Characidium declivirostre Steindachner, 1915

Armbruster, Jonathan W., Lujan, Nathan K. & Bloom, Devin D., 2021, Redescription of the Guiana Shield Darter Species Characidium crandellii and C. declivirostre (Crenuchidae) with Descriptions of Two New Species, Ichthyology & Herpetology 109 (1), pp. 102-122 : 109-113

Characidium declivirostre Steindachner, 1915

Figures 2A , 7–9

Characidium declivirostre Steindachner 1915: 31 , Río Coquenan, tributary of Río Caroni, Venezuela.

Melanocharacidium pectorale (sic).— Conway et al., 2012: figs. 6C, D, H, and J, and other mentions [fin pad description].

Specimens examined.— Venezuela: AMNH 91168 , 1 (0), Bolivar, Orinoco River basin, Río Carapo , at first rapids along right bank , 5.69750, –63.54167 (coordinates per Armbruster and Taphorn, 2013); AMNH 91169 , 4 (0), Bolivar, Orinoco River basin, Río Carapo , at third rapids above camp along left bank , 5.71333, –63.53333 (coordinates per Armbruster and Taphorn, 2013); AMNH 91170 , 17 (0), Río Paragua, at Gusano Rapids, ca. 1–1.5 hours upriver from Río Carapo mouth , ~ 5.5037, –63.5941 (coordinates placed at first large rapids complex upstream of Río Carapo, 1 March 1990; AMNH 232950 , 8 (0), Amazonas, Orinoco River basin, Río Cuao at Guacamaya Raudal , 13 km upstream from Raudal del Danto , 5.12861, –67.52556; AMNH 232982 , 2 (0), Amazonas, Orinoco River basin, Río Cuao at Raudal Piapoco, 21.5 km upstream from Raudal del Danto , 5.18250, –67.51361; AMNH 233000 , 26 (0), Amazonas, Orinoco River basin, Río Cuao, Raudal Cielo, west side of Isla del Cielo, 15 minutes by foot from SAS-01-04 basecamp, 7 March 2001; AMNH 233025 , 1 (0), Amazonas, Orinoco River basin, Río Cuao at Raudal Pauji, W side of Isla del Cielo, ca. 10 minutes by boat downstream from SAS-01-04 basecamp, 5.14861, –67.53639; AMNH 233084 , 14 (0), Amazonas, Orinoco River basin, Caño Pawa ca. 30 minutes by foot upstream from mouth into Río Cuao , ca. 30 minutes downstream from Puerto Nuevo , 5.29194. –67.32889; AUM 22310 , 1 (0), Bolivar , Orinoco River basin, Río Chaviripa , at base of falls, ca. 2 km SE Caicara-Puerto Ayacucho Rd. , 7.11389, –66.47306; AUM 22318 , 1 (0mo/me, 1gm), 34.9 mm SL, Bolivar, Orinoco River basin, Tributary of Río Caripo , ca. 16 km ESE Los Pijiguaos, Caicara-Puerto Ayacucho Rd. , 6.57361, –66.93417; AUM 36625 , 4 (3mo/me, 4gm), 43.4–50.2 mm SL, Bolivar, Orinoco River basin, Río San Ignacio , km 258 , 5.01015, –61.13694; AUM 36638 , 8 (3mo/3me, 4gm), 37.0– 56.2 mm SL, Bolivar, Orinoco River basin, tributary to Río Yuruani , km 255 , 5.01888, –61.11447; AUM 36660 , 2 (0), 41.7–45.7 mm SL, Bolivar, Orinoco River basin, Río Samey , 57.5 km WSW of Santa Elena, 6 km S. of El Piaje, on foot path, 