Likewise, E. castroaguirrei is characterized by an oblong and laterally thicker body (37.6-58.5% HL); greater body depth (33.8-42.1% SL), and eye diameter (23.3-31.6% HL); in addition to 16 significant morphometric characters: length of the second dorsal-fin spine 23.2-34.2% SL; depressed second dorsal-fin spine (extending to base of third to fourth dorsal-fin rays); length of second anal-fin spine 11.1-20.6% SL, depressed second anal-fin spine extending to the base of third to fourth anal-fin rays not reaching the distal point of last anal-fin ray; pelvic-fin spine length 49.7-65.0% in the first pelvic-fin ray length.


Introduction
The commonly named 'mojarras' of the family Gerreidae comprise a group of fish distributed along the subtropical and tropical coasts areas of the world's ocean.In the Americas, the family consists of 22 valid species, classified in four genera (Nelson et al., 2004): Gerres Quoy & Gaimard, 1824;Diapterus Ranzani, 1840;Eucinostomus Baird &Girard, 1855, andEugerres Jordan &Evermann, 1927.Most of the species inhabit coastal lagoons and estuaries with sandy or muddy bottoms bordered by mangroves; however, they occasionally enter river mouths.It has been thought that E. mexicanus, is the only one gerreid species confined to freshwater habitats in the southeastern Mexico and northern Guatemala (Deckert, 1973;Deckert & Greenfield, 1987;Castro-Aguirre et al., 1999;Miller et al., 2006).
A comparative examination on the skeleton of the freshwater gerreids was conducted to identify the most distinctive bone characters between the species, the examined specimens were prepared by cleared-stained techniques following the procedure of Taylor & van Dyke (1985), and preparing fish skeletons based on Burns & Everly (2000).The results are presented following the body skeleton organization and the bony nomenclature of Rojo (1991), González-Acosta (2005) and Becker et al. (2009).
Twenty one linear measurements between external landmark pairs were taken with dial calipers to the nearest 0.1 mm from 157 specimens [125 of E. mexicanus and 42 of E. castroaguirrei] (Fig. 1).In this analysis, we followed in general the meristic and morphometric methods presented by Hubbs & Lagler (1964) with additions by González-Acosta (2005).Other measurements not included in the Figure 1: interorbital length, third anal spine length, third dorsal spine length, distance between the tips of the second dorsal, and standard length (SL) were also considered.A Student's t-test was used to compare the means of the morphometric proportions.Principal component analysis [PC] was used to examine the variation in morphometric proportions to eliminate the effects of size.The PC analysis was performed on a correlation matrix of log-transformed data, using 20 body measurements (excluding SL) from the external morphology of each specimen.All morphometric data were standardized to remove the size component from the shape measurements and to minimize bias due to variability in standard length, according to Elliot et al. (1995).Likewise, measurements were log-transformed to homogenize the variances (Sokal & Rohlf, 1981).Diagnoses and descriptions are given for the taxa included.

