Merodoras nheco, Horácio Higuchi, José L. O. Birindelli, Leandro M. Sousa & Heraldo A. Britski, 2007

Horácio Higuchi, José L. O. Birindelli, Leandro M. Sousa & Heraldo A. Britski, 2007, Merodoras nheco, new genus and species from Rio Paraguay basin, Brazil (Siluriformes, Doradidae), and nomination of the new subfamily Astrodoradinae., Zootaxa 1446, pp. 31-42 : 33-41

Merodoras nheco , new species

Figure 1

Gênero novo. Britski et al., 1999: 116 [citation and diagnosis].

Holotype.- MZUSP 90591 (40.8 mm), Brazil: Estado do Mato Grosso do Sul: município de Rio Verde de Mato Grosso, Rio Anhuma, tributary of Rio Negro (Paraguay basin) (19° 10' 07"S; 55° 18' 26"W), 28 August 1998, A. Machado-Allisson, B. Chernoff, P. W. Willink, O. Froehlich & A. Catella.

Paratypes.- Brazil: Estado do Mato Grosso do Sul: MZUSP 47180 (4, 25.5 - 35.3 mm); município de Coxim, Rio Piquiri (17° 16'S, 55°52'W), March 1994, T. Lipparelli. MZUSP 60053 (2, 45.6 - 48.1 mm); same data as holotype. Estado do Mato Grosso: MZUSP 35906 (3, 29.7 - 35.3 mm); município de Itiquira, Santo Antônio do Paraíso farm, ponds between Rio Piquiri and Rio Itiquira, 01 October 1979, J. H. B. Medeiros & J. C. Oliveira. MZUSP 83556 (23.0 mm); município de Itiquira, seasonal ponds between Rio Piquiri and Rio Itiquira (17 o 28' 13''S, 55 o 14' 46''W), 29 September 2003. MZUSP 84414 (28, 45.2 - 54.4 mm); ANSP 185103 (2, 43.6 - 50.8 mm); Tributary at Região do Rombado (16 o 26'S, 56 o 25'W), município de Poconé, 17 February 2000, C. Oliveira, F. A. G. Rondon et al.. AMNH 236346 (29.9 mm); Rio Pixaime, 20 November 1992, H. R. Axelrod. MZUSP 92610 (20, 21.0 - 61.1 mm); Flooded area, 1 km from Vila Mimoso, Pantanal de Paiaguás (16 o 17'S, 55 o 48'W), município de Barão de Melgaço, 30 September 2006, F. A. Machado, F C. T. Lima et al.. MZUSP 92611 (4, 26.5 - 61.5 mm); Rio Mutum, between Vila Mimoso and Joselândia, Pantanal de Paiaguás (16 o 19' 30''S, 55 o 49'59''W), município de Barão de Melgaço, 30 September 2006, F. A. Machado, F C. T. Lima et al.

Diagnosis.- Same as genus.

Description.- Specimens examined ranged from 23.0 to 54.4 mm SL. See Table 1 for counts and measurements and Fig. 1 for dorsal, lateral, and ventral views of holotype ( MZUSP 90591, 40.8 mm SL). Body weakly elongate, wide and somewhat depressed anteriorly, deepest at dorsal-fin origin, gently tapering to slender caudal peduncle. Ventral surface weakly flattened.

Anterior nares surrounded by short tubular skin located close to nostril tip; posterior nares larger than anterior, with a skin flap, not tubular, located at antero-superior portion of the orbit. Cephalic shield ornamented with distinct grooves and ridges. Interorbital fontanel divided by epiphyseal bar into a small rounded anterior portion, surrounded by mesethmoid and frontals, and large rounded posterior portion surrounded just by frontals. Nuchal foramina absent. Nuchal shield formed by anterior, middle and posterior nuchal plates (Fig. 2). Nuchal plates with straight to slightly concave lateral margins. Anterior nuchal plate wide, pentagonal, sutured to epioccipital Fig. 2). Epioccipital with no posterior process. Nuchal portion of cephalic shield flat to weakly angled in transversal cut. Six branchiostegal rays (n=3). Five pairs of ribs (n=3), gradually decreasing in size posteriorly. Thirty-one (n=2) or thirty-two (n=1) vertebrae, sixth fused and seventh partially fused into the complex vertebra.