4.42296, –61.59586; AUM 36674 , 7 (2mo/me, 7gm), 33.0– 47.5 mm SL, Bolivar, Orinoco River basin, Río Mapuari , km 268, 38 km N of Santa Elena, 4.94401, –61.12099; AUM 37137 , 2 (0), 29.6– 32.7 mm SL, Bolivar, Orinoco River basin, Río Paragua , 66.9 km SSW of La Paragua , at Uraima Falls, 6.30117, –63.62427; AUM 39296 , 5 (0), 19.1–20.5 mm SL, Amazonas, Orinoco River basin, Río Manapiare , 14.5 km NW of San Juan de Manapiare , 5.42863, –66.13616; AUM 39468 , 2 (0mo/me, 2gm), 34.5–36.3 mm SL, Amazonas, Orinoco River basin, Río Ventuari , above Salto Tencua , 58 km ESE of San Juan de Manapiare , 5.04777, –65.61583; AUM 39539 , 1 (0mo/me, 1gm), 38.6 mm SL, Amazonas, Orinoco River basin, Río Ventuari , at Raudales Tencua , 56 km ESE of San Juan de Manapiare , 5.04968, –65.62722; AUM 39601 , 1 (0), Amazonas, Orinoco River basin, Río Manapiare , 17 km NW of San Juan de Manapiare , 5.44198, –66.1507; AUM 40191 , 44 (6mo/me, 20gm, 4cs), 18.4–57.2 mm SL, Amazonas, Orinoco River basin, tributary to Orinoco , 30 km S of Puerto Ayacucho , 5.38659, –67.61556; AUM 43381 , 3 (0mo/me, 1gm), 58.4 mm SL, Amazonas, Orinoco River basin, Río Iguapo , 12.6 km ENE of La Esmeralda , 3.19988, –65.43582, 7 March 2005; AUM 43461 , 1 (0), Amazonas, Orinoco River basin, Río Casiquiare , bedrock in stream, 73 km NE of San Carlos de Río Negro , 2.35258, –66.57521; AUM 43497 , 6 (0), Amazonas, Amazon River basin, Río Siapa , raudales, 154 km E of San Carlos de Río Negro , 1.60339, –65.71587; AUM 43519 , 1 (1mo/me, 1gm), 58.7 mm SL, Amazonas, Amazon River basin, Caño Aracamoní , 156 km ESE of San Carlos de Río Negro , 1.55669, –65.72533; AUM 43924 , 11 (0), Amazonas, Orinoco River basin, Río Iguapo , 12.8 km ENE of Esmeralda , 3.19544, –65.43908; AUM 43943 , 1 (0), Amazonas, Orinoco River basin, Caño Tama Tama , 31 km WNW of Esmeralda , 3.21294, –65.82502; AUM 44072 , 1 (0), Amazonas, Orinoco River basin, Río Orinoco , Punto de Maraya , 80.8 km E of San Fernando de Atabapo , Isla Maraya, 4.02303, –66.97189; AUM 53418 , 1 (0mo/me, 1gm), 53.9 mm SL, Amazonas, Orinoco River basin, Cano Pasa , 98 km S of the Puerto Ayacucho airport , 5.06119, –67.77957; AUM 54195 , 1 (0), Amazonas, Orinoco River basin, Río Cataniapo , at Gavilan community , 5.54902, –67.38808; AUM 54207 , 26 (0), Amazonas, Orinoco River basin, Río Cataniapo , at Sardi community , 5.53375, –67.37395; AUM 56680 , 27 (5mo/me, 11gm, 2cs), 50.5–55.5 mm SL, Amazonas, Orinoco River basin, Río Cuao , at Raudales Danto , 69.3 km S of Puerto Ayacucho , 5.04409, –67.56045; AUM 56762 , 2 (0), Amazonas, Orinoco River basin, Caño Soromoni , 11.8 km WNW of La Esmeralda , 3.1938, –65.65197; NMW 62442 , 4 (3gm), syntypes, Orinoco River basin, Río Coquenan, tributary of Río Caroni .