Results
In their critical studies of the genus Eugerres, Deckert (1973) and Deckert & Greenfield (1987) did not identify meristic or morphometric variations among populations of 'E.mexicanus ' [s.l.] from Mexico and Guatemala; they identified to 'E. mexicanus ' [s.l.] as the only New World gerreid restricted to freshwater.Recently, González-Acosta (2005)   specimens (including type materials) from several localities in their known range of distribution.
Taxonomic analyses conducted on 'Gerres' mexicanus syntypes [sic] NMW 72289 (165.6 mm SL) and NMW 78820 (142.8-152.5 mm SL) allowed us confirm the identity of Eugerres mexicanus (sensu Steindachner, 1863) and distinguish it from the E. aff.mexicanus morph included in the 'E.mexicanus ' [s.l.] complex (González-Acosta et al., 2007), and described herein as E. castroaguirrei new species.As well as, morphometric comparisons permit differentiation of two forms as distinct species (Tables 1-2).Thus, E. mexicanus can be distinguished by the length of the second dorsal and anal fin spines in relation to the standard and head lengths (Fig. 2a), respectively.Likewise, the depressed second dorsal-fin spine reaches the base of the fifth or sixth dorsalfin rays and the second anal-fin spine, when depressed extends to distal the point of the last anal-fin ray.
Freshwater species were morphologically different.Proportional measurements (Tables 1-2) indicate that Eugerres castroaguirrei, has a moderate orbit diameter and second dorsal and anal spine length; and a longer head, snout, and postorbital length.Thus, the new species differs from E. mexicanus, except in terms SL-body depths ratio (Tables 1-2).
Based on the results of the principal components analysis of 20 morphometric variables (excluding SL), the distinction between Eugerres mexicanus and E. castroaguirrei is largely described by PC-I vs. PC-II (Fig. 2b).The variables that loaded most heavily on PC-I (< 50% of the variance) included: body depth, length of dorsal and anal fin bases, postorbital length, head length, interorbital length, head height, pelvic fin length, caudal peduncle height and length, upper jaw length, and snout length (Table 3).Those that loaded most heavily on PC-II included: length of second dorsal and anal-fin spines, and length of third dorsal and anal-fin spines (Table 3).
Coloration.Coloration in life specimens yellowish-silver; four dark, wide stripes on sides; lower dark stripes discontinuous (vs.9-11 continuous dark stripes in other Eugerres species); dorsal and caudal fins with blackish margins; pectoral, pelvic and anal fins yellowish-white.Ground color on preserved specimens silvery brown back; four well-defined longitudinal dark stripes on sides of the body; belly silver to yellowish-white; ventral stripes not well-defined; fins yellowish to brownish-white; black margin on first dorsal fin.
Sexual dimorphism.Not observed.

Distribution and habitat.
Eugerres castroaguirrei inhabits the ríos Grijalva-Usumacinta basin in the highlands of Chiapas and Tabasco, Mexico and northern Guatemala (Fig. 4).This species has a sympatric distribution with its congener E. mexicanus.

Ecological notes. Eugerres castroaguirrei
has not yet been studied; thus, biological and ecological data are lacking.Specimens of the new species are fished by local communities and consumed as food.Further studies are necessary to obtain additional data.The holotype was captured using a seine net 90 cm bellow surface in channel with sandy-slime bottom, pH 8.0, conductivity 478 μS/cm, dissolved solids 242 mg/l, dissolved oxygen 6.9 mg/l, and 26 ºC.Proposed common name.The proposed English common name is "Lacandon mojarra", after its principal area of distribution.
The proposed Spanish common name is "mojarra lacandona".

Remarks.
Here we establish that Eugerres castroaguirrei was a previously overlooked species in the literature on the Family Gerreidae and freshwater ichthyofauna of Central America (e.g., Meek & Hildebrand, 1925;Jordan & Evermann, 1927;Deckert & Greenfield, 1987;Castro-Aguirre et al., 1999;Miller et al., 2006) Diagnosis.Eugerres mexicanus is distinct from their marine estuarine congeners by their body oblong and laterally thicker and the dorsal-fin origin posterior to the insertion of the pectoral and pelvic fins; form its freshwater congener can be distinguished based on the combination of the following characteristics: length of second dorsal-fin spine 18.9-43.3%SL (vs.23.3-34.2%SL); depressed second dorsal-fin spine extends to base of fifth or sixth dorsal-fin rays; length of second anal-fin spine 16.7-30.2%SL (vs.11.1-20.6%SL); depressed second anal-fin spine extends to distal point of  2 and 4, respectively.
Coloration.Coloration in life based on 26 specimens obtained from Tenosique, Tabasco, México (UABC 2666; MNHN 2006-0767; NMW 95101): body silvery white, brown on back; four dark, thicker stripes on sides; lower stripes dark with irregular pattern; dorsal and caudal fins blackish-white; black margin on first dorsal fin; pectoral, pelvic, and anal fins yellowishwhite.Ground color of preserved specimens silvery-black or brown; lateral dark stripes gently convex (sometimes indistinct); belly silvery to yellowish-white; ventral stripes not well-defined; fins yellowish to brownish-white; spinous portion of dorsal fin with black margin.
Sexual dimorphism.Not observed.

