Rhinotocinclus marginalis, Reis & Lehmann, 2022

Reis, Roberto E. & Lehmann A., Pablo, 2022, A new genus of armored catfish (Siluriformes: Loricariidae) from the Greater Amazon, with a review of the species and description of five new species, Neotropical Ichthyology (e 220002) 20 (2), pp. 1-100 : 73-76

Rhinotocinclus marginalis , new species urn:lsid:zoobank.org:act:89E47C73-8B40-4676-86A4-C97EFBB8034C

( Fig. 40 ; Tab. 8 )

Holotype. MCP 54748 , 19.6 mm SL, rio Iriri at Cachoeira Grande, ca. 15 km upstream from confluence with rio Xingu , Piranhaquara, Pará, 03°50’35.5”S 52°44’08.3”W, 10 Oct 2012, M. H. Sabaj, M. A. Arce & L. M. Sousa .

Paratypes. Rio Xingu basin, Pará, Brazil: ANSP 193022 , 55 (not measured), MCP 48609 , 8 , 17.8–19.9 mm SL (1 measured, 19.9 mm SL) + 1 cs, same data as holotype . INPA 31155 , 29, 15.0– 20.3 mm SL (4 measured, 18.2–20.3 mm SL) + 2 cs, rio Iriri, ca. 4 h below mouth of rio Novo , Altamira, 04°14’14”S 53°24’34”W, 22 Aug 2008, H. López-Fernandez, N. Meliciano, G. Ortí & C. Röepke. INPA 33916 , 10, 15.3–20.5 mm SL (4 measured, 18.5–20.5 mm SL), Altamira National Forest , Itaitúba, approx. 05°50’S 54°55’W, 19 Jun 2009, R. Leitão & F. Ribeiro . INPA 59657 , 3, 18.8–20.8 mm SL (3 measured), rio Iriri, above mouth of rio Novo , ltamira, 04°28’11”S 53°41’38”W, 21 Aug 2008, H. López-Fernandez, N. Meliciano, G. Ortí & C. Röepke . INPA 59658 , 9, 16.1–19.1 mm SL, ROM 112205 , 2 tissue samples, rio Iriri at Barrinha , Altamira, 04°09’05”S 53°23’28”W, 19 Aug 2008, H. López-Fernandez, N. Meliciano, G. Ortí & C. Röepke. LBP 16685 , 6, 20.6–25.1 mm SL, unnamed creek in Altamira, 03°24’19.8”S 52°05’48”W, 30 Sep 2012, C. Oliveira, R. Britzke, L. M. Sousa & D. A. Bastos (preserved in ethanol).

Diagnosis. Rhinotocinclus marginalis is distinguished from all congeners, except for R. bockmanni , R. dani , R. hera , R. pentakelis , and R. loxochelis n. sp. by having the dominant color pattern formed by dark bars on body well separated and distinct (Fig. 5D; vs. dominant color pattern formed by wide dark bars partially coalesced or closed together, or formed by a dark stripe from the snout tip, through the eye and extending to end of caudal peduncle); having two separate light lines from snout tip diverging to each nostril (Figs. 6E,F; vs. light lines on snout absent, Y- or V-shaped); and by having teeth with yellow cusps (Fig. 5C; vs. cusps light ochre or brown). Rhinotocinclus marginalis is further distinguished from R. britskii , R. discolor , R. eppleyi , R. isabelae , R. kwarup , R. longirostris , R. pilosus , R. polyochrus , R. variola , and R. yaka by lacking an adipose fin (vs. adipose fin present). Rhinotocinclus marginalis is further distinguished from congeners, except R. bockmanni , R. dani , R. loxochelis n. sp., and R. pentakelis , by having small platelets at adipose-fin position (vs. small platelets absent). Rhinotocinclus marginalis is distinguished from R. bockmanni by lacking a triangular dark spot at the dorsal-fin membrane (vs. dorsal-fin dark spot present), and by having a conspicuous dark bar 2 on body (vs. bar 2 absent or inconspicuous). It is distinguished from R. hera by having small platelets at adipose-fin position (vs. small platelets absent) and yellow teeth cusps (vs. light ochre cusps); and from R. pentakelis by the shallower caudal peduncle (9.1– 10.1% vs. 10.1–11.7% SL and 23.3–26.3% vs. 26.6–29.7% HL). Rhinotocinclus marginalis is further distinguished from R. loxochelis n. sp. by the regularly arranged dark bars on body (vs. dark bars on body somewhat fragmented and inclined, such that they connect to form a zig-zag pattern), the shallower caudal peduncle 9.1–10.1% vs. 10.3–11.2% SL) and shorter pectoral-fin spine (26.7–30.3% vs. 30.4–32.1% SL); and from R. dani by the dark bars on body barely passing lateral dark stripe (vs. dark bars 2 and usually 3 reaching to the ventral midline), and fewer premaxillary (12–18, mode 16; Tab. 1) and dentary (11–16, mode 13; Tab. 2) teeth (vs. more numerous premaxillary, 19–28, mode 21, and dentary, 16–22, mode 19, teeth).