Diagnosis.— Characidium declivirostre can be distinguished from all crenuchids except C. crandellii , C. duplicatum , new species, and C. wangyapoik , new species, by having venter without scales from isthmus to pelvic girdle (vs. maximally to insertion of posteriormost pectoral-fin ray) and from most species of Characidium by having a very large pectoral fin with first three, unbranched pectoral-fin rays thickened, and first pectoral-fin ray bent at an oblique angle (vs. pectoral-fin rays not thickened and first ray either straight or slightly curved). Characidium declivirostre differs from C. crandellii and C. duplicatum , new species, by having three unbranched pectoral rays (vs. four); from C. crandellii by having ten circumpeduncular scales (vs. 12), by having two or more thin dark bands consisting of spots on rays in the dorsal and pectoral fins (vs. a median wide dark band with pigment concentrated on membranes in the dorsal fin and the pectoral fin either entirely dark or dark with a lighter middle), by having an almost square dorsal fin with slightly concave distal edge (vs. falcate), by having the adipose fin above the middle of the anal-fin base (vs. anterior edge of the adipose fin above or behind a vertical through the insertion of the posterior most anal-fin ray), by having dentary and premaxillary teeth narrow and peg-like, and maximally tricuspid with only the central cusp well developed (vs. wide and tri- to pentacuspid, central cusp longest), by having zero to two (total) teeth in the second dentary row (vs. 8 or more), by a preanal length/SL ratio of 76.9–80.6% (vs. 72.9–76.6%), and an anal–apex length/SL ratio of 93.9–97.8% (vs. 89.4– 94.0%); from C. duplicatum , new species, by having one leading unbranched pelvic ray (vs. two); and from C. wangyapoik , new species, by having the ventral portion of the flanks with less than ten almost square blotches (vs. 10 or more narrow bars), by having lateral-line canals at least 33% of scale length and pores not covered by preceding scales (vs. lateral-line canals very short in scale, less than 25%, and pores covered by preceding scales).

Description.— Measurements ( Table 1 ) and meristics (reported below) based on 20 specimens. Dorsal profile of body forms convex arc from tip of snout to adipose fin (slight change in conformation at insertion of posteriormost dorsal-fin ray) then concave arc to caudal fin. Ventral profile straight to anal fin, then concave arc to caudal fin. Body depth greatest at dorsal-fin origin and least at middle of caudal peduncle. Body oval in cross section anteriorly (flattened ventrally) and oval with dorsal and ventral surfaces somewhat flattened on caudal peduncle. Eye 57.6–78.8% snout length, decreasing in size with SL ( Fig. 3 ), oval, angled with anterior vertex ventral to posterior vertex; dorsal rim of orbit slightly higher than interorbital surface. Snout pointed. Gill membranes united across isthmus. Tubercles present in both sexes, distributed dorsally on head and anterodorsal scales.

Scales cycloid, generally flat with about 6 striae in first postdorsal scale. Lateral line complete with canal in scales running approximately 33% of scales and pores visible laterally just posterior to previous scales; 30 (1), 31 (13), 32 (3), 33 (2), or 35 (1) lateral-line scales; lateral-line scales distinctly smaller anteriorly; naked area between anterior lateral-line scales and pectoral-fin base; lateral line continues onto scales covering caudal base. 3 (3) or 4 (17) scales above lateral line and 2 scales below lateral line, 10 circumpeduncular scales. Scales covering anterior 20% of caudal fin. 8 (10), 9 (9), or 10 (1) predorsal scales. Venter unscaled on isthmus and posteriorly to about two to three scales anterior to pelvic-fin origin.

