Two new species of the genus Corydoras (Ostariophysi: Siluriformes: Callichthyidae) from northwestern Argentina, and redescription of C. micracanthus Regan, 1912

The group herein denominated as the Corydoras micracanthus species-group is supported by presenting dorsal and pectoral spines length reduced; caudal fin slightly emarginated; low body depth; parieto-supraoccipital process and nuchal plate not in contact and small eyes for the genus. It comprises Corydoras micracanthus, herein redescribed, and two new species described in the present paper, all from northwestern Argentina. Corydoras gladysae, a new species from the Calchaquí river, is distinguished from other species of the genus by the caudal-fin shape, slightly emarginated and by presenting the shortest dorsal and pectoral spines length (mean = 9.2 % and 14.8 % of SL, respectively). Corydoras petracinii, a new species from the Las Costas river, is distinguished from its congeners by the following combination of characters: dorsal spine short (mean = 16.6 % of SL), pectoral spine short (mean = 18.3 % of SL), body moderately elongate (body depth 29.5 % SL mean), caudal fin slightly forked and trunk flanks with 5–7 subsquare differenced blotches in the middle region. Corydoras micracanthus, from the Mojotoro river basin, is defined by its higher number of dorsolateral body plates (24–26) and trunk color pattern, presenting 4 to 6 subsquare blotches well differentiated in the flanks. The inclusion of these species into the genus Corydoras is discussed.

Unlike most catfish that have nocturnal habits, the Corydoras species has a diurnal activity (Nijssen & Isbrücker, 1980, 1986Staruss, 1985) and is one of the more known genera by the fishkeepers since most of the species of this genus are considered ornamental fish (Burguess & Quinn, 1992).
Corydoras micracanthus described by Regan (1912) on the basis of specimens obtained in the province of Salta, Argentina, was only known by the original description and it has not been reviewed again until today possibly because its presence in aquariums and the ichthyological collections is very poor; nevertheless Nijssen & Isbrücker (1980) designated a lectotype and paparelctotypes without making a mention of distinguishing features or redescription.
On the basis of new collections made in the Salta province, in northwestern Argentina, two new species of genus Corydoras are described in this article and C. micracanthus is redescribed. These three species are proposed as a group denominated here «C. micracanthus group» characterized by dorsal and pectoral spines short; caudal fin slightly emarginated; body depth low; supraoccipital and nuchal plate not in contact and small eyes for the genus. Additional data of ecology, biology and aquarium behavior are given.

MATERIALS AND METHODS
Measurements were taken with a digital caliper (± 0.1 mm) from point to point. To take the morphometric and meristic data we followed Reis (1997) with modifications of Britto (2000). The following additional data was also taken: longer dorsal soft ray. In the description, the frequency of each count (mode) is provided between parenthesis and the value corresponding to holotype is given with an asterisk. The holotypes of the species herein described, and part of the comparative material used, were maintained in aquariums during 6 months before its fixation in formaldehyde, with the intention of enriching the present study with behavior data, and photographs of live specimens. The type material is deposited in Museo Argentino de Ciencias Naturales «Bernardino Rivadavia», Buenos Aires, Argentina (MACN) and Museo de Ciencias Naturales de Salta, Argentina (MCNI).
The abbreviations used are: HL: head length; SL: standard length; SD: standard deviation; m.a.s.l.: meters above sea level.
In addition, Corydoras gladysae can be differentiated from other species of the genus by the following combination of features: 24-25 dorsolateral body plates (mode = 25); 21-22 ventrolateral body plates (mode = 22); small eye (7.3-12.5% HL) located dorso-laterally on head; dorsal fin rounded shaped; dorsal fin very low height in males and females (average 18.1% SL), body depth (29.0-35.7% SL); posterior expansion of the parieto-supraoccipital process triangular and elongated; separation of the predorsal plate by one or two dorsolateral body plates; trunk and head covered by a thin skin, with tiny dispersed odontodes and caudal fin with four or five series of small dark brown blotches restricted to rays, forming poorly defined vertical stripe pattern.
