Characidium duplicatum, Armbruster & Lujan & Bloom, 2021

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 : 113-116

Characidium duplicatum , new species

urn:lsid:zoobank.org:act:11B1C6B7-9273-4C91-959A-31FDF3C0797B

Figure 10

Leptocharacidium sp. — Hardman et al., 2002: 235 [locality record].

Holotype.— CSBD F-3614 (ex AUM 62835 ), 1 (1mo/me, 1gm), 39.4 mm SL, Guyana, Region 8 (Potaro-Siparuni), Potaro- Essequibo River basin, Kuribrong River , in rapids at Grass Shoals , 05.40791, –059.53179, J. W. Armbruster, D. C. Werneke, E. A. Liverpool, D. P. Fernandes, D. C. Taphorn, 12 March 2014.

Paratypes.— Guyana: AUM 28124 , 1 (1mo/me), 25.2 mm SL, Potaro-Siparuni (Region 8) , Essequibo River basin, Potaro River, Amatuk cataract and beach just below cataract, 5.30389, –59.31111, L. M. Page, J. W. Armbruster, M. H. Sabaj, M. Hardman, J. H. Knouft, W. S. Prince, 25 October 1998 ; AUM 28135 , 5 (5mo/me, 5gm, 1cs), 19.6–22.6 mm SL, Potaro-Siparuni (Region 8) , Essequibo River basin, Potaro River, Waratuk cataract, 5.25889, –59.40028, L. M. Page, J. W. Armbruster, M. H. Sabaj, M. Hardman, J. H. Knouft, W. S. Prince, 26 October 1998 ; AUM 62835 , 1 (1mo/me, 1gm), 38.9 mm SL, Potaro-Siparuni (Region 8) , Essequibo River basin, Kuribrong River, in rapids at Grass Shoals , 5.40791, –59.53179, J. W. Armbruster, D. C. Werneke, E. A. Liverpool, D. P. Fernandes, D. C. Taphorn, 12 March 2014 ; AUM 72308 , 1 (0mo, 1me, 1gm), 29.9 mm SL, Potaro-Siparuni (Region 8) , Essequibo River basin, Kuribrong River, at Ram Sheep Rapids , 5.44236, –59.50201, D. C. Taphorn, J. W. Armbruster, D. C. Werneke, E. A. Liverpool, D. P. Fernandes, M. Benjamin, 13 March 2014 ; ROM 61496 , 13 (5mo/me, 7gm), 19.5–28.9 mm SL, Potaro-Siparuni (Region 8) , Essequibo River basin, Potaro River, Amatuk Falls , side channel of Potaro River near portage, 5.30421, –59.31051, E. Holm, 2 October 1990 ; ROM 110060 , 1 (1mo/me, 1 gm), 36.2 mm SL, Potaro-Siparuni (Region 8) , Essequibo River basin, Sheetrock Creek at crossing of road between Wailang and Mona Falls , 5.47494, –59.43769, N. K. Lujan, E. Liverpool, D. Gordon, M. Benjamen, L. Ziccardi, O. Williams, 29 April 2013 .

Other specimens examined.— Guyana: AUM 38991 , 1 (1mo/ me, 1gm), 25.2 mm SL, Upper Takutu-Upper Essequibo (Region 9) , Essequibo River basin , Essequibo River at Kassi- Attae Rapids, 5.5 km SE mouth of Kuyuwini River , 2.22654, –58.29379, J. W. Armbruster, M. H. Sabaj, M. Hardman, D. Arjoon, N. K. Lujan, L. S. de Souza, 10 November 2003 ; AUM 39024 , 7 (6mo, 7me, 6gm, 1cs), 25.0–30.0 mm SL, Upper Takutu-Upper Essequibo (Region 9) , Essequibo River basin , Essequibo River at Yukanopito Falls, 44.5 km SW mouth of Kuyuwini River , 1.91461, –58.52046, J. W. Armbruster, M. H. Sabaj, M. Hardman, D. Arjoon, N. K. Lujan, L. S. de Souza, 9 November 2003 .

