Pseudacanthicus nyktos, Lopes & Sousa & Chamon, 2024

Pseudacanthicus nyktos , new species urn:lsid:zoobank.org:act:27E6D1DE-9993-4F99-A35A-DDCC4EB5A30D

( Fig. 1-7; Tab. 1)

Holotype. INPA-ICT 060701, 190.18 mm SL, Brazil, Pará , Vitória do Xingu, rio

Xingu, 02°49’13.6”S 52°00’42.7”W, 15 Sep 2012, L. M. Sousa.

Paratypes. All from Brazil, Pará, rio Xingu basin: ANSP 194624, 1, not measured, Senador José Porfírio , 02°38’51.5”S 52°01’39”W, 24 Set 2013, M. H. Sabaj Pérez, A. Gonçalves, N. K. Lujan, D. B. Fitzgerald, P. M. Ito, A. Oliveira & R. Robles. ANSP 194793 , 1 , not measured, Vitória do Xingu , 02°53’18.7”S 51°56’24.54”W, 22 Set 2013, M. H. Sabaj Pérez, L. M. Sousa, A. Gonçalves, N. K. Lujan, D. B. Fitzgerald, P. M. Ito, A. Oliveira & R. Robles. ANSP 194905 , 1 , not measured, Vitória do Xingu , same locality of the former batch, 22 Set 2013, M. H. Sabaj Pérez, L. M. Sousa, A. Gonçalves, N. K. Lujan, D. B. Fitzgerald, P. M. Ito, A. Oliveira & R. Robles. ANSP 196656 , 1 , not measured, Senador José Porfírio, 02°51’32.4”S 51°58’47.1”W, 3 Mar 2014, M. Arce, A. P. Gonçalves, J. A. S. Zuanon, D. B. Fitzgerald, R. Robles, D. R. G. Ribeiro, A. R. Martins, T. S. Jesus, L. M. Sousa, D. R. Costa (Dani), D. R. Costa (Ronca), N. S. Balao (Nelson) & A. S. Oliveira (Tonho). INPA 31467 , 3 , 44.85–250.2 mm SL, Vitória do Xingu , Itaboinha, 02°53’22”S 51°56’26”W, 4 Nov 2008, L. Rapp Py-Daniel and INPA staff. INPA 31804 , 11 , 71.09–109.89 mm SL, Vitória do Xingu , 02°53’22”S 51°56’26”W, 4 Nov 2008, L. Rapp Py- Daniel and INPA staff.; INPA 31805, 1, 202.3 mm SL, Belo Monte, ilha do Merencio (Pontão), 03°06’17”S 51°43’33”W, 5 Nov 2008, L. Rapp Py-Daniel and INPA staff. INPA 31806 , 1, 198.95 mm SL, Belo Monte, Paraná do Rio, 03°06’06”S 51°42’49”W, 4 Nov 2008, L. Rapp Py- Daniel and INPA staff. LIA 8171 View Materials , 3 View Materials , 49.36–160.39 mm SL, Vitória do Xingu , 02°49’13.6”S 52°00’42.7”W, 13 Set 2012, L. M. Sousa and INPA staff. LIA 8172 View Materials , 1, 140.30 mm SL, Vitória do Xingu , 02°49’13.6”S 52°00’42.7”W, 28 Oct 2013, L. M. Sousa and INPA staff. LIA 8173 View Materials , 1 View Materials c&s, 82.2 mm SL, 1 alc., 124.2 mm SL, 1 skel., 154 mm SL, specimens donated by the fish company Aquário PITY. LIA 8174 View Materials , 7 View Materials , 40.1–163.8 mm SL, specimens donated by ornamental fisherman. MZUSP 107205 , 3 , 86.5 – 116.2 , locality of Pontão, (property of Mrs. Maria and Mr. Waldomiro), Belo Monte, 03°06’49”S 51°43’23”W, 12 Jul 2010, J. Muriel-Cunha, O. Oyakawa, I. Fichberg & C. Chamon GoogleMaps .