Description. Morphometric data presented in Table 1. Body short, robust. Snout rounded, pointless. Head profile convex from the snout to eye; approximately straight from eye to parietosupraoccipital process. Profile of body straight or slightly convex from tip of parietosupraoccipital process to last dorsal-fin ray. Postdorsal-fin body profile slightly concave to adipose-fin spine; noticeably concave from this point to the caudal-fin base. Ventral profile of body slightly convex from isthmus to anal-fin origin. Profile concave from first anal-fin ray to caudalfin base. Body roughly cylindrical in cross section at pectoral girdle, gradually becoming more compressed toward caudal fin; maximum body width in humeral region.
Eye round, small for the genus, located dorsolaterally on head; orbit delimited dorsally by frontal and sphenotic, ventrally by infraorbitals; Anterior and posterior nares very close to each other. Anterior naris tubular. Posterior naris non tubular, very close to the orbit anterior margin, only separated by flap of skin. Mouth located ventrally. Maxillary barbel generally reach anteroventral limit of gill opening. One indivudual presents the left anterior maxillary barbel forked. Mental barbel very short. Small rounded papillae covering entire surface of all barbels, upper and lower lips. Supraoccipital process externally visible and predorsal plate exposed just in front of first dorsal-fin ray. Posterior expansion of supraoccipital process triangular and elongated, separated from nuchal plate by one or two dorsolateral body plates. Trunk and head covered by a thin skin, with dispersed tiny odontodes. Dorsal fin shape rounded; its origin just on third dorsolateral body plate. Dorsal spine shorter than first seven dorsal fin rays. Dorsal fin rays I,7 or I,8; third and forth rays are the longest. Anterior border of dorsal spine smooth; posterior border with minute serrations. Adipose fin short with curved spine, approximately triangular in shape; the membrane base extends on 2 or 3 dorsolateral body plates, its origin separated from base of last dorsal-fin ray by 7 to 9 dorsolateral body plates. Anal fin ellipsoid or rounded, its origin located slightly anterior to adipose fin origin in most specimens; and at same distance in some. Anal-fin rays: i,5,i*(2), i,6(3), i,7(1), ii,3,i(1), ii,4(2). Pectoral fin approximately triangular with rounded margins; its origin located just posterior to gill opening. Ossified portion of pectoral spine shorter than firsts branched rays; its posterior border serrated; Pectoral fin reaches pelvic fins origin. Pectoral-fin rays: I,7 or I,8. Pelvic fin ellipsoid, its origin just below third ventrolateral body plates, at vertical through base of second branched dorsal-fin ray; it does not reach anal fin; base of pelvic fin ends just anterior to anus. Pelvic-fin rays: i,5; one specimen i,6. Caudal fin smoothly emarginated, almost truncated; posterior edge rounded. Principal caudal-fin rays: 13 (3), 14*(3), 15(3); upper procurrent caudal-fin rays: iv (3), v (3), vi*(3); lower procurrent caudal-fin rays: iv (3), v (3), vi*(3).

Color in life.
Ground color of head brown. Opercle dark brown with golden green reflections. Preopercular edge light brown to reddish. Ventral region of head light. Dark brown blotch between opercle and parieto-supraoccipital process. Barbel sometimes with little blotches, almost imperceptibles.
Ground color of trunk light brown with small dark brown blotches distributed on flanks and dorsal region. Large dark brown blotches over lateral line region on body plates sometimes grouped forming a wide discontinuous and irregular strip. Dorsal region presents three great dark blotches slightly distinguished form ground color; first blotch occupies the base of the dorsal fin first rays, second blotch appears at the posterior end of the dorsal fin; and third blotch in adipose fin base. Ventral surfaces of body yellowish white. Dorsal fin hyaline with dark anterior margin and points or small blotches to rays, sometimes forming one or two incomplete oblique stripe. Caudal fin with four or five series of small dark brown blotches restricted to rays, forming poorly defined vertical stripe pattern. Caudal fin membranes hyaline. Pelvic fin generally hyaline, eventually with small dark brown points or blotches restricted to rays. Adipose fin spine brown. Adipose fin hyaline, sometimes with spot in membrane. Anal fin with spots or blotches in middle zone, sometimes forming one or two short, irregular and oblique strips. Pectoral fin with small dark brown points or blotches restricted to rays with disposition and size similar to dorsal fin. Iris dark grey or black.