Dorsal fin with 2 unbranched and 9 branched rays (ii,9); first unbranched ray about one-half length of second, closely adhered; first branched ray longest and last shortest; fin roughly rectangular. Pectoral fin with 3 unbranched and 9 (1), 10 (17), or 11 (2) branched rays (iii,9–11); unbranched rays and first branched ray with thick pads of tissue anteriorly; first unbranched ray forming curve; first branched ray longest and last shortest; pectoral fin oriented obliquely on body with insertion of posteriormost ray located posterodorsally to origin. Pelvic fin with 1 leading unbranched ray, 6 branched rays, and 1 posterior unbranched ray (i,6,i); first branched ray longest; 2 to 3 pelvic axillary scales present with complex covering approximately half of pelvic-fin base. Anal fin with 2 unbranched rays and 5 (15) or 6 (5) branched rays (ii,5–6); first unbranched ray approximately one-third length of second, closely adhered; fin margin almost straight with second unbranched or first branched ray longest; anal fin fits into space made available by steep, concave profile of venter starting at anal-fin origin. Caudal fin with 1 unbranched and 9 branched rays in upper lobe and 7 (1) or 8 (19) branched and one unbranched ray in lower lobe (i,9,7–8,i); forked with upper and lower lobes coequal. Adipose fin present with base centered on vertical through middle of anal-fin base. Rays of paired, dorsal, and anal fins with thick flaps of skin dorsally (paired fins) and laterally (dorsal and anal fins) that overlap successive rays of adpressed fin; flaps widest and longest anteriorly, decreasing in size and width posteriorly and often absent on middle and posterior rays.

Teeth tricuspid, narrow and peg-like with median cusp round and lateral cusps poorly developed. 5 (1), 6 (9), 7 (3), 8 (4), 9 (2), or 10 (1) premaxillary teeth. 6 (2), 7 (1), 8 (6), 9 (3), 10 (3), 11 (1), or 12 (4) teeth in outer dentary row. Inner dentary row made up of many small, unicuspid teeth; teeth may be absent or not readily visible in smaller specimens. Ectopterygoid usually with 2 rows of approximately 10 (lateral) and 5–10 (medial) minute teeth, some specimens having only lateral row.

Branchiostegal rays 4, 1 attached to posterior ceratohyal, 3 attached to anterior ceratohyal. Gill rakers 1–3 on dorsal limb, 1 on angle, 6–7 on ventral limb of anterior branchial arch. 1 supraorbital present; crescent shaped; from dorsal midpoint of orbit to dorsal 1/3 of anterior scleral ossicle. All elements of infraorbital series, except infraorbital 1, without laminar component. Parietal branch of supraorbital sensory canal extending to middle of parietal. Parietal fontanel reduced to tiny triangle at posterior borders of parietals. Frontal foramina above supraorbital canal 3 wide, circular.

Total number of vertebrae 33 (1), 34 (1), 35 (2), 36 (2). Vertebral centra 2–4 fused, without ventral processes. Rib of centrum 4 distally expanded, extending anteriorly toward lateral process of centrum 2. Posterior chamber of swim bladder extremely reduced, about length of one vertebral centrum. Supraneurals between neural spine of fourth centrum and anterior dorsal-fin pterygiophore 4 (3), 5 (3). Epurals 2 (1), 3 (5). Uroneural present, about 50% as long as urostyle.

Coloration in life.— ( Fig. 7 ) Base color tan with very slight yellow tinge (particularly on head and fins). Dorsal surface with six saddles considerably darker than light intervening regions (light regions from head to dorsal fin have spots centrally [dorsoventrally] and distally [anteroposteriorly] located on scales); first dorsal saddle located on nape, second anterior to dorsal-fin origin, third directly under dorsal fin, fourth in interdorsal region, fifth posterior to adipose-fin base, and sixth at end of caudal peduncle and continuing on to scales covering base of caudal fin. Faint lateral stripe made of spotted scales present along lateral line (spots always located along center lines, but may be posterior on scale (anterior body), central on scales (middle body), or anterior on scales (posterior body); lateral stripe covers lateral-line scales and one row above. Approximately 8 roughly square blotches located below lateral line (usually 2–2.5 scale rows wide, but some blotches smaller, particularly in smaller specimens). These blotches terminate in lateral stripe. Faint, iridescent yellow-green area noticeable above pectoral-fin base.

Head mottled with dark patches on tan to yellow background. Dark patch located on center of opercle, patch fading anteriorly; posterodorsal corner of opercle tan. Large, diffuse, dark blotches present below and behind eye. Wide stripe present from eye to snout. Dark bands present between orbits and down snout. Amount of melanin on head variable, some individuals with large light gray to yellow patches and some almost entirely dark gray.