Head somewhat short (24.1 - 27.5 % of standard length), depressed, broadly rounded in dorsal view, dorsal profile gently concave from dorsal spine to nostril tip. Eyes small (diameter 17.3 - 22.8% of head length), located approximately in first quarter between snout tip and dorsal spine origin. Jaws equals, terminal. Premaxillary and dentary with narrow transverse band of small acicular teeth. Maxillary small, located aside base of premaxillary and supporting maxillary barbel.

Three pairs of barbels (maxillary, inner and outer mental), simple, long, surfaced with small papillae. Maxillary barbel longest, almost reaching tip of postcleithral process. Outer mental barbel almost reaching tip of coracoid process. Inner mental barbel shorter, reaching posterior edge of coracoid suture.

Dorsal fin origin located at slightly greater than one-third body length from snout tip. Dorsal-fin spine strong and very gently curved over entire length with distal cartilaginous (break-away) tip. orsal-fin spine smooth, without serrae on both faces. Pectoral-fin spine strong, dorsoventrally flattened, gently curved along anterior margin with distal cartilaginous tip (length greater than that of dorsal-fin spine). Anterior margin of pectoral-fin spine with strong teeth becoming slightly larger and more antrorse distally; posterior margin with strong retrorse teeth shorter than those along anterior margin. Pelvic fin weakly rounded when extended; origin slightly anterior to vertical through tip of adpressed pectoral spine. Anal fin rounded when erect, reaching caudal-fin procurrent rays when adpressed. Caudal fin truncate. Upper and lower procurrent caudal-fin rays small, not modified as plates. Adipose fin small with distal free margin thin and rounded.

Cleithrum laterally expanded and visible in dorsal view, exposed portion sculptured with shallow grooves to base of postcleithral process. Postcleithral process lanceolate, narrow, elongate with one row of spines along its entire length and pointed tip reaching or surpassing vertical through origin of dorsal spine. Coracoid process extends posteriorly to just beyond last branched ray of pectoral fin, shorter than postcleithral process. Pectoral girdle entirely exposed ventrally (covered only by a thin layer of epithelium), with the ventral portion of coracoid expanded, restricting cavity for arrector ventralis inferior to a tubular canal on anterior edge of coracoid (Fig. 3). Axillary pore inconspicuous.

Head and body covered by minute, sparse tubercles, most abundant on dorsal portions of head and body, and on adipose fin. Tubercles almost always punctate, appearing as minute dots.

Lateral line incomplete, with three tympanal scutes plus 3-8 midlateral scutes, finishing between vertical through posterior insertion of pelvic fin and origin of anal fin. Lateral line pores absent beyond last midlateral scutes. First two tympanal scutes very small and inconspicuous, without medial thorns in adults. Third tympanal scute well developed, connected to posterior nuchal plate and reaching postcleithral process, with medial thorn in adults (thorn sometimes present in juveniles). Infranuchal scute attached to posterior nuchal plate dorsally and rib of sixth vertebra internally, reaching postcleithral process ventrally. Post-infranuchal midlateral scutes taller than wide with distinct medial thorn; thorn ventrally oriented in large specimens; accessory thorns present in upper and lower wings in adults. Scutes becoming slightly shallower posteriorly, covering about one-half of body depth anterior to pelvic-fin origin. Gas bladder with abbreviated cordiform shape (width slightly greater than length) and occupying the anterior half of body cavity (Fig. 4). Longitudinal septum occupying two-thirds of gas bladder length, with transverse septum reduced. Gas bladder divided into only two lateral chambers, with no distinction between anterior and posterior chambers. Pleura covering the internal walls of gas bladder, continuing anteriorly to septum, restricting the communication between lateral chambers to a small foramen (Fig. 4d). Walls of gas bladder distinctly speckled on internal and external faces.

Coloration in alcohol.- Overall color pale brown or tawny. Small dark brown spots or blotches covering top and sides of head, all fins, barbels, dorsal and pectoral spines and postcleithral process. A pair of irregular dark brown stripes flank lateral line above and below, and another stripe runs dorsolaterally from base of dorsal fin to adipose fin, turning upwards and covering the middle portion of the later. In some specimens, the stripes flanking the lateral line form a marbled pattern at base of caudal fin. Ventral surfaces pale with dark speckles on gular, branchiostegals, abdomen, and pelvic region, largely absent from exposed portion of pectoral girdle. Ventral surface, from head to caudal peduncle, darkly smudged.

Distribution.-Known only from flooded area of Paraguay basin called “Pantanal Matogrossense,” in western Brazil (Fig. 3).