Color in formol. Ground color of head grey or dark brown. Opercular reddish grey or brown. Preopercular edge yellowish. Ventral region of head yellowish white. Barbel sometimes with very small blotches.
Ground color of trunk brown with small dark blotches distributed in flank and dorsal area. Large dark blotches over lateral line region on body plates sometimes grouped forming a wide discontinuous and irregular strip. Dorsal region present three great dark blotches slightly distinguished from ground color; first blotch occupies the base of the dorsal fin first rays, second blotch appears at the posterior end of the dorsal fin; and third blotch in adipose fin base. Ventral surfaces of body yellowish white. Dorsal fin hyaline with dark anterior margin and points or small blotches to rays, sometimes forming one or two incomplete oblique strips. Caudal fin with four or five series of small dark brown blotches restricted to rays, forming poorly defined vertical stripe pattern. Caudal fin membranes hyaline. Pelvic fin generally hyaline, eventually with small dark brown points or blotches restricted to rays. Adipose fin spine brown. Adipose fin hyaline, sometimes with spot in membrane. Anal fin with spots or blotches in middle zone, sometimes forming one or two short, irregular and oblique strips. Pectoral fin with small dark brown points or blotches restricted to rays with disposition and size similar to dorsal fin. Iris dark grey or black.

Distribution, biology and ecology data.
Currently, this species is only known from Calchaquí river, in the sub-Andean range system, at Cachi and Payogasta localities, in Cachi Department, Salta, Argentina (Fig. 6). This river is a tributary to the superior river basin of the Juramento river, that ends in Paraná river. According to Gonzo (2003), the Calchaquí river is the main collector of the Calchaquíes Valleys. This river and its tributaries conform a dendritic network of Atlantic drainage with a surface estimated at 14.175 km 2 and a perimeter of 640 km. Its layout fits into the disposition of mountainous cords and crosses intermountainous valleys of southern course. This river begins at the Nevado del Acay, at 5716 m.a.s.l., where it reaches its maximum height. Its length is 252 kilometers. The drainage network has a remarkable contrast between the East and west the latter with a greater number of affluents of permanent regime, oriented from West to East, and whose volumes are fed by defrosting water (Gonzo, 2003). The diversity of fish of this river is scarce, but most of them are endemic species, of limited distribution, sometimes only in certain sections of the water curse.
Corydoras gladysae was a very rare species in the samplings made and requires a great fishing effort to capture it. The more abundant sympatric species are: Bryconamericus rubropictus (Berg, 1901), Ixinandria steinbachi (Regan, 1906), Heptapterus mustelinus (Valenciennes, 1840), Jenynsia maculata Regan, 1906 and Trichomycterus spegazzinii (Berg, 1897). It is important to remark the permanent presence in this river of an introduced species, the «rainbow trout»: Onchorhyncus mykiss (Walbaum, 1792); this is an active predator that includes small fish in its diet, a reason why it could be affecting the endemic fish populations at the area.
The water from the Calchaquí river at Cachi and Payogasta has a high salinity due to the contribution of tributaries, as well as alkalinity, the pH measured at Payogasta during the month of August was 8.04. Although aquatic vegetation was not observed in the main channel of this river, the margins as well as secondary branches and adjacent drains show aquatic plants and filamentous algae. Gramineae family plants predominate along the borders and a species of this family was even observed growing totally submerged in one of the places where Corydoras gladysae was found.
