Ancistrus megacanthus, Widholzer & Borsoi & Reis & Lehmann A., 2024

Widholzer, Ronaldo L., Borsoi, Jessica, Reis, Roberto E. & Lehmann A., Pablo, 2024, A new species of Ancistrus (Siluriformes: Loricariidae), with a redescription of Ancistrus brevipinnis and further evidence of hidden diversity in the laguna dos Patos system, Brazil, Neotropical Ichthyology (e 230078) 22 (1), pp. 1-23 : 4-8

Ancistrus megacanthus , new species

urn:lsid:zoobank.org:act:6E583531-02DB-4557-8F3E-837D56B190D3

( Figs. 1‒2 ; Tab. 1 )

Holotype. MCP 19582 , male, 96.8 mm SL, Brazil, Rio Grande do Sul, São Gabriel, rio Vacacaí on road RS-630, tributary to rio Jacuí , laguna dos Patos system , 30°27’18”S 54°22’26”W, 14 Jan 1997, L. R. Malabarba, J. A. Gomes & V. A. Bertaco.

Paratypes. Brazil, Rio Grande do Sul, rio Vacacaí basin , laguna dos Patos system : MCP 54919 , 5, 47.7‒74.5 mm SL (all measured) + 1 c&s, 54.5 mm SL, collected with holotype. MCP 16294 , 6, 85.3‒124.4 mm SL (all measured), MZUSP 128046 , 1, 120.2 mm SL (measured), UFRGS 29411 , 1, 101.1 mm SL (measured), Caçapava do Sul, arroio Pessegueiro , ca. 30º28’S 53º37’W, 15 Apr 1993, A. Ramires. MCP 54883 , 1 (tissue sample), 39.4 mm SL (measured), São Gabriel, arroio Cambaizinho on road BR-290, 30°20’23”S 54°03’07”W, 5 Jul 2022, R. Widholzer & M. Haas. MCP 54898 , 2, 33.7‒44.1 mm SL (1 measured, 44.1 mm SL), São Gabriel, arroio Cambaizinho on road BR-290, 30°20’23”S 54°03’07”W, 4 Nov 2022, R. Widholzer & M. Haas .

Genseq-2 coI. Sequence deposited in GenBank ( MCP 54883 GenBank accession code OR250758).

Diagnosis. Ancistrus megacanthus differs from non-Andean congeners (except A. abilhoai Bifi, Pavanelli & Zawadzki, 2009 , A. agostinhoi Bifi, Pavanelli & Zawadzki, 2009 , A. brevifilis Eigenmann, 1920 , A. brevipinnis , A. cirrhosus (Valenciennes, 1836) , A. cuiabae Knaack, 1999 , A. hoplogenys (Günther, 1864) , A. kellerae de Souza, Taphorn & Armbruster, 2019 , A. leucostictus (Günther, 1864) , A. lithurgicus Eigenmann, 1912 , A. luzia Neuhaus, Britto, Birindelli & Sousa, 2022 , A. maximus de Oliveira, Zuanon, Zawadzki & Rapp Py-Daniel, 2015 , A. mullerae Bifi, Pavanelli & Zawadzki, 2009 , A. multispinis , A. nudiceps (Müller & Troschel, 1849) , A. patronus de Souza, Taphorn & Armbruster, 2019 , A. saudades de Souza, Taphorn & Armbruster, 2019 , A. taunayi , A. stigmaticus Eigenmann & Eigenmann, 1889 , A. trinitatis (Günther, 1864) , and A. yutajae de Souza, Taphorn & Armbruster, 2019 ) by adult males – those with welldeveloped snout tentacles – having the pectoral-fin spine long, exceeding half length of the pelvic-fin leading ray (vs. pectoral-fin spine short, not reaching half length of the pelvic-fin leading ray). It differs from those species (except A. yutajae ), and including all congeners from Rio Grande do Sul ( A. brevipinnis , A. multispinis , and A. taunayi ), by having dorsal and caudal fins plain dark brown to black, without lighter spots or bands (vs. lighter spot or bands on dorsal and caudal fins), and differs from A. brevipinnis and A. yutajae by having a dark brown to black background body color (vs. light brown body). Additionally, the new species is distinguished from A. taunayi by having small white to yellow dots on dorsal surface (vs. dorsal surface with light brown to pale yellow vermiculated spots). Ancistrus megacanthus is further distinguished from A. taunayi by having four branched rays on the anal fin (vs. three branched rays).

Description. Morphometric data in Tab. 1 . Body and head depressed. Dorsal profile of body and head convex from tip of snout to dorsal-fin origin, slightly concave to straight from that point to origin of adipose fin, concave from end of adipose fin to origin of caudal fin. Ventral profile of body straight, body height begins to decrease at pelvic girdle. Body covered with bony plates except along dorsal-fin base and ventral surface between tip of snout and anal-fin origin. Median series with 22‒24(13, 23*) lateral plates; lateral-line canal complete and uninterrupted. Three* predorsal plates, seven* plates at base of dorsal fin, four* plates between dorsal and adipose fins and 8*(6), 7(5) or 6(2) plates between adipose fins and caudal fins. Fin rays and body plates covered by small, caudally directed odontodes in regular rows, larger on fin spines.

