Hypostomus agna (Miranda-Ribeiro)

Osvaldo T. Oyakawa, Alberto Akama & Angela M. Zanata, 2005, Review of the genus Hypostomus Lacépède, 1803 from rio Ribeira de Iguape basin, with description of a new species (Pisces, Siluriformes, Loricariidae)., Zootaxa 921, pp. 1-27 : 4-10

Hypostomus agna (Miranda-Ribeiro)

(Fig. 1A, 2, 3; Table 1)

Plecostomus agna Miranda-Ribeiro, 1907 :188 (type locality: Rio da Ribeira, Iporanga).

Plecostomus agna Miranda-Ribeiro, 1908a:4 [reference, distribution]. Miranda-Ribeiro, 1911:45. Miranda-Ribeiro, 1918:711 [reference]. Gosline, 1945:81 [reference, distribution]. Fowler, 1954:174 [reference, distribution].

Hypostomus agna Isbrücker, 1980:18 [reference, distribution]. Burgess, 1989:430 [reference, distribution]. Isbrücker, 2001:28 [reference]. Weber, 2003:355 [reference, distribution]. Armbruster, 2004:79 [reference].

Material examined: Brazil. São Paulo: - MZUSP 40198, 4, 150.2-300.0 mm SL; Barra do Turvo; Sema/Sudelpa, no date. - MZUSP 42587, 3, 158.5-185.3 mm SL; rio Ribeira de Iguape, Ribeira; Expedição MZUSP, 6-13 March 1991. - MZUSP 42799, 1, 171.3 mm SL; rio Ribeira de Iguape, Ribeira; Expedição MZUSP, 8-27 March 1991. - MZUSP 45190, 1, 139.1 mm SL; rio Ribeira de Iguape, Eldorado; M. Damato, 5 March 1993. - MZUSP 45427, 2, 229.4-289.4 mm SL; rio Ribeira de Iguape, near mouth of rio Pilões and 8 km upstream of rio Batatal, Eldorado; M. Damato, 1 March 1993. - MZUSP 45434, 4, 159.5-223.3 mm SL; rio Ribeira de Iguape, near rio Pilões, Eldorado; M. Damato, 2 March 1993. - MZUSP 53339, 1, 168.5 mm SL; rio Ribeira de Iguape, near Iporanga; S. Buck, 21 September 1996. - MZUSP 45446, 3, 154.8-202.8 mm SL; rio Ribeira de Iguape, close to rio Pilões, Eldorado; M. Damato, 3 March 1993. - MZUSP 45463, 1, 184.9 mm SL; rio Pilões, tributary of rio Ribeira de Iguape, Eldorado; M. Damato, 6 March 1993. - MZUSP 45167, 1, 186.3 mm SL; rio Batatal, tributary of rio Ribeira de Iguape, Eldorado; M. Damato, 3 March 1993. - MZUSP 38583, 5, 174.0-295.0 mm SL; rio Pardo, Iporanga - Barra do Turvo Road, Barra do Turvo; O. T. Oyakawa et al., 5-6 February 1988. - MZUSP 55485, 2, 101.3-152.3 mm SL; waterfalls of rio Palmital, 1000 m above mouth into rio Ribeira de Iguape, Apiaí; M. R. Santos & M. Morato, 22 January 1995. - MZUSP 55215, 2, 126.1-257.0 mm SL; rio Pardo, Barra do Turvo; M. R. Santos & M. Morato, 25 January 1995. - MZUSP 55206, 1, 300.0 mm SL; rio Catas Altas, Ribeira; M. R. Santos & C. E. Espírito Santo, 14 October 1995.

Diagnosis. Hypostomus agna is distinguished from its congeners inhabiting Ribeira de Iguape river basin by the absence or poorly developed keels (versus keel well developed), mid-dorsal series of plates longitudinally aligned, not interrupted by the first plate of dorsal series (Fig. 1A) (versus mid-dorsal series of plates not aligned, interrupted by the first plate of dorsal series (Fig. 1B)), caudal peduncle trapezoidal in cross-section (versus oval), higher number of plates around supraoccipital (3 to 5 versus 1 to 2), and higher number of teeth in the premaxillary (mode = 51 versus mode =25-39) and dentary (mode around 52 versus mode = 26-42). Hypostomus agna can be further distinguished from H. interruptus and H. tapijara by having the basal lamina of first proximal radial of anal fin covered by skin (versus basal lamina of first proximal radial of anal fin usually uncovered).