Etymology. This species is dedicated to its first collector Gladys Ana María Monasterio de Gonzo, Argentine ichthyologist of the Universidad Nacional de Salta (UNSa), who for more than 20 years has been contributing to the knowledge of the diversity, distribution and biology of fishes in the Salta province, Argentina.
Aquariological observations. It is a species that lives in a fast-current river, and in the aquarium it does not swim in open waters. It is not as elegant as other species of the genus and its caudal fin is rarely displayed unfolded. It is usually seen staying in holes between stones. Accepts small worms like tubifex and dried food that reach the aquaria bottom in small amounts.
Description. Morphometric data is presented in Table 1. Body moderately elongate, robust. Snout rounded, pointless. Head profile convex from the snout to eye; approximately straight from eye to parieto-supraoccipital process. Profile of body straight or slightly convex from tip of parieto-supraoccipital process to last dorsalfin ray. Post-dorsal fin body profile slightly concave to adipose-fin spine; slightly concave from this point to the caudal-fin base. Ventral profile of body slightly convex from isthmus to anal-fin origin. Profile concave from first anal-fin ray to caudal-fin base. Body roughly cylindrical in cross section at pectoral girdle, gradually becoming more compressed toward caudal fin; maximum body width located in humeral region.
Eye round, small for the genus, located dorsolaterally on head; orbit delimited dorsally by frontal and sphenotic, ventrally by infraorbitals; Anterior and posterior nares very close to each other. Anterior naris tubular. Posterior naris non tubular, very close to the orbit anterior margin, separated only by a thin skin. Mouth located ventrally. Maxillary barbel generally not reach anteroventral limit of gill opening. Mental barbel very short. Supraoccipital process externally visible and predorsal plate exposed just in front of first dorsal-fin ray. Posterior expansion of supraoccipital process triangular and elongated, separated from nuchal plate by minimum space. Trunk and head covered by a thin skin, with dispersed tiny odontodes.
Dorsal fin approximately triangular; its origin just on third or fourth dorsolateral body plate. Dorsal spine shorter than first five dorsal fin rays; the third and fourth rays the longest. Dorsal fin rays I,8; Adipose fin short with curved spine, approximately triangular shape; the base of membrane extend over two or three dorsolateral body plates, its origin separated form base of last dorsal-fin by seven to nine dorsolateral body plates. Anal fin approximately triangular with rounded posterior side; its origin (drawing vertical line perpendicular to antero-posterior axis) slightly anterior to origin of adipose fin in most specimens, sometimes it is at the same distance. Anal-fin rays: ii,5* (1). Pectoral fin approximately ovoid; its origin located just posterior to gill opening; distal extreme of pectoral fin overpass the origin of pelvic fin in adults. Ossified portion of pectoral spine shorter than first five branched rays. Pectoral-fin rays: I,7, or I,8. Pelvic fin ellipsoid; its origin just below third ventrolateral body plates, at vertical through base of second or third dorsal-fin branched ray; it does not reach anal fin base. Pelvic fin base ends just anterior to anus. Pelvic-fin rays: i,5*(1). Caudal fin slightly emarginated. Principal caudal-fin rays 14*(1); upper caudal fin procurrent rays: iii; lower caudal fin procurrent rays: iii.
Dorsolateral body plates: 22*(1) or 23 (5); ventrolateral body plates: 20*(1) or 21(2); dorsolateral body plates on the dorsal fin base: 4(1), 5(3), 6*(2); dorsolateral body plates from adipose fin to caudal fin: 2(1), 3(2), 4*(2). Ground color of trunk light brown or golden yellow with little darker blotches distributed in flanks and dorsal region. Flanks of trunk present in middle region 5 to 7 subsquare differentiated blotches. Dorsal region presents four dark blotches slightly distinguished form the ground color; first blotch occupies the base of the dorsal fin first rays, second blotch appears at the posterior end of the dorsal fin, third blotch in adipose fin base; and fourth blotch over the caudal peduncle, just before the beginning of the caudal fin. Ventral surfaces of body yellowish white. Dorsal fin with dark anterior margin and little blotches to rays, forming oblique strip. Caudal fin with joined dark blotches forming 3 or 4 vertical irregular stripe pattern. Caudal fin membranes hyaline. Pelvic fins without dots or blotches. Adipose fin hyaline, sometimes with blotch in membrane. Adipose spine with dark brown basal area. Anal fin hyaline, without blotches or dots. Pectoral fins pale yellow or hyaline, occasionally very little spots almost imperceptible. Iris dark grey; peripupilar ring golden orange, pupil black.