Dorsal fin with alternating dark and light bands; bands better developed anteriorly. Distal band dark, pigment concentrated on rays. Second band gray (membranes) and tan (rays). Third band black, pigment concentrated on membranes. Fourth band wide gray (membranes) and tan (rays) band. Fifth (most proximal) band black, band covering rays and membranes. Pectoral fin with yellow to gray base color with dark spots centered on rays in two to three distal and one basal band; bands irregular; some spots bleeding onto membranes; ventrally, pectoral-fin colors more diffuse, generally gray with fleshy pads of unbranched and first branched rays almost white. Pelvic fin colored similarly to pectoral with single median band including melanophores on interradial membranes, and diffuse dark band at base of fin. Anal fin as pelvic fin, but no basal band, and band much wider anteriorly. Adipose fin yellow to gray proximally and dark distally with dark color confluent with saddle below it. Base color of caudal fin dark with pigment particularly concentrated at branching points of lepidotrichia; 2 yellow spots present at base (just above and below midline); dorsal and ventral spots may bleed into one another across center; rest of fin with large yellow blotches.

Coloration in alcohol.— ( Fig. 8 ) As in life but yellows and grays change to tan to light brown, spots on scales less distinct and numerous, and iridescence absent. Juveniles colored similarly to adults, but may have more and narrower ventral blotches, generally lighter in color overall. Some populations considerably lighter with greater contrast between light and dark areas, narrower bands, and smaller spots on fins.

Distribution.— ( Fig. 9 ) Characidium declivirostre is known from throughout the Orinoco River basin and the upper Negro River basin. The type locality is the Río Coquenán (also spelled Kukenan), a tributary of the upper Caroni River in Venezuela, We collected and examined material from near this locality (AUM 37137). Its range likely also includes leftbank clear or blackwater tributaries of the Orinoco River in Colombia and rivers draining the southern slope of the western Guiana Shield in Brazil. Our collections of this species were mostly from bedrock shoals in medium to largesized streams.

Remarks.— In their investigation of the microanatomy of paired-fin pads in ostariophysan fishes, Conway et al. (2012: figs. 6C, D, H, and J) described the pectoral- and pelvic-fin pads of this species (AUM 40191, misidentified therein as Melanocharacidium pectorale ). The pads were described as Type I pads where the epidermis is thickened without contribution from the subepidermal layers. They found the epidermis to be thicker ventrally than dorsally (60 vs. 30 lm), the subdermal layer thicker dorsally than ventrally (30 vs. 10 lm), keratinized unculi present on the ventral surfaces of the fin-ray pads, and alarm substance cells and mucocytes present and common on dorsal and ventral surfaces except the ventral surfaces of the pads.

Figures

FIG. 2. (A) Landmarks used in geometric morphometrics. Specimens were propped to maintain bodies orthogonal to the lens (Characidium declivirostre, AUM 36674, 44.0 mm SL); photo by J. W. Armbruster. (B) Wireframes of changes along PC1; gray is average shape and black is a shape that corresponds to a shape with þ0.1 Procrustes units. (C) Wireframes of changes along PC2 colored as in B. (D) Results of PCA of geometric morphometrics; specimens of C. crandellii are split by basin with a syntype (Amazon basin) indicated in black.

FIG. 7. Dorsal, lateral, and ventral views of Characidium declivirostre in life, AUM 56680, 55.5 mm SL; Raudales Danto, Río Cuao, Amazonas, Venezuela. Photos by N. K. Lujan.

FIG. 8. Dorsal, lateral, and ventral views of a specimen of Characidium declivirostre from the same stream as the type locality, AUM 36625, 50.1 mm SL. Scale ¼ 1 cm. Photos by J. W. Armbruster.

FIG. 9. Distributions of Characidium declivirostre, C. duplicatum, new species, and C. wangyapoik, new species. Type localities indicated by T’s (approximated for C. declivirostre).

FIG. 3. Plot of the ratio of orbital diameter to head length versus standard length for all measured specimens of fast-water Characidium of the Guiana Shield showing allometry in eye size.