Corydoras thanatos, Tencatt & Ohara & Sousa & Britto, 2022

Tencatt, Luiz Fernando Caserta, Ohara, Willian Massaharu, Sousa, Leandro Melo de & Britto, Marcelo Ribeiro de, 2022, Science and hobby joining forces for the discovery of three new Corydoras (Siluriformes: Callichthyidae) from the rio Tapajós basin, Brazil, with comments on Corydoras sp. CW 111, Neotropical Ichthyology (e 220070) 20 (3), pp. 1-40 : 4-15

Corydoras thanatos , new species urn:lsid:zoobank.org:act:3DD6AB86-416E-4C6C-B16C-015042A2A013

( Figs. 1-3 ; Tab. 1)

Corydoras CW 146. —Lucanus, 2021:29 [AMAZONAS magazine, guide about Corydoras from Serra do Cachimbo ].

Holotype. MNRJ 53287 , 33.2 mm SL, Brazil, Pará State , Novo Progresso Municipality , stream with unknown name tributary to the rio Jamanxim, rio Tapajós basin, 08°23’06”S 55°19’43”W, 7 Jul 2019, M. R. Britto, W. M. Ohara and L. F. C. Tencatt.

Paratypes. All from Brazil, Pará State, Novo Progresso Municipality, rio Jamanxim basin, rio Tapajós drainage, collected by M . R. Britto, W. M. Ohara and L. F. C. Tencatt . CITL 382 , 32 , 12.6–18.5 mm SL, rio Jamanxim , 08°23’01”S 55°19’08”W, 7–8 Jul 2019 . INPA 59776 , 2 , 12.9–14.2 mm SL; NUP 23540, 1, 33.2 mm SL, stream with unknown name, 08°23’33”S 55°22’36”W, 7–8 Jul 2019 . CITL 383 , 7 of 9, 15.0–37.0 mm SL, 2 cs of 9, 30.0– 33.8 mm SL ; INPA 59777 , 3 , 29.0– 33.3 mm SL ; INPA 59778 , 1 , 21.9 mm SL ; MZUSP 126860 , 7 , 21.5–32.3 mm SL, collected with the holotype .

Diagnosis. Corydoras thanatos can be distinguished from its congeners, except for the species from the lineages 6, 7 and 9 sensu Alexandrou et al. (2011) plus C. difluviatilis Britto & Castro, 2002 , C. hastatus Eigenmann & Eigenmann, 1888 , and C. pygmaeus Knaack, 1966 , by having posterior margin of pectoral-fin spine with most serrations directed towards the tip of the spine (vs. most serrations directed towards origin of spine); from C. difluviatilis and C. pygmaeus it differs by the presence of contact between nuchal plate and posterior process of the parieto-supraoccipital (vs. absence of contact between nuchal plate and the posterior process of the parieto-supraoccipital); from C. hastatus by the absence of a large-sized dark blotch on caudal-fin base (vs. caudal-fin base region with a dark brown or black large blotch, roughly diamond-shaped or rhomboid, or arrow-shaped, typically bordered anteriorly and posteriorly by conspicuously light areas); it can be distinguished from the species within lineage 7 by having dark brown or black markings on fins, except for the pelvic fin (vs. all fins devoid of dark markings); it differs from the species within lineages 6 and 9, except for C. coppenamesis Nijssen, 1970 , C. lymnades Tencatt, Vera-Alcaraz, Britto & Pavanelli, 2013 , C. garbei Ihering, 1911 , and C. gossei Nijssen, 1972 , by having anterior laminar expansion of infraorbital 1 strongly well developed, conspicuously expanded towards the anteroventral portion of snout, significantly covering its lateral surface (vs. anterior laminar expansion of infraorbital 1 ranging from poorly to well developed, not conspicuously expanded towards the anteroventral portion of snout, leaving most of its lateral surface exposed); the new species differs from C. gossei by presenting dark brown or black markings on head (vs. presence of pale yellow to white blotches on head); from C. lymnades by having a robust body (vs. slender); from C. coppenamesis , C. lymnades , and C. garbei by having flank midline covered by small-sized dark brown or black blotches; flank midline variably with distinct longitudinal series of blotches, generally more evident on posterior half of flank (vs. flank midline with longitudinal dark brown or black stripe in C. coppenamesis ; with a distinct series of longitudinally aligned moderate- to large-sized dark brown or black blotches in C. lymnades and C. garbei ).