Color in formol.
Ground color of head grayish dark brown. Opercular zone dark reddish grey. Preopercular edge and ventral zone light grey. Barbel brown get lighter toward the ends.
Ground color of trunk light grayish brown with little darker blotches distributed in flanks and dorsal region. Flanks of trunk present in middle region 5 to 7 subsquare differentiated dark brown blotches. Dorsal region presents four dark blotches slightly distinguished form the ground color; first blotch occupies the base of the dorsal fin first rays, second blotch appears at the posterior end of the dorsal fin, third blotch in adipose fin base; and fourth blotch over the caudal peduncle, just before the beginning of the caudal fin. Ventral surfaces of body yellowish white, sometimes dark centre by transparency of internal organs. Dorsal fin with dark anterior margin and little blotches to rays, forming oblique strip. Caudal fin with joined dark blotches forming 3 or 4 vertical irregular stripe pattern. Caudal fin membranes hyaline. Pelvic fins without dots or blotches. Adipose fin hyaline, sometimes with blotch in membrane. Adipose spine dark grey. Anal fin hyaline, without blotches or dots. Pectoral fins pale yellow or hyaline, occasionally very little spots almost imperceptible. Iris dark grey or black.

Distribution, biology and ecology data.
This species has only been found in its type locality, a nameless stream at 200 m from its ending into the San Lorenzo river (S 24°47'8.94'' -W 65° 28'10.61') (Fig.7). This river is part of the Juramento river basin that belongs to a fluvial network that ends in Paraná river at Santa Fe province. This stream, that begins in the hills known as Lomas de Medeiros, goes south to join the San Lorenzo river. It remains dry in almost all it surface a great part of the year increasing its caudal in the summer, the rainy season. Nevertheless, near its junction in the San Lorenzo river, it has surface water all the year (pers. obs.), probably due to a subterranean water flow from this last river. The Lomas de Medeiros hills were the ancient alluvial cone of Wierna river (Igarzábal & Medina, 1991).
This species was captured together with Corydoras cf. paleatus, but it is less abundant than this species. Other fish species captured in this habitat were: Hoplias malabaricus (Bloch, 1794); Jenynsia spp.; Heptapterus mustelinus. A great amount of water in the San Lorenzo river at «La Quebrada de San Lorenzo» is used to provide of tap water for the local population which is rapidly increasing. This leads this river to be down stream almost completely dry in winter (dry season), leaving just a few «wells» with water.
Currently there is a highway under construction that passes a few meters alongside this habitat. This situation is destroying it because of the earth extraction from the river and the original water course modification. Also, great amounts of garbage were observed in the surroundings of the rivers. All the alterations of the natural habitat and the proximity of the city together with the poor distribution and frequency of this species puts it and its habitat in a serious danger for its conservation over the long term.

Corydoras micracanthus
C. micracanthus is distinguished from C. petracinii by presenting a bigger modal number of dorsolateral body plates (25 vs. 23): by the shape of the dorsal fin, rounded rectangular (vs. triangular); by the dorsal fin first soft ray not branched in 85 % of individuals examined (vs. always branched in C. petracinii); by the coloration pattern of the trunk flanks that consists on 4-6 large blotches (vs. 5-7 blotches in C. petracinii).
