Hypancistrus margaritatus, Tan, Milton & Armbruster, Jonathan W., 2016

Tan, Milton & Armbruster, Jonathan W., 2016, Two new species of spotted Hypancistrus from the Rio Negro drainage (Loricariidae, Hypostominae), ZooKeys 552, pp. 123-135 : 127-131

Taxon classification Animalia Siluriformes Loricariidae

Hypancistrus margaritatus sp. n. Figures 3, 4

Holotype.

CSBD F1701/ AUM 35610 (dual-accessioned), 45.6 mm SL, Amazon Basin, Rio Negro drainage: Rio Branco, Takutu River. Takutu River ca. 2.75 km W Saint Ignatius. Rupununi (Region 9), Guyana, South America. 3.35500°N, 59.83077°W, 5-6 Nov 2002, J.W. Ambruster, M.H. Sabaj, D.C. Werneke, C.L. Allison, M.R. Thomas, C.J. Chin, D. Arjoon, L. Atkinson.

Diagnosis.

Hypancistrus margaritatus is distinguished from all congeners by its color pattern of dense, small, light-colored spots on a dark base color. Three other described species have a color pattern of light spots on a dark base color: Hypancistrus inspector , Hypancistrus lunaorum , and Hypancistrus contradens . Hypancistrus margaritatus has consistently-sized spots on head and trunk about the size of the nasal aperture (vs. smaller spots on head than trunk in Hypancistrus inspector , spots usually smaller than half the nasal diameter in Hypancistrus lunaorum , and spots ranging in size from equal to or larger in diameter of nasal aperture in Hypancistrus contradens ). Hypancistrus margaritatus also has more spots, 23 in a lateral, horizontal series from snout tip to base of caudal peduncle 23 in holotype (vs. 8-20 spots in Hypancistrus contradens ; 8-16 in Hypancistrus lunaorum ). Hypancistrus margaritatus has more densely-packed spots relative to Hypancistrus lunaorum , with spots on lateral surface of the body having a minimum interspot to spot diameter ratio ranging from 1.0-1.6 (vs. 1.9-4.4 in Hypancistrus lunaorum ).

Description.

Morphometric data given in Table 1. Holotype 45.6 mm SL. Ventral surface from anus to head lacking plates. Lateral plates unkeeled. Cheek plates with 14 hypertrophied odontodes on one side and 18 on other. Frontal, infraorbitals, nasal, preopercle, compound pterotic, and suprapreopercle supporting odontodes. Small ridge of opercle exposed with four odontodes.

Caudal fin emarginate, lower lobe longer than upper. Ventral surface flat. Head tall. Snout short. Distance between verticals through tip of snout and anterior margin of orbit greater than distance between verticals through anterior margin of orbit and posteromedial tip of supraoccipital. Head with steep angle in profile, roughly 45 º, from tip of snout to anterior margin of eye. Body depth increases gradually from anterior margin of the eye to dorsal-fin origin, then decreases gradually from dorsal-fin origin to insertion of adipose fin, then caudal peduncle depth increases slightly from insertion of adipose fin to caudal-fin origin.

Supraorbital crest pronounced. Orbital opening oriented at less than 45° from sagittal plane. Interorbital isthmus between supraorbital crests flat. Supraoccipital crest low. Eye large with iris operculum. Gill opening restricted in L-conformation, with half of opening vertical and opening laterally and half of opening horizontal and opening ventrally.

Dorsal-fin spine short; depressed dorsal fin reaching slightly beyond pre-adipose plate to origin of adipose fin. Depressed pectoral-fin spine reaching beyond base of pelvic-fin rays; depressed pelvic-fin spine reaching beyond posterior insertion of anal fin. Dorsal fin II,7; caudal fin I,14,I; four dorsal procurrent caudal-fin spines; four ventral procurrent caudal-fin spines; anal fin i4; pectoral fin I,6; pelvic fin i,5. Fin spines and rays supporting odontodes.