Diagnosis. Pseudacanthicus nyktos differs from all congeners by a dark brown to black body with light gray vermicular spots on the dorsal and ventral surfaces, and fins (vs. body with dark gray to dark brown background with black dots or spots and fins with intense orange to red color in P. leopardus , P. pitanga and P. pirarara ; body and fins with dark gray background color and white dots or spots in P. serratus and P. fordii , more conspicuous in ventral region; body with a gray color and black dots or spots in P. histrix and P. spinosus ; and body with a dark brown color without spots or dots; with white transverse bars on the dorsal and caudal fins in P. major ). It can also be diagnosed from all congeners (except P. leopardus ) by a lower mode number of teeth in premaxilla and dentary: 3 and 5, respectively (vs. 4 and 15 in P. fordii ; 4 and 15 in P. serratus , 4 and 7 in P. spinosus ; 4 and 13 in P. pitanga ; 7 to 14 in P. major ; 8 and 14 in P. pirarara ). Pseudacanthicus nyktos can also be diagnosed from all congener by the following characters: crownshaped anterior edge of supraoccipital with three tips, one larger central tip and two smaller lateral tips (vs. anterior end of supraoccipital with one central tip in remaining species, except in P. major ); thin and elongated nasal shape (vs. wide and short nasal shape in the other species, except in P. leopardus ); rectangular lateral ethmoid with lateral projection absent (vs. triangular with lateral projection well-developed); anterior process of metapterygoid short and straight (vs. anterior process with a projection).

Description. Morphometric and meristic data summarized in Tab. 1. Dorsal profile of the body is slightly convex from tip of snout to vertical through dorsal-fin origin; concave, nearly straight from that point to caudal-fin origin. Region above the nostrils up to supraoccipital process slightly straight. Ventral profile of body straight from snout tip to caudal-fin origin. Ventral surface from pectoral girdle to urogenital papilla naked. Greatest body width at pectoral girdle. Trunk strongly keeled; five rows of longitudinal keels; one keel along each series of body plates. Greatest body depth at dorsal-fin origin, shallowest at caudal peduncle, between adipose fin and first procurrent caudal-fin ray.

Head deep, rounded anteriorly; snout and cheek completely covered by numerous small plates ( Fig. 2), except for small naked area on tip of snout. Snout rounded in dorsal profile. Nasal thin and elongated. Frontal slightly elongated, extending anteriorly halfway up nostril, with great contact with the orbit laterally. Short sphenotic. Ventral contact between sphenotic and sixth infraorbital slight or absent. Compound pterotic broad and moderately fenestrated with anterior border in contact with anterior margin of the orbit. Posterior area to compound pterotic with one or two small plates. Anterior region of the supraoccipital process with a larger mesial tip and two smaller lateral tips, forming a crown ( Fig. 3). Posterior area of supraoccipital forming a pointed V-shape crest. Opercular plates eversible, supporting well-developed odontodes.

Mouth medium size, with similar width and length, covered by small papillae decreasing in size towards the outer edge. Central buccal papillae reduced or absent. Labial filaments absent, maxillary barbel short; base of barbel united to upper and lower lips, tip free ( Fig. 4). Premaxillary teeth 3(15), 4(10)*, 5(5); dentary teeth 4(1), 5(13), 6(11)*, 7(5), 8(1). Teeth narrow and elongated; ends of teeth curved and forked, with small lateral cusp ( Fig. 5).