Diagnosis.— Characidium duplicatum can be distinguished from all other crenuchids except Leptocharacidium by having two anterior unbranched pelvic rays, and from all crenuchids except C. crandellii , C. declivirostre , and C. wangyapoik , new species, by having venter without scales from the isthmus to pelvic girdle (vs. maximally to posteriormost pectoral-fin ray insertion), and from most species of Characidium by having a very large pectoral fin with first four unbranched pectoral-fin rays thickened, and first pectoral-fin ray bent at oblique angle (vs. pectoral-fin rays not thickened and first ray either straight or slightly convex). Characidium duplicatum further differs from C. declivirostre and C. wangyapoik , new species, by having four unbranched pectoral-fin rays (vs. three); from C. crandellii by having nine branched pectoral-fin rays (vs. 10– 11); pelvic fin ii,5,i (vs. i,6,i), by having ten circumpeduncular scales (vs. 12), by having two or more thin dark bands consisting of spots on dorsal- and pectoral-fin rays (vs. a median wide dark band with pigment concentrated on dorsal- and pectoral-fin membranes, membranes either entirely dark or lighter at center), by having an almost square dorsal fin with slightly concave distal margin (vs. falcate), by having adipose fin above middle of anal-fin base (vs. origin of adipose fin above or behind vertical through posteriormost anal-fin ray insertion), by having dentary and premaxillary teeth narrow, peg-like, maximally tricuspid with only central cusp well developed (vs. wide and tri- to pentacuspid, central cusp longest), by having no teeth in the second dentary row (vs. 8 or more), and by an anal–apex length/SL ratio of 94.5–100.0% (vs. 89.4–94.0%). In addition, the anterior border of the pectoral girdle is convex such that it is deepest at the midline in C. duplicatum , while the anterior border is slightly notched (deepest lateral of midline) in C. declivirostre and C. wangyapoik , new species, and deeply notched in C. crandellii .

Description.— Measurements based on 20 specimens ( Table 2 ); meristics based on 23 specimens. Dorsal profile of body convex arc from tip of snout to posterior end of supraoccipital, then beginning steeper and longer concave arc from supraoccipital to end of dorsal fin (highest point of arc at dorsal-fin origin); body then relatively straight and angled ventrally to end of caudal peduncle. Ventral profile straight to end of pelvic base, rises slightly to anal fin, then concave arc to caudal fin. Body deepest at dorsal-fin origin and shallowest at middle of caudal peduncle. Body oval in cross section anteriorly and oval with dorsal and ventral surfaces somewhat flattened on caudal peduncle. Eye diameter 78.8– 118.0% snout length, decreasing in size with SL ( Fig. 3 ), oval, angled with anterior vertex ventral to posterior vertex; dorsal rim of orbit significantly higher than interorbital surface. Snout broadly rounded. Gill membranes united across isthmus, but width of membrane greater in larger specimens. Tubercles absent.

Scales cycloid with approximately 10 parallel striae in first postdorsal scale. Lateral line complete with canal in most scales occupying approximately 33–50% of scales and pores exposed just posterior to previous scale (some scales with pore at or near posterior end of scale); 28 (1), 29 (10), 30 (3), or 31 (3) 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-fin base. 4 scales above lateral line and 1 (3) or 2 (14) scales below lateral line, 10 circumpeduncular scales. Scales covering anterior 20% of caudal fin. 8 (5), 9 (9), 10 (2), or 11 (1) predorsal scales. Venter unscaled on isthmus and posteriorly to approximately 2 scales before pelvic-fin origin.

Dorsal fin with 2 unbranched and 8 (1), 9 (20) or 10 (2) branched rays (ii,8–10); first unbranched ray slightly less than one-half length of second, closely adhered; first branched ray longest and antepenultimate shortest, making fin slightly concave. Pectoral fin with 4 unbranched and 8 (2), 9 (19), or 10 (2) branched rays (iv,8–10); unbranched rays with thick pads of tissue anteriorly; first unbranched ray strongly curved posteriorly; fourth unbranched ray longest and last branched ray shortest; pectoral fin oriented obliquely on body with posteriormost insertion located dorsal to origin. Pelvic fin with 2 leading unbranched rays, 5 branched rays, and 1 posterior unbranched ray (ii,5,i); second branched ray longest; 2 to 3 pelvic axillary scales present with complex covering ~ half of pelvic-fin base. Anal fin with 2 unbranched rays and 3 (1) or 5 (22) branched rays (ii,3 or 5); first unbranched ray less than 1/3 length of second, closely adhered; fin slightly falcate with second branched ray longest, first unbranched ray considerably shorter, rays then becoming shorter to last; anal fin fits into concavity made by steep, concave margin of ventral profile starting at anal-fin origin. Caudal fin with 1 unbranched and 10 (22) branched rays in upper lobe and 9 (22) branched and 1 unbranched ray in lower lobe (i,10,9,i; one specimen had caudal fin too damaged to count rays); forked with upper and lower lobes equal. Adipose fin present with base centered on vertical over 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, sometimes absent on posterior rays.

Teeth tricuspid, narrow and peg-like with median cusp round and lateral cusps poorly developed. 5 (3), 6 (14), or 7 (6) premaxillary teeth. 3 (1), 5 (4), 6 (6), 7 (10), 8 (1), or 9 (1) teeth in outer dentary row. No teeth observed on inner dentary row. Ectopterygoid with single row of approximately 10 minute teeth.