Description. Morphometric data is presented in Table 3. Body low and long for the genus. Snout rounded, pointless. Head profile convex from upper-lip to tip of posterior process of parieto-supraoccipital dorsal. Profile of body slightly convex along dorsal-fin base. Postdorsalfin body profile slightly concave to adipose fin spine; concave from this point to caudal-fin base. Ventral profile of body slightly convex from isthmus to anal-fin origin. Profile concave from first anal-fin ray to caudal-fin base. Body roughly cylindrical or ovoid in cross section at pectoral girdle, gradually becoming more compressed toward caudal fin; maximum body width located in humeral region.
Eye round, small for the genus, located dorsolateraly on head; orbit delimited dorsally by frontal and sphenotic, ventrally by infraorbitals. An- terior and posterior nares very close. Anterior naris tubular and posterior naris without tube. Mouth located ventrally. Maxillary barbel generally not reach anteroventral opening of gills; anterior maxillary barbel slightly longer than the posterior maxillary barbel. Mental barbel very short. Small rounded papillae covering entire surface of all barbels, upper and lower lips. Parieto-supraoccipital process externally visible and predorsal plate exposed just in front of first dorsal-fin ray. Supraoccipital process posterior expansion triangular and elongated, separated form predorsal plate by minimum space. Trunk and head covered by a thin skin, with tiny papillae dispersed and odontodes on posterior margin of body plates.
Dorsal fin shape rounded, its origin just over second or third dorsolateral body plate. Dorsal spine shorter than first five dorsal fin rays, posterior border smooth. Dorsal fin rays: I,7-8; the third ray is longest; posterior border of dorsal spine with serrations. Adipose fin approximately triangular; its origin separated from base of last dorsal fin ray by 7-10 dorsolateral body plates (modal=8). Adipose fin preceded by short, curved, well-ossified spine; base of membrane over 2 or 3 dorsolateral body plates. Anal fin approximately triangular with posterior side rounded; its origin (drawing vertical line perpendicular to antero-posterior axis) slightly anterior to adipose fin origin in majority of specimens. Anal fin rays: 7, first half of anal fin first ray is hard. Pectoral fin approximately triangular with its interior margin rounded; its origin just posterior to gill opening. Ossified portion of pectoral spine shorter than first three branched rays; distal extreme of pectoral fin reaches origin of pelvic fins. Pectoral fin rays: I,7 or I,8. Pelvic fin ellipsoid, its origin just under third ventrolateral body plates, at vertical through base of second branched dorsal-fin rays; it not reach anal fin base. Pelvic fin base ends just anterior to anus. Pelvic fin rays: i,5, one specimen i,6. Caudal fin slightly emarginated, upper lobe slightly longer than lower lobe. Principal caudal-fin rays: i,14,i; upper caudal fin procurrent rays: iii; lower caudal fin procurrent rays: iii.

Color in life.
Ground color of head brown. Opercle dark brown with golden green reflections. Preopercular edge light brown to reddish. Ventral region of head light. Dark brown irregular blotch between opercle and parietosupraoccipital process. Barbel yellowish or light brown.
Ground color of trunk light brown or golden yellow with little darker blotches distributed in flanks and dorsal region. Trunk with 4-6 subsquare differentiated large brown blotches at junction of body plates. Dorsal region presents four dark brown blotches slightly distinguished form the ground color; first blotch occupies the base of the dorsal fin first rays, second blotch appears at the posterior end of the dorsal fin, third blotch in adipose fin base; and fourth blotch over the caudal peduncle, just before the beginning of the caudal fin. Ventral surfaces of body yellowish white. Dorsal fin with dark anterior margin and little blotches to rays, forming oblique strip. Caudal fin with joined dark blotches forming 2 or 3 vertical irregular stripe pattern. Caudal fin membranes hyaline. Pelvic fins without dots or blotches. Adipose fin hyaline, sometimes with blotch in membrane. Adipose spine with dark brown basal area. Anal fin hyaline, without blotches or dots. Pectoral fins pale yellow or hyaline, occasionally very little spots almost imperceptible. Iris dark grey; peripupilar ring golden orange, pupil black. Fig. 6. Geographic distribution of Corydoras micracanthus (squares), C. gladysae (black stars), C. petracinii (white star) and C. cf. paleatus (circle). Arrows = type localities.