Description. Proportional measurements in Tab. 8 . Dorsal profile of head concave from snout tip to area between nares, convex from that point to parieto-supraoccipital tip and straight from that point to origin of dorsal fin. Dorsal profile of body mostly straight and descending from dorsal-fin origin to insertion of caudal fin. Trunk horizontally ovoid to roundly triangular and caudal peduncle vertically ovoid in cross section, vaguely flattened ventrally and compressed caudally. Body progressively narrowing posteriorly from cleithrum, more so behind dorsal fin.

Head slightly convex between orbits; dorsal margin of orbit slightly elevated. Snout elongated, depressed, its anterior margin rounded in dorsal view, with small depression anterior to naris. Eye comparatively large, positioned dorsolaterally, with small dorsal iris operculum. Posterior tip of parieto-supraoccipital with patch of enlarged odontodes. Slightly enlarged odontodes on snout border, especially on rostral and postrostral plates and on lower surface of pectoral and pelvic spines; enlarged odontodes curved and posteriorly oriented. Odontodes on head and trunk otherwise of uniform size and distribution. Canal cheek plate bent and elongated posteroventrally, almost contacting cleithum. Lips rounded, narrow, covered with minute papillae; papillae slightly decreasing in size towards lip margin. Lip margin with uniformly distributed papillae forming delicate fringe. Maxillary barbel with large free distal portion. Teeth moderately robust, bifid. Larger, medial cusp blade-like and slightly rounded, not elongated. Smaller, lateral cusp minute and pointed. Premaxillary teeth 12–18 (14); dentary teeth 11–16 (13); accessory teeth absent.

Body entirely covered by dermal plates except for ventral surface of head around lips, area around pelvic-fin insertion, and area around anus. Lateral plates arranged in five longitudinal series on trunk. Dorsal plate series complete, beginning at origin of dorsal fin, with 19–20 plates; mid-dorsal series incomplete, with 7–8 plates; middle series complete, with two ossified tubes and 22–24 plates. Lateral line on middle plate series with two ossified tubes, 20–22 pored plates followed by 2–3 terminal plates without canal. Midventral series incomplete with 19–20 plates. Ventral series complete and continuous from pelvic-fin origin to caudal-fin base, with 20-–21 plates. Predorsal plates forming two transverse rows anterior to nuchal plate. Coracoid completely exposed ventrally, twice longer than cleithrum; cleithrum exposed laterally with medial area and arrector fossa covered by skin. Lateral abdominal plates 3–4 (3/3); plates transversely elongate, clearly arranged in line between coracoid and pelvic-fin origin. Middle abdominal plates 1–2 series, between the lateral abdominal plates. Preanal plate large, single or double, bordered anteriorly by two or three plates. First anal-fin pterygiophore exposed in front of anal-fin as small, plate-like bone supporting odontodes. Total vertebrae 28, in one dissected specimen.