Distribution and habitat.
Inhabit freshwater habitats along the ríos Grijalva-Usumacinta, and Coatzacoalcos basins in southeastern México (Chiapas, Tabasco, and Veracruz) and northern Guatemala (Fig. 4).Eugerres mexicanus is endemic to the Neotropical region and the Usumacinta Province (Miller, 1966(Miller, , 1982(Miller, , 1986;;Miller & Smith, 1986;Miller et al., 2006), with a noteworthy distribution in highlands with elevations of 100 to 300 m (Castro-Aguirre et al., 1999); some of these localities are in Chiapas and the northern part of Guatemala (upper Usumacinta).
Ecological notes.Eugerres mexicanus has not been wellstudied.However, Collette & Russo (1981) established that 'E.mexicanus ' [s.l.] specimens are eaten by Batrachoides goldmani Evermann & Goldsborough, 1902, a species with a similar distribution.The Mexican mojarra has omnivorous feeding habits, involving the consumption of insects, plants, crustaceans, and mollusks; and reach sexual maturity at 160.0 mm SL (González-Acosta, 2005).Helminthes parasites of this species had been recently reported (Salgado-Maldonado, 2006).The species is caught abundantly by local fishermen with danger of becoming overexploited.Further studies are necessary to evaluate the potential for aquaculture of this species (González-Acosta, 2005).Additional biological data for this freshwater fish are also necessary.

Common names.
Eugerres mexicanus is commonly known in English as Mexican mojarra or White mojarra; in Spanish as mojarra mexicana or mojarra blanca; and along the Grijalva-Usumacinta basin as Pichincha.

Osteology of freshwater gerreids
Neurocranium.In the freshwater gerreid species the neurocranium is characterized by a triangular-shaped supraoccipital crest with an anterior process convex and with a broad base, which differs from marine estuarine species where the anterior process of the supraoccipital is right angled or straight with a narrower base (González-Acosta, 2005).The anterior process of supraccipital in E. mexicanus is nearly straight (Fig. 5a), whereas E. castroaguirrei is convex (Fig. 5b).
The parietal in E. mexicanus is bell-shaped, the epioccipital has a short rear projection, the internal crest of the frontal is reduced and the pterosphenoid thinner (Fig. 5a).Whereas in E. castroaguirrei the parietal is triangular-shaped, the epioccipital has an expanded rear projection, the internal crest of the frontal is thickened and the pterosphenoid slender (Fig. 5b).
The infraorbital 1 in E. mexicanus is trapezoidal-shaped but with the anterior margin almost straight; the ventral margin is somewhat concave and markedly serrated; the caudal process is enlarged and pointed; the dorsal apophysis is dactylar-shaped with a pointed apex; a foramen is absent (Fig. 5c).In contrast, E. castroaguirrei has a first infraorbital bone trapezoidal-shaped with their anterior margin rounded; the ventral margin is concave and finely serrated; the caudal process is rectangular and pointed; the dorsal apophysis is blunt; a foramen is present (Fig. 5d).
Branchiocranium.The premaxilla in E. mexicanus is slender, presents an ascending process developed with margins almost straight; the articular process is digitiform but with a wide basis, the caudal process is rounded rectangular and the symphysial process is angular and notched (Fig. 6a).In E. castroaguirrei the premaxilla is thickened, presents an ascending process developed with margins curved, the articular process is digitiform with a reduced base, the caudal process is markedly rectangular and the symphysial process is rounded without a notch (Fig. 6b).
Marine estuarine species of Eugerres are characterized by the presence of molariform-like teeth in the pharyngeal and pharyngobranchial plates (González-Acosta, 2005), condition that differ in freshwater species where the pharyngeal teeth are granular; in particular, E. mexicanus has an irregular arrangement of the dentition in the pharyngeal plates (Fig. 6c), whereas E. castroaguirrei is distinguishable by the presence of a dentition serialy arrangement in the pharyngeal plates (Fig. 6d).