Snout rounded, with wide bare anterior margin, posteriorly limited by dermal plates of varying sizes. Cutaneous tentacles present on lateral, anterior and middorsal portions of snout of males, sometimes branched and forming “Y” pattern. In females, tentacles smaller and usually arranged in pairs on sides of snout. Eye large 13.4‒22.1% (17) of head length, located laterodorsally on head. Orbit delimited dorsally by sphenotic and frontal, anteriorly by prefrontal plate, ventrally by infraorbitals IV and V, and posteriorly by infraorbital VI. Naris located dorsally on head, with tubular expansion of skin, delimited dorsally by frontal, anteriorly by infraorbital II and III, laterally by prefrontal plate and nasal bone. Interorbital region slightly convex to flat. Exposed part of operculum almost always triangular, rounded anteriorly. Dermal plates of different sizes and shapes arranged in post-opercular area, contiguous to compound pterotic. Nine to 12(13) strong retractable odontodes with sickle-shaped claws at extremity in opercular region.

Lower lip not reaching anterior margin of pectoral girdle. Lower lip with papillae randomly distributed throughout surface. Maxillary barbel short and free. Short mandibulary ramus, 12.6‒19.6% (17) of head length; premaxillary tooth row of same length or slightly shorter. Teeth numerous, 38‒67(17) in premaxilla and 32‒68(13) in dentary, bicuspid, with reduced lateral cusp and larger, wider mesial cusp.

Dorsal-fin spine elongated, soft rays almost reaching or reaching to adipose-fin origin when depressed; pectoral-fin spine long, exceeding half-length of pelvic-fin unbranched ray on adult males; reaching first third of pelvic fin in females. Pelvic fin reaching middle of anal-fin length. Caudal fin truncated, with lower leading ray longer than upper. All branched fin rays doubly branched near apical end. Dorsal fin with II+7(17) rays; pectoral fin I+6(17) rays; pelvic fin i+5(17) rays; anal fin i+4(17) rays; caudal fin i+14+I(17) rays.

Sixth vertebral centrum (first after Weberian apparatus) with enlarged rib. Seventh vertebra without rib and supporting first dorsal-fin pterygiophore. Eight following vertebral centra with thin ribs and supporting dorsal-fin pterygiophores. Total vertebrae 28(1).

Coloration in alcohol. Dorsal and ventral region of body dark brown to black. Well-defined white to yellowish white dots, usually smaller than pupil diameter, on dorsal and lateral region of body, sometimes in ventral region, usually not visible in poorly preserved specimens. Slightly lighter brown bar on terminus of caudal peduncle. Rays and interradial membranes of paired fins with dark coloration similar to body and no or inconspicuous light dots. Dorsal, adipose and caudal fins plain dark brown to black. Upper and lower tip of caudal-fin leading rays and outer branched rays with small light termination ( Fig. 1 ).

Coloration in life. Body dark brown to black with pale yellow dots, usually smaller than pupil diameter, along body; paired fins with yellow dots, dorsal fin with few inconspicuous light dots on unbranched ray, adipose and caudal fins unmarked. Lighter brown bar on terminus of caudal peduncle. Tip of upper and lower leading rays and outer branched rays of caudal fin with conspicuous yellow spots ( Fig. 2 ).

Sexual dimorphism. Adult males have more numerous and larger snout tentacle than females and larger pectoral-fin spines, usually exceeding half-length of the pelvic-fin unbranched ray and reaching the first third of pelvic fin in females.

Geographical distribution. Ancistrus megacanthus is known from three localities in the rio Vacacaí sub-basin of the rio Jacuí basin, laguna dos Patos hydrographic system ( Fig. 3 ).

Ecological notes. The new species occurs in creeks at altitudes close to 100 m above sea level, with swift waters and rocky to gravelly bottom ( Fig. 4 ). These water courses are in the Pampa biome, in a savannah formation with predominance of herbaceous vegetation and trees restricted to river banks, and with soils characterized by being clayey and gravelly, typical of the Precambrian Sul-Riograndense Shield terrains.

Etymology. Ancistrus megacanthus , latinized from Greek MEGAS (megas), large, great and AKANTHOS (akanthos), thorn, spine, in reference to the large pectoral-fin spines of the new species. A noun in apposition.

Conservation status. Ancistrus megacanthus is known from three localities in the rio Vacacaí drainage, with an Extension of Occurrence ( EOO) estimated by the maximum convex polygon around the Vacacaí-Mirim sub-basin of 11,177 km 2. Main diffuse threats in the area are the transformation of the lotic environment to lentic habitats (e.g., by the construction of dams) and the use of agricultural pesticides. As no specific threats to the species were detected, it is provisionally assessed as Least Concern ( LC) according to the International Union for Conservation of Nature ( IUCN) categories and criteria ( IUCN Standards and Petitions Subcommittee, 2022).

Figures

FIGURE 1 | Ancistrus megacanthus, dorsal, lateral and ventral views of holotype, MCP 19582, male, 96.8 mm SL, Brazil, Rio Grande do Sul, São Gabriel, rio Vacacaí on road RS-630, tributary to rio Jacuí, laguna dos Patos system.

FIGURE 2 | Life color pattern of Ancistrus megacanthus. Specimens not preserved. Hisonotus armatus and Hemiancistrus punctulatus in background.

FIGURE 3 | Distribution of Ancistrus species in northern laguna dos Patos system.Ancistrus aff.brevipinnis (yellow), A. brevipinnis (turquoise), and A. megacanthus (red). T = type-locality.

FIGURE 4 | Arroio Cambaizinho, showing typical habitat of Ancistrus megacanthus.