Lips papillose, forming oral disk approximately half the width of the head. Maxillary barbels long, not reaching past the posterior edge of the lower lip when extended posteriorly, thin, and pointed; barbel rugose proximally, almost smooth distally.

Median plates 24; mid-dorsal plates 23; mid-ventral plates 24; rows of plates on caudal peduncle five. Dentary teeth five on one side and six on other, long and wide. Premaxillary teeth eight, smaller than dentary teeth. Medial cusps longer and wider than lateral cusps, with cusps separate and angled away from one another (vs. parallel and adjacent). Central buccal papilla absent.

Color. Light yellow spots on dark brown base color in life (Figure 3). Brownish-gray base color with small white spots in 70% ethanol. Gray base color mostly uniform, with slightly paler saddle areas at origin of dorsal fin, in middle area of dorsal fin, between dorsal fin and adipose fin, and posterior to adipose fin extending onto dorsal procurrent caudal fin rays. Light spots evenly-sized across body, approximately size of nasal aperture, and smaller in diameter than the length of lateral plates. Light spots relatively evenly spaced and present on all plated regions of body, usually with minimum interspot distance 1.0-1.6 times spot diameter. 21 spots in horizontal series from snout tip to end of caudal peduncle, 23 spots in series along sagittal plane from snout tip to dorsal procurrent caudal fin rays. Dark background color and evenly spaced small white spots also on fin spines and rays; fin-ray membranes hyaline but dusky with sparse melanophores. Six white spots on pectoral-fin spine, decreasing gradually in number to two to three on shortest fin rays. Four to five white spots on pelvic-fin spine, decreasing in number to three spots on shortest fin rays. Spots on dorsal-fin spine and rays in rows roughly parallel to slope of dorsal surface of body between dorsal-fin origin and insertion. Five to six spots on dorsal-fin spine and anterior rays, decreasing in number to three on last dorsal-fin rays. Posterior margin of dorsal fin darker gray than rest of the dorsal fin. Adipose-fin spine with a single spot. Adipose-fin membrane with two small spots. Adipose-fin membrane hyaline, more transparent posteriorly. Ventral naked area posterior to pectoral fin insertion and anterior to anus pale yellow. Mouth and ventral area anterior to pectoral fin insertion yellowish. Eye color dark dorsally with dense melanophores and small white spots, white ventrally with dark shaded spots of sparse melanophores. Odontodes present on fin spines and cheeks straw-colored with dark brown tips.

Range.

Hypancistrus margaritatus is described from a single specimen collected in the Takutu River, a tributary of the Rio Branco of the Rio Negro drainage.

Etymology.

The specific epithet is Latin for "adorned with pearls," referring to the dense white spots on the body.

Remarks.

The holotype and only known specimen of the species was dual-accessioned at AUM and CSBD because the specimen will be kept at AUM for the near term, but will eventually be repatriated to Guyana.

A Hypancistrus from the Takutu River that may represent Hypancistrus margaritatus has been designated as L404 in the aquarium pet trade ( Stawikowski 2006) and is uncommonly sold (MT pers. obs.). L404 appears to be conspecific with Hypancistrus margaritatus . In designating the L-number L404, Stawikowski (2006) noted the largest imported specimens were 11 cm in length. The pictured invidual may be larger than the holotype of Hypancistrus margaritatus . It also has many more spots (~35 spots from snout tip to caudal peduncle vs. 21 in the holotype of Hypancistrus margaritatus ). An image of L404 was also provided in Seidel (2008). The image shows approximately 28 spots from snout tip to caudal peduncle, intermediate between that of the holotype of Hypancistrus margaritatus and the Stawikowski (2006) image. If L404 is Hypancistrus margaritatus , spot number may increase with body size in this species, while the relative distance between spots does not increase.

Hypancistrus margaritatus appears to be rare, as only one specimen was collected during four expeditions that yielded 55,156 fish specimens ( de Souza et al. 2012). The specimen was collected from within a hole in a lateritic boulder in a run of the mainstem Takutu River. This species is listed as Hypancistrus sp. in de Souza et al. (2012).