Dorsal fin supports.
In both freshwater species of Eugerres, the first and second dorsal-fin spines are supported by a singular pterygiophore structure, in which the first and second dorsal supports are fused and supported by a proximal radial or first pterygiophore.In E. mexicanus the first dorsal spine is small and the second spine long, equaling about 51.7% the pterygiophore length (Fig. 7a); whereas in E. castroaguirrei, the first dorsal spine is also small or reduced but the second spine is long, equal to about 55.5% of the pterygiophore length (Fig. 7b).
Anal fin supports.As in the dorsal fin, the first and second anal-fin spines are supported by the apparently fused first and second pterygiophores.In E. mexicanus this pterygiophore had a rounded anterior margin and their length comprising 79.4% of the second anal-fin spine length (Fig. 7c).In contrast, the pterygiophore in E. castroaguirrei has a rectangular anterior margin and their length is about 97.5% of the second anal-fin spine length, with a rectangular anterior margin (Fig. 7d).

Discussion
The Usumacinta Province, located in the northern part of the Neotropical Region, is characterized by numerous derivatives of marine species adapted to freshwater (e.g., Batrachoides goldmani Evermann & Goldsborough, 1902, Hyporhamphus mexicanus Álvarez, 1959and Strongylura hubbsi Collette, 1974), with impoverished primary and secondary ichthyofauna.There are numerous low-elevation rivers and lakes along the Atlantic slope characterized by the warmer temperatures found in tropical climates (Miller, 1982).The evolutionary radiation of fish in this region indicates old ichthyofauna with a high degree of endemism at the generic and suprageneric levels (Briggs, 1994).
The discovery of new freshwater taxa is significant and could change our understanding of continental ichthyofauna (Lundberg et al., 2007).Examples of this in the area are the endemic 'Chiapas catfish' Lacantunia   (Raven & Axelrod, 1979;Savage, 1982).Its discovery has increased the diversity of the fish in the area.The region is negatively affected by the impact of human activities, including the introduction and invasion of exotic species, the destruction of habitats by deforestation, the impact of hydroelectric and agricultural developments on natural drainage basins (Lozano-Vilano et al., 2007).
Eugerres castroaguirrei represents the second known vicarious freshwater species of Gerreidae in the New World.The new species is locally sympatric with E. mexicanus and shares its distribution with representative species from other families of marine origin [Ariidae: Cathorops aguadulce (Meek, 1904), Potamarius nelsoni (Evermann & Goldsborough, 1902)  It has been hypothesized that vicarious species are derivatives fish with marine ancestry that migrated from their original habitat at different times (Lovejoy & Collette, 2001).Studies of molecular characteristics should provide further information regarding genetic distinctions between E. castroaguirrei and their relatives, as well as the timing of the divergence of the marine estuarine and freshwater groups.
The comparative analysis based on the osteology of the species supported their taxonomic distinction on the basis of the differential morphology of some bones from several body regions: neurocranium (supraoccipital process), orbitotemporal region (parietal, epioccipital and pterosphenoid), oromandibular region (premaxilla, pharyngobranchial plates), orbital region (infraorbital 1), and the appendicular skeleton (dorsal and anal fin pterygiophores).Differences observed between the bone anatomy of the freshwater group and those reported for marine estuarine species (Andreata, 1988;Andreata & Barbiéri, 1981;González-Acosta, 2005), support the idea to include in further studies to the freshwater gerreid species in a different taxonomic category at genus level (González-Acosta et al., 2007).

Fig. 2
Fig. 2. a) Graphical index for discrimination of Eugerres castroaguirrei (squares, n = 42) and E. mexicanus (triangles, n = 122), provide by the biplot of second anal-fin spine length against anal-fin base.b) Plot of principal components (PC) scores for E. castroaguirrei (EC) and E. mexicanus (EM) from the southeastern of Mexico and northern Guatemala.

Table 2 .
Morphometric data for Lectotype, two Paratypes, and 122 non-type specimens of Eugerres mexicanus.Standard length, body depth and head length are expressed in millimeters.Numbers in parentheses: mean; SD: Standard deviation.

Table 3 .
Character loading on principal components I-II for 16 measurements taken on 42 specimens of E. castroaguirrei and 125 of E. mexcanus from the Grijalva-Usumacinta River Basin.Bold values represent the variables that loaded most heavily on PC-I and PC-II.
Etymology.The name proposed (castroaguirrei) honors the late Dr. José Luis Castro Aguirre, who made several important contributions to our understanding of Mexican fish fauna.