Color in formol.
Ground color of head grayish brown. Opercle dark brown or reddish grey. Preopercular edge light yellow. Ventral region of head light. Dark brown irregular blotch between opercle and parieto-supraoccipital process. Barbel yellowish or light yellow.
Ground color of trunk light brown or pale yellow with little darker blotches distributed in flanks and dorsal region. Trunk with 4-6 subsquare differentiated large dark blotches at junction of body plates. Dorsal region presents four dark blotches slightly distinguished form the ground color; first blotch occupies the base of the dorsal fin first rays, second blotch appears at the posterior end of the dorsal fin, third blotch in adipose fin base; and fourth blotch over the caudal peduncle, just before the beginning of the caudal fin. Ventral surfaces of body pink white. Dorsal fin with dark anterior margin and little blotches to rays, forming oblique strip. Caudal fin with joined dark blotches forming 2 or 3 vertical irregular stripe pattern. Caudal fin membranes hyaline. Pelvic fins without dots or blotches. Adipose fin hyaline, sometimes with blotch in membrane. Adipose spine with dark brown basal area. Anal fin hyaline or pale yellow, without blotches or dots. Pectoral fins pale yellow or hyaline, occasionally very little spots almost imperceptible. Iris dark grey or black.
After checking out the comparative material, the localizations in Gonzo (2003) of Corydoras paleatus for Mojotoro, La Caldera, Lesser and Vaqueros rivers and for Gallinato and Pucheta creeks, Coronel Alfonso Peralta dam (Campo Alegre) and water-drainage channel, would correspond to Corydoras micracanthus and not to C. paleatus.
The Mojotoro river is part of the active basin of the Bermejo river. It includes a dense fluvial network that drains an area of approximately 850 km 2 , located in the northern region of the «Valle de Lerma». The torrentiality is the main characteristic of these rivers (Igarzábal & Medina, 1991).
The water at these environments generally has a pH near 7, soft and with low salinity. It is usually very crystalline, with an increase of the caudal and turbidity of the water in the rainy season, in summer, mostly on January and February.
Due to the precipitation regime of the zone, with the dry season on winter, the volume of the rivers and streams fluctuate much throughout the year, with its maximum in the summer, estival period from December to April (Igarzábal & Medina, 1991). The aquatic vegetation is less frequent due to the high torrentiality of these ambients, although in water-drains and some streams of low torrentiality the «watercress» and others aquatic plants are abundant together with marginal plants. These environments seem to be favorable to Corydoras micracanthus because they are much more abundant there, which could be due to a greater availability of foods and minor presence of predators like Hoplias malabaricus or catfishes that may be present in bigger water courses, and also this species may be more protected from birds sight by the aquatic plants. The more abundant fish species in these ambients are Jenynsia spp.; Characiformes of small size like Bryconamericus thomasi Fowler, 1940;Trichomycterus spegazzinii;Heptapterus mustelinus; and Ixinandria steinbachi.

DISCUSSION
The distribution of these three species of Corydoras is restricted to small areas in the Argentinean northwest were they are endemic. C. micracanthus is the one with the major distribution. This group of species is characterized by a short length of the dorsal and pectoral spines, caudal fin slightly emarginated and small eyes for the genus.
The inclusion of these species in the genus Aspidoras Ihering was evaluated at the beginning because they present a short ossified portion of the pectoral and dorsal spines, and a low body height, characteristic features of this genus. However the parieto-supraoccipital bone shape and fontanel corresponds to Corydoras. Also, the genus Aspidoras presents a diagnostic character defined by a foramen in the supraoccipital which is not present in these species. These last features justified the inclusion of these in the genus Corydoras, considering the length of the spines as something more variable, probably due to an adaptation to fast flowing waters. The supraoccipital shape may be an ancestral character with less selective pressure of natural selection. This would make this feature a strong character for the determination of the genus Corydoras. Even though with a more detailed study of the genus Corydoras and a better definition of it, it is probable that some of these species might be placed in new genus. In Aspidoras, the posterior process of the supraoccipital is short and separated from the nuchal plate by one to three pairs of dorsolateral body plates. Corydoras and Brochis species, in contrast, typically have the posterior process of the supraoccipital well developed and fully contacting the nuchal plate, with the tips of the bones sometimes overlapping (Britto, 2000).