Branchiostegal rays 4; 1 ray attached to posterior ceratohyal; 3 rays attached to anterior ceratohyal. Gill rakers 1–3 on dorsal limb, 1 on angle, 3–5 on ventral limb of anterior branchial arch. One supraorbital present; approximately crescent shaped with ventral side almost straight; 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 absent; however, parietals do not meet along midline, perhaps due to small size of cleared and stained specimens. Frontal foramina above supraorbital canal 3 wide, circular.

Total number of vertebrae 33 (2). Vertebral centra 2–4 fused, without ventral processes. Rib of centrum 4 distally expanded, extending anteriorly toward lateral process of centrum 2. Swim bladder was not examined. Supraneurals between neural spine of fourth centrum and anterior dorsal-fin pterygiophore 4(2). Epurals 3(2). Uroneural present, about 3/4 as long as urostyle.

Color in life.— ( Fig. 10 ) Body with yellow base color dorsally fading to gray ventrally. Dorsal surface covered with eight dark saddles, first on posterior of head, three between head and dorsal-fin origin (middle band lighter), one under dorsal fin, one between dorsal and adipose fins, one beginning under posterior half of adipose and continuing on caudal peduncle, and one at end of the caudal peduncle. Lighter spaces between saddles with yellow pigment covered by brown dorsally. Lateral stripe faintly visible, formed by scales in stripe with spot of color that leaves anterior edges of scales yellow-gray. Scales with spots covering all except anterior edge of scales present in all regions of body; spots more distinct on caudal peduncle. Several long wide blotches below lateral line with those between posteriormost pectoral-fin ray insertion and anal-fin origin most distinct. Dorsal fin with two distalmost bands formed from spots on rays with interspaces on rays yellow and fin membranes gray; proximal band of dorsal fin similar to others, but anterior end of band with large spot that covers both rays and membranes. Pectoral fin with three bands consisting of spots on branched rays; unbranched rays with central column of black surrounded by gray; interradial membranes gray; large dark spot present above most of pectoral fin and yellow spot present just above pectoral-fin origin; anterior base of pectoral fin to opercle gray with some patches of black melanophores. Pelvic fin colored similar to dorsal fin but with two bands; base of pelvic fin yellow. Anal fin with one medial dark band made of spots centered on rays and bleeding into interradial membranes; rest of anal fin gray. Adipose fin with yellow-gray base and dark tip contiguous with dorsal saddles below. Caudal fin with mostly gray membranes and alternating dark and yellow patches; dark patches longer than yellow patches on central rays and distally on all rays; dorsal- and ventralmost three or four rays with two to three large yellow spots surrounded by black; yellows and blacks fading distally. Head mostly mottled with black and dusky yellow; black interorbital bar present, continuing ventrally along anterior border of eye; large, dark crescent anterior of eye with small connection to interorbital bar; large black spot below eye that widens and fades ventrally; large dark spot on opercle, preopercle, and posterior infraorbitals. Some iridescent green and yellow spots present posterior to eye.

Color in alcohol.— ( Fig. 10 ) Similar to life except with iridescence absent and grays and yellows converted to tan. Holotype and specimen collected with it (AUM 62835) considerably darker than all other specimens.

Distribution.— ( Fig. 9 ) Found throughout the Essequibo River basin, but has been rarely encountered during our surveys. Most locations are in the lower Potaro and Kuribrong, but two localities are in the upper Essequibo upstream of the mouth of the Kuyuwini River.

Remarks.— Because Characidium duplicatum is distributed in both the lower and upper Essequibo with no collections in between, we excluded the upper Essequibo localities from the type series to be more certain that the types contain a single species.

Although similar to Characidium declivirostre in color pattern, C. duplicatum has extra unbranched pectoral and leading pelvic rays. The fourth unbranched pectoral and second unbranched pelvic rays are more similar in appearance to the first unbranched rays than to other branched rays, suggesting that the extra unbranched fin rays were gained via conversion of anterior branched rays. This is further supported by the fact that the pectoral and pelvic fins have the same total number of rays (usually 13 and 8, respectively) in C. duplicatum and C. declivirostre . The fourth unbranched pectoral ray and second unbranched pelvic rays are also the longest in their respective fins, while the first branched ray is the longest in C. declivirostre and C. wangyapoik , new species. The only other crenuchid with two unbranched pelvic rays is Leptocharacidium omospilus .

Etymology.— Duplicatum is Latin for double and is a neuter adjective. In reference to the presence of two unbranched anal-fin rays.

Figures

FIG. 10. Lateral view (top) of Characidium duplicatum, new species, in life and dorsal, lateral, and ventral views after preservation, CSBD F-3614, holotype, 39.4 mm SL. Photos by J. W. Armbruster.

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.

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).