Ecological Note.- Merodoras nheco lives in lentic waters, most precisely in lakes in flooded areas of Pantanal. In most specimens, we found many parasites (Pentastomida) in their gas bladder, between the inner walls and its internal pleura. Pentastomida parasites are known to have a reptile as definitive host. According to Francisco Machado (pers. com.), the “ jacarés-do-pantanal ” (“southern-caiman,” Caiman crocodilus yacare ) feed on Merodoras nheco when the lakes dry in the winter, and the fishes became easy to capture.

Etymology.- Specific name toponymic with reference to the town of Nhecolândia, the place where this species was discovered. The Portuguese nh is pronounced like the French or Italian gn or the Spanish ñ. The word nheco apparently does not have any particular significance other than having probably been a nickname for the town founder.

Discussion

Taxonomic comments on the subfamily Astrodoradinae .

The taxonomy and relationships among the species and genera in the subfamily Astrodoradinae remain poorly defined, and the species are often difficult to identify. For example, Higuchi (1992) recognized only one valid species in Amblydoras whereas Sabaj and Ferraris (2003) recognized six valid species in this genus. Fernández-Yépez (1950, 1968) proposed Hildadoras for two distantly related species: H. orinocensis (= Oxydoras sifontesi , sensu Sabaj & Ferraris, 2003), and H. bolivarensis (= Amblydoras bolivarensis , sensu Sabaj & Ferraris, 2003). The best diagnoses of the genera and species are yet found in Eigenmann (1925), who based his diagnoses on few external features and/or gas bladder anatomy (e.g., Scorpiodoras , which was described on the basis of having “posterior air-bladder chamber banjo- or scorpion-shaped” - Eigenmann, 1925: 324).

Comments on morphology in Merodoras .

The gas bladder with abbreviated cordiform shape is present in all species of Astrodoradinae , and also in Acanthodoras and Agamyxis . In all of these taxa the transverse portion of the internal septum is reduced, making the anterior lateral chambers nearly continuous (i.e. not clearly separable into anterior and posterior portions). In Scorpiodoras , Astrodoras and Hypodoras (Sabaj, pers. com.), the gas bladder has a terminal diverticulum, whereas in the others species of Astrodoradinae the gas bladder is simple, with no diverticulum. In Merodoras nheco , the transverse portion of the septum is very weakly developed, and the anterior chamber is divided by pleura, making the gas bladder appearing to be composed by two lateral chambers (Fig. 4). This condition has not been reported in any other doradids so far.

The generalized pelvic girdle in catfishes is composed of a wide basipterygium with an anterior process bifurcated (anterolateral and anteromesial processes) (Higuchi, 1992; Britto, 2002). In Merodoras nheco , there is usually an additional third process, which is smaller than and mesial to the anteromesial process of the basipterygium (present in three of the five c&s specimens; Fig. 5).

The pectoral girdle of Merodoras nheco is characterized by the expansion of the coracoid that restricts the arrector ventralis inferior opening to a small mesial fossae (Fig. 6), and covers most of ventral surface of pectoral girdle, making it ventrally exposed. Among doradids, this shape is only shared with Amblydoras and Physopyxis . In Physopyxis , the fossae is absent, and the muscles are completely enclosed by the coracoid, in a putatively more derived state.

In Doradids, the dorsal-fin spine is entirely smooth only in Merodoras and Anadoras . In Scorpiodoras , Astrodoras and Hypodoras , the dorsal-fin spine has well-developed serrae along the entire length of the anterior face. In Amblydoras and Physopyxis the anterior serrae are present only along the proximal portion of the anterior margin of the dorsal spine. In Amblydoras the serrae along the anterior margin of the dorsal spine decrease with ontogenetic development.

Comments on relationships of Merodoras .

According to Higuchi (1992), the genus is most closely related to Amblydoras , with which it shares concave anterior edges of hypobranchials 1 and 2, and a very thin parasphenoid. The two genera share with Physopyxis a ventrally exposed pectoral girdle, with greatly reduced opening for arrector ventralis inferior, and a truncate caudal fin. Other characters, however, like the presence of anterior serrae on the proximal portion of anterior face of the dorsal-fin spine in Amblydoras and Physopyxis (vs. dorsal spine without serrae in Merodoras ), do not support Higuchi’s hypothesis. The inclusion of additional species may influence relationships in a cladistic analysis, as documented by several authors (i.e., Schaefer, 1998). Therefore, a more robust analysis, including more characters and taxa within Astrodoradinae , is needed to provide a well-supported hypothesis for relationships within this group of thorny catfishes.