In Corydoras difluviatilis Britto & Castro (2002), C. micracanthus and C. gladysae the posterior process of the supraoccipital although well developed, does not contact the nuchal plate, leaving a small gap between these bones, which is covered by fleshy skin. C. difluviatilis is distinguished from the species herein denominated the C. micracanthus group by a relatively pointed snout (vs. relatively rounded snout).
This absence of contact between supraoccipital and nuchal plate is an ancestral character state for the Callichthyidae family. Considering this and the distribution of the species of the C. micracanthus group, plus C. difluviatilis, these possibly form a basalmost monophyletic group within the genus Corydoras.
These three species herein denominated the C. micracanthus group live above 1000 m.a.s.l. and had been registered almost up to 2500 m.a.s.l., in the case of C. gladysae. These fish as other benthonic fish of the area such as Ixinandria steinbachi (Loricariidae), has a greenish brown color in the flanks and dorsal regions of body, very similar to the ground color of the detritus and algae that grows over and covers the stones at the bottom. These fish also have dark brown blotches that get confused, when seen, with the shadows generated by the hollows in between the stones (pers. obs.). It has been observed that when these fish feel threatened they quickly look for refuge between the stones and remain steady, which could make them virtually «invisible» to predators such as birds or other fish, making these features a camouflage strategy (crypsis). Cryptic coloration is an antipredative feature (Wickler, 1968;Lowe-McConnel, 1987) that allows to these individuals not to give any signals to predators. Axenrot & Kullander (2003) observed in many Corydoras species that the fish, after an initial evasive reaction to the threat, remain steady. These fish propel themselves almost exclusively with their tail, not using their pectoral fins as other fish. In fact, the pectoral and dorsal fins have little mobility. The pectoral fins are in a ventral location and slightly rotated, forming an approximate plane parallel to the anteroposterior axis. These fins, together with the dorsal fin have the function of directing and maintaining swimming direction. The pectoral fins also bring more adhesion to the substrate, and help the fish to swim through the bottom in the rapid-flowing water. It has been observed that when these fish swim fast they do it in straight lines and taking sharp curves, generally near the bottom but not in contact with it. When they are feeding they do it slowly, its body inclined with the head pointing towards the bottom exploring with its barbels the substrate looking for food. When they remain steady they usually stand at the bottom on the pectoral and ventral fins, leaving the tail and rest of the body free from the contact with the bottom. This may let them start swimming faster than if they were in contact with the body ventral side on contact with the bottom.
The short length of the fish's spines and the low body height are probably an evolutive con-vergence related with the great torrentiality of the environments where they live. This is supported because even though these species have these features in common they do not seem to be so closely related when you examine other features such as, i.e., the quantity of the lateral plates in C. micracanthus vs. C. gladysae. Also the caudal fin, slightly emarginated, is a typical feature of the species that need a strong propulsion and are not constantly swimming. This hypothesis is also supported because the species with more reduced spines live in the faster flowing rivers.
Corydoras gladysae was previously cited by Gonzo (2003) as Corydoras micracanthus (non Regan, 1912) although C. gladysae differs from C. micracanthus by a less amount of dorsolateral body plates (23 vs. 25), a shorter dorsal spine (media: 9.2 % SL vs. 13.9 % SL), approximately straight dorsal profile of the head from the eye to the parieto-supraoccipital process (vs. convex dorsal profile of the head in C. micracanthus), by the presence of dots or dark brown blotches in the pectoral, pelvic and anal fin (vs. absence of dots or blotches in the pectoral, anal and pelvic fins in C. micracanthus) and by the brown