Diapoma thauma, Menezes & Weitzman, 2011

Menezes, Naércio A. & Weitzman, Stanley H., 2011, A Systematic Review Of Diapoma (Teleostei: Characiformes: Characidae: Stevardiinae: Diapomini) With Descriptions Of Two New Species From Southern Brazil, Papéis Avulsos de Zoologia 51 (5), pp. 59-82 : 67-73

Diapoma thauma , new species

Figs. 17-24 , Table 2

Specimens examined: All specimens from Brazil, Rio Grande do Sul.

Holotype: MCP 44105 , male, SL 32 mm, rio das Antas near mouth of rio da Prata basin, 29°04’01”S, 51°22’48”W.

Paratypes: MCP 37900 , (SL 26-35), collected with holotype . MCP 23047 , 2 (SL 41 and 42 mm), rio Turvo , on road between Vila Flores and Antônio Prado, rio Jacuí basin, 28°52’19”S, 51°26’57”W . MCP 37581 , 2 ( SLK 34 and 37 mm), rio dos Sinos, beach João Fernandes , about 4 km from Caraá, rio Jacuí basin, 29°46’27”S, 50°26’08”W . MCP 41011 , 12 (SL 26-45 mm), rio Antas Prata , rio das Antas basin, 28°58’16”S, 51°27’20”W . UFRGS 9708 , 24 (21-36 mm), rio Carreiro , Dois Lajeados, 28°56’24”S, 51°46’47”W , MZUSP 104077 (SL 34 and 35 mm, C&S), 28 (SL 22.5-37 mm), rio Carreiro , Dois Lajeados (approximately same coordinates as UFRGS 9708 ) .

Diagnosis: Most similar to Diapoma terofali , both species without the opercular extensions present in D. speculiferum and D. pyrrhopteryx , but easily distinguished from that species by having fewer gill rakers and anal-fin rays, as stated in the diagnosis of D. terofali .

Description: Morphometric data presented in Table 2.

Body compressed, elongate, shorter and less deep than that of D. terofali (compare respective data on Tables 1 and 2). General body shape, dorsal and ventral body profiles, shape of snout and mouth, position of fin origins and extension of maxilla as described above for D. terofali .

Dorsal-fin rays ii, 8 in all specimens, n = 80. Posteriormost ray unbanched in all specimens, n = 80. Adipose fin present. Unbranched anal-fin rays iv or v (one specimen with vi), branched rays 24-27, 25.8 (26), n = 80. Moderately developed anal-fin lobe includes anterior unbranched rays and first 6 or 7 branched rays. Anal fin of males with bilateral hooks on last unbranched ray and first 9-10 branched rays distributed as shown in Figure 21. Cleared and stained specimen ( MZUSP 104077, SL 35 mm) has 3 hooks on last unbranched ray, 2 hooks on eighth and nineth branched rays, 3 on first, third, fourth and sixth branched rays and 4 on second and seventh branched rays. Pectoral-fin rays i, 9-11, 9.8 (10), unbranched ray i in all examined specimens. Tip of longest pectoral-fin rays extending to, or slightly beyond, pelvic-fin origin in males, not reaching pelvic-fin origin in females, but no statistical difference between sexes in length of pectoral fin. Pelvic-fin rays i, 6, n = 80. Sexually mature males with hooks on pelvic fin rays, distributed as shown in figure 22. The same specimen with 8 hooks on first unbranched ray, 13 on first, 20 on second, 17 on third, 14 on fourth, 12 on fifth branched rays and 11 on sixth unbranched ray. Distal tip of longest pelvic-fin rays extending slightly beyond anal-fin origin in adult males, barely reaching anal-fin origin in adult females.

Scales cycloid with few radii (3-5) on exposed field on body and radii more numerous (10-15) on enlarged scales bordering pouch opening. Lateral line incomplete, anterior segment with 8-14, 10.1 (9), n = 46 pored scales and 23-30, 26 (28), n = 46 non-perforated scales. Lateral series scales 35-40, 37 (37), n = 46. Horizontal scale rows between dorsal-fin origin and anal-fin origin 10-12, 11 (11), n = 63. Predorsal scales 13-16, 14 (14), n = 79. Horizontal scale rows around caudal peduncle 13-15, 14 (14), n = 46. Premaxillary teeth in two rows ( Fig. 19 ), outer row with 2-5, 3.7 (4) tricuspid teeth, n = 80, inner row with 4-6, 4.4 (4) tricuspid or quadricuspid teeth, n = 80. Maxillary ( Fig. 19 ) with 2-5, 3.1 (3) tricuspid teeth, n = 80. Dentary ( Fig. 19 ) with 4-5, 4.2 (4) tricuspid to quadricuspid teeth, n = 80 and 5-11, 7.4 (8) smaller tricuspid teeth, n = 80. Total number of gill rakers on first gill arch 16-19, 17.4 (17), n = 80. Branchiostegal rays 4 in two cleared and stained specimens, 3 originating from anterior ceratohyal and 1 from posterior ceratohyal.

Color in alcohol: Nearly identical to that of D. terofali described above, but with dark humeral blotch and lateral body stripe more conspicuous ( Fig. 17 ). Anteroventral margin of humeral blotch about 2 scales distant from posterior edge of upper portion of opercle.

Sexual dimorphism, reproductive mode and gonad anatomy: Anal- and pelvic- fin hooks ( Figs. 21 and 22 ) present in males only. Some morphometric differences between males and females ( Table 2) are statistically significant (values of p in bold), but upon regression analysis caudal-peduncle depth was the only character to show significant difference between sexes ( Fig. 23 ).

Histological sections through ovary of a female ( MCP 23047) indicated presence of ovoid spermatozoa ( Fig. 24 ).

Distribution: This species has been collected to date in tributaries of the rio Jacuí basin, northwest of Porto Alegre, Rio Grande do Sul, Brazil ( Fig. 16 )

Etymology: The name thauma is from the Greek meaning wonder, or marvel with reference to the beauty of the fish when alive.

Remarks: Specimens used for histological sections of gonads ( MCN 18903) from rio dos Sinos, Caraá, Rio Grande do Sul, (approximately 29°45’S, 50°26’W) have all the morphological features of the specimens on which the species description is based.

Figures

FIGURE 16: Map of southern South America showing collecting sites and type localities (black symbols) of the species of Diapoma. Squares, Diapoma pyrrhopteryx; triangles, D. speculiferum; diamonds, D. terofali; circles, D. thauma. Some symbols may represent more than one locality.

FIGURE 17: Diapoma thauma, MCP 37900, adult male above, SL 32.0 mm and adult female below, SL 41.0 mm.

FIGURE 18: Diapoma thauma, MZUSP 104077, C&S, sexually active male, SL 35.0 mm; opercular bones, lateral view, left side. OP, opercle; POP, preopercle; IOP, interopercle; SOP, subopercle.

FIGURE 19: Diapoma thauma, MZUSP 104077, C&S, sexually active male, SL 35.0 mm; jaws and dentition, lateral view, left side, anterior at left.

FIGURE 20: Diapoma thauma, MZUSP 104077, C&S, sexually active male, SL 35.0 mm; caudal-fin squamation of ventral portion of caudal-fin lobe, lateral view, left side.

FIGURE 22: Diapoma thauma, UFRGS 8956, C&S, sexually active male, SL 35.0 mm; pelvic-fin rays, ventral view, left side, showing pelvic-fin hooks.

FIGURE 23: Caudal peduncle depth as a function of standard length for Diapoma thauma.

FIGURE 24: Diapoma thauma, MCN 18903; histological section of ovary of adult female; arrow points to spermatozoa.