Dorsal-fin rays I,7; spine slightly arched. Dorsal-fin origin slightly posterior to vertical through pelvic-fin origin. Dorsal-fin spinelet present, plate-like, roundly triangular dorsally and V-shaped anteriorly. Spinelet articulated to first dorsal-fin pterygiophore and dorsal-fin spine locking mechanism functional. Adipose fin absent; single azygous plate at adipose-fin position in 1 out of 3 cs specimens. Pectoral-fin rays I,6. Large spine slightly arched; tip of adpressed spine reaching between distal fourth and tip of pelvic fin. Pectoral-fin axillary slit present, with large slanted opening ventral to tip of posterior process of cleithrum. Pelvic-fin rays i,5, fin short, with tip of adpressed fin almost reaching or reaching to anal-fin origin in males, falling short of that point in females. Adult males with small, delicate fleshy flap along posterodorsal margin of thickened first pelvic-fin ray. Odontodes on ventral surface of thickened first pelvic-fin ray bent and oriented mesially. Anal-fin rays i,5. Caudal-fin rays i,14,i, with upper and lower unbranched rays subequal.

Color in alcohol. Dorsal portions of head and trunk light brown, and cream to pale yellow ventrally. Two separate light mark from snout tip diverging towards nostrils. Compound pterotic and most of parieto-supraoccipial behind eyes dark brown. Posterior portion of parieto-supraoccipital lighter than surrounding areas, but not forming inverted Y-shaped mark. Trunk with five conspicuous dark brown bars; bars 1 merged with darkened predorsal area. Bars extending transversely from dorsal midline to middle lateral series of plates. Dark bars connected by lateral dark stripe from compound pterotic to caudal-fin base; dark bars barely passing ventrally to such stripe. Ventral surface mostly unpigmented, but small concentrations of chromatophores on cheeks, lateral abdominal plates, and caudal peduncle. Tooth cusps light yellow. Fins with transverse, brown bands formed by concentration of chromatophores on rays; bands more numerous on leading rays; membranes mostly hyaline. Dorsal fin without dark triangular spot at anterior portion of membrane, spine with 2–3 dark brown spots, branched rays with 2–3 dark bands, especially in distal half. Pectoral-fin spine with 4–5 dark spots, branched rays with 1–2 irregular dark bands. Pelvic fin hyaline or with one indistinct dark band. Anal fin with 1–2 dark bands. Caudal fin with dark transverse blotch at base and 1–2 irregular, indistinct brown bands.

Sexual dimorphism. Males have a conspicuous urogenital papilla immediately behind the anus and a low skin fold along the first, unbranched pelvic-fin ray, both characteristics being absent in females. Males also possess a larger nostril than females, and the snout profile greatly elevated immediately in front of the eyes. Males also have slightly longer pelvic fins, which reach or almost reach to the anal-fin origin and fall short of that point in females.

Geographical distribution. Rhinotocinclus marginalis is known from tributaries to the lower rio Xingu and rio Iriri, in the state of Pará, Brazil ( Fig. 36 ).

Etymology. Rhinotocinclus marginalis , from the Latin margo, meaning margin, and - alis, in allusion to the distribution of the species in the northern margin of the Brazilian Shield. An adjective.

Conservation status. The extinction risk of Rhinotocinclus marginalis is assessed as low despite the limited knowledge of its geographic distribution. The species is known from six localities in the lower Xingu and the Iriri rivers, with an Extension of Occurrence (EOO) estimated by the convex polygon of those localities of approximately 6,570 km 2. Despite logging and gold mining are common throughout the area, these threats are not believed to put the population in risk. In addition, the entire area of distribution of the species is located inside preservations areas (Altamira National Forest, Terra do Meio Ecological Station, Riozonho do Afrísio and Iriri Extractive Reserves, and at least three Indigenous Lands). For this reason, R. marginalis is preliminarily categorized as Least Concern (LC) according to the International Union for Conservation of Nature (IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2019).

Figures

FIGURE 36 | Geographic distribution of Rhinotocinclus species of the R. bockmanni group in southeastern Amazon basin. Rhinotocinclus bockmanni (black dots); R. dani (red dots); R. hera (pink dots); R. loxochelis n. sp. (blue dots); R. marginalis n. sp. (white dots); R. pentakelis (yellow dots). T = Type-locality.

FIGURE 40 | Rhinotocinclus marginalis, holotype, MCP 54748, 19.6 mm SL, rio Iriri at Cachoeira Grande, ca. 15 km upstream from confluence with rio Xingu, Piranhaquara, Pará, Brazil.