Dorsal fin i,8 rays; first dorsal-fin ray with odontodes less conspicuous than those of the pectoral fin; locking mechanism absent. Four to five predorsal plates. Pre-dorsal plates triangle-shape. Pectoral-fin rays i,6; unbranched ray covered with hypertrophied odontodes. Tip of adpressed pectoral fin almost reaching vertical through medial pelvic-fin unbranched ray. Pelvic-fin rays i,5; unbranched pelvic-fin ray reaching vertical through anal-fin base when adpressed. Anal-fin rays i,5. Caudal fin i,14,i, truncate; juveniles with elongated filaments on the unbranched caudal-fin rays. Dorsal plates 21(1), 22(1), 23(8)*, 24(9), 25(5), 26(3), 27(4); mid-dorsal plates 21(2), 22(2), 23(13)*, 24(14); median plates 21(1), 22(3), 23(8) 24(8)*, 25(10), 26(1); mid-ventral plates 21(2), 22(2), 23(10), 24(11)*, 25(5), 26(1); ventral plates 10(4), 11(10), 12(4)*, 13(3), 14(3), 15(3), 16(1), 17(1); supracaudal plates 5(8), 6(19), 7(3), 8(1)*.

Four to five procurrent rays positioned anterior to undivided caudal-fin rays. Nine bifid neural spines supporting the dorsal fin. Twenty-eight total vertebrae. Eleven vertebrae from the first simple neural spine. Reduced swimbladder capsule, not reaching the sixth vertebra ( Fig. 6). Seven to eight infraorbitals. Fourth infraorbital widely contacting posterior margin of orbit. Sixth infraorbital contributing only to the posteroventral margin of orbit. Lateral-line pores restricted to the hypural plate.

Coloration in alcohol. Similar to live specimens, but with less vivid or absent vermicular stains on the ventral and dorsal regions and fins. Body color light brown or dull gray. Ventral surface light gray or beige. Eyes dull gray.

Coloration in life. Dorsal surface with dark brown to black color pattern on body and fins. Light gray vermicular blotches all over the body. Ventral surface color pattern light gray or light brown, with pronounced vermicular blotches. Eyes completely black ( Fig. 7).

Sexual dimorphism. Adult males present more developed odontodes on the unbranched pectoral-fin ray, operculum and cheeks.

Geographical distribution. Pseudacanthicus nyktos is primarily found in the lower rio Xingu, ranging from Belo Monte ( Fig. 8) to near the junction with the rio Amazonas, near Porto de Moz (based on oral records from fishermen). However, ornamental fishermen have observed both adult and juvenile specimens of P. nyktos outside its native range, particularly in the rio Xingu region near Altamira. This expansion into new areas is likely due to anthropogenic influences, specifically aquarium trade companies’ release of unsold specimens over the past decade in Altamira City.

Ecological notes. Pseudacanthicus nyktos primarily inhabits the main channel of the river and is frequently associated with submerged logs. It can be caught at a depth of up to 20 m. Observation in captivity has demonstrated that the new species present a carnivorous habit that predominantly feeds on small aquatic invertebrates. Their territorial nature often leads to intraspecific aggression when they share the same environment. Notably, during the reproductive period, males typically confine females within burrows until egg-laying is complete (LMS, pers. obs.).

Etymology. The specific epithet nyktos derives from Greek meaning “night”, in allusion to the dark color of the specimens in comparison with other species of the genus and also to the deep locations where they can be found, where there is no light. An adjective.

Conservation status. Pseudacanthicus nyktos exhibits a distribution as reported by local fishermen within the lower rio Xingu region, an area situated beyond the impact zone of the Belo Monte Hydroelectric Power Plant. Populations of this species currently face no identified imminent threats, which would align with the Least Concern (LC) categorization, as per the criteria established by the International Union for Conservation of Nature (IUCN, 2022).

Remarks. Pseudacanthicus nyktos holds significant importance as a resource for local fishermen along the rio Xingu. These fishermen commonly refer to the species as “assacu-preto” (L-185), drawing an association with the Amazonian tree Hura crepitans , known locally as “assacu”, which features thorns on its trunk, resembling the conspicuous odontodes found in most Pseudacanthicus species. The capture of “assacu-preto” typically involves the use of air compressors and hoses that enable fishermen to breathe while diving. However, it’s important to highlight that safety procedures for such dives remain less than satisfactory.