Description: Standard length of examined specimens 101.3 to 269.3 mm SL. Counts and proportional measurements in Table 1. Dorsal profile gently rising from snout tip to dorsal-fin origin, and gently descending from this point to the end of caudal peduncle. Caudal peduncle robust and roughly trapezoidal in cross-section; strongly flattened on ventral portion. Dorsal plates between end of dorsal-fin base and adipose-fin spine flattened. One or two, usually one, preadipose plate.

Pre-dorsal region of trunk located between pterotic-supracleithrum and vertical through dorsal-fin origin covered by three horizontal series of plates that extend posteriorly to caudal fin. Median series of plates bearing the lateral-line canal. Mid-dorsal series situated above and mid-ventral series situated below median series. Dorsal series of plates starting at vertical through dorsal-fin origin. Ventral series of plates usually starting at vertical through pelvic-fin origin (starting at midlength of pelvic-fin base in a few examined specimens). Plates of the mid-dorsal series longitudinally aligned, not interrupted by first plate of dorsal series (Fig. 1A). Abdomen usually covered with minute platelets, leaving large naked areas around the pelvic fin and anterior to anus.

Plates on dorsal and lateral portion of body relatively smooth in all specimens examined. Weakly developed keels present in dorsal, mid-dorsal, median, and mid-ventral series of plates in larger specimens and no apparent keel, unless for the mid-ventral keel in smaller specimens (approximately 185.0 mm SL). Keel situated on dorsal series of plates, when present, with its origin aligned to vertical through base of second branched dorsal-fin ray. Mid-ventral series with keel more developed on first four plates.

Head wide and rounded anteriorly. Dorsal and ventral regions of head completely covered with dermal ossifications, except for a roughly ovoid naked area on snout tip. Specimens around 175.0 mm SL with naked area extending from snout tip to margin of upper lip. Ornamentation of pterotic-supracleithrum similar to the remaining surface of head and with odontodes densely distributed. Dorsal margin of orbit not distinctly elevated and not continuing in ridge on pterotic-supracleithrum. No ridge on the supraoccipital or on predorsal plates. Usually three, sometimes four or five plates bordering posterior margin of supraoccipital bone. Space between orbits straight or somewhat concave. Eyes large.

Mouth distinctly wide. Anterior-most papillae of inner face of lower lip roundish and small, followed by patch of larger and sparsely distributed papillae and with a more posterior patch of smaller and closely positioned papillae. Teeth long and bicuspid; medial cusp approximately 2.5 times the length of outer cusp and curved inward. Premaxillary teeth inserted in a relatively straight line; dentary teeth inserted in a somewhat concave arch facing mouth cavity; contralateral dentaries forming a relatively wide angle. Maxillary barbels short, approximately half eye diameter, and without papillae.

Dorsal-fin origin situated on vertical anterior to pelvic-fin origin and approximately on posterior third of pectoral-fin spine. Dorsal fin relatively small; tips of adpressed last two dorsal-fin rays ending on the first or second plate before adipose-fin spine. Margin of dorsal fin relatively straight or slightly convex. Adipose-fin spine robust and somewhat curved ventrally. Distal half of pectoral fin spine of specimens 175.0 mm SL or larger covered dorsally with small odontodes slightly curved forward. Tip of adpressed pectoral fin ending slightly beyond origin of pelvic fin. Tip of pelvic fin usually posterior to origin of anal-fin rays. Basal lamina of first proximal radial of anal fin covered by skin in all specimens examined. Caudal-fin margin slightly concave, lower spine somewhat longer than upper.

Color in alcohol. Dorsal and lateral surface of body with light brown ground coloration. Body covered with roundish dark brown spots, decreasing gradually in size and becoming more concentrated towards snout. Ventral surface of body pale or yellowish. Overall ground coloration of all fins pale to light brown, scattered with roundish dark brown spots. Spots on fins slightly larger than those on body.

Distribution and notes. Hypostomus agna occurs exclusively in the Ribeira de Iguape river basin (Fig. 3). The species was found to live mainly in the channel of the largest rivers, as exemplified by rio Ribeira de Iguape itself, rio Batatal, rio Pardo, and rio Catas Altas. Hypostomus agna is one of the largest species of loricariids of this basin, just smaller than H. tapijara .