Description. Morphometric data in Tab. 1. Head laterally compressed with convex dorsal profile, roughly triangular in dorsal view. Snout short, rounded. Head profile convex from tip of snout to anterior nares, ascending nearly straight or slightly convex from this point to dorsal-fin origin; interorbital region slightly concave in some specimens. Profile slightly convex along dorsal-fin base. Postdorsal-fin body profile slightly concave to adipose-fin spine, concave from this point to caudal-fin base. Ventral profile of body nearly straight or slightly convex from isthmus to pectoral girdle, and slightly convex from this point until pelvic girdle. Profile nearly straight or slightly convex from pelvic girdle to base of first anal-fin ray, ascending abruptly concave until caudal-fin base. Body roughly elliptical in cross section at pectoral girdle, gradually becoming more compressed toward caudal fin.

Eye rounded, located dorsolaterally on head. Orbit delimited anteriorly by lateral ethmoid, anterodorsally by frontal, posterodorsally by sphenotic, posteroventrally by infraorbital 2, and anteroventrally by infraorbital 1. Anterior and posterior nares close to each other, only separated by flap of skin. Anterior naris tubular. Posterior naris close to anterodorsal margin of orbit, separated from it by distance similar to naris diameter. Mouth small, subterminal, width similar to bony orbit diameter. Maxillary barbel moderate in size, not reaching anteroventral limit of gill opening. Outer mental barbel slightly longer than maxillary barbel. Inner mental barbel fleshy, base of each counterpart slightly separated from each other. Small rounded papillae covering entire surface of all barbels, upper and lower lips, snout and isthmus.

Mesethmoid moderate in size, with anterior tip poorly developed, smaller than 50% of bone length (see Britto, 2003:123, character 1, state 1; fig. 1B); posterior portion wide, partially exposed and bearing small odontodes. Nasal capsule delimited posteriorly and dorsally by frontal, anteriorly by mesethmoid, and ventrally and posteriorly by lateral ethmoid. Nasal relatively wide, laterally curved, inner margin with relatively well-developed laminar expansion contacting frontal and mesethmoid; outer margin with poorly-developed laminar expansion typically contacting lateral ethmoid. Lateral ethmoid moderately expanded anteriorly, with anterodorsal expansion contacting nasal, and anterior margin contacting posterior portion of mesethmoid. Frontal elongated, narrow, width less than half of entire length; anterior projection short, size clearly smaller than nasal length. Frontal fontanel large, slender, and somewhat ellipsoid; posterior tip extension slightly surpassing anterior margin of parieto-supraoccipital. Sphenotic somewhat trapezoid, contacting parieto-supraoccipital dorsally, pterotic-extrascapular posteriorly, second infraorbital posteroventrally and frontal anteriorly ( Fig. 2A ). Pterotic-extrascapular roughly pipe-shaped, with posteriormost portion contacting first lateral-line ossicle, posteroventral margin contacting cleithrum, and anteroventral margin contacting opercle and variably infraorbital 2; posterior expansion almost entirely covering lateral opening of swimbladder capsule, leaving slender area on its dorsal margin covered only by thick layer of skin. Parieto-supraoccipital wide, posterior process long and contacting nuchal plate; region of contact between posterior process and nuchal plate covered by thick layer of skin.

Two laminar infraorbitals with minute odontodes. Infraorbital 1 conspicuously large, ventral laminar expansion generally strongly well developed; some specimens with well-developed expansion; anterior portion with strongly well-developed laminar expansion, surpassing anterior margin of nasal capsule; inner laminar expansion moderately developed ( Fig. 2A ). Infraorbital 2 small, relatively slender, with posterior laminar expansion ranging from moderately to well developed; posteroventral margin contacting posterodorsal ridge of hyomandibula, posterodorsal edge contacting sphenotic and generally pterotic-extrascapular; posterodorsal edge not in contact with pterotic-extrascapular in some specimens; inner laminar expansion ranging from poorly- to moderately developed ( Fig. 2A ). Posterodorsal ridge of hyomandibula close to its articulation with opercle relatively slender, exposed, and bearing small odontodes. Dorsal ridge of hyomandibula between pterotic-extrascapular and opercle exposed and bearing odontodes. Interopercle partially covered by thick layer of skin, with posterior portion exposed and bearing odontodes; subtriangular, anterior projection ranging from moderately to well developed. Preopercle elongated, relatively slender; minute odontodes on external surface. Opercle dorsoventrally elongated; relatively compact in shape, with width equal to or slightly larger than half of its entire length; free margin slightly convex, without serrations and covered by small odontodes.

Four branchiostegal rays decreasing in size posteriorly. Hypobranchial 1 deep; hypobranchial 2 somewhat triangular, tip ossified and directed towards anterior portion, posterior margin cartilaginous; ossified portion well developed, its size about twice cartilaginous portion. Five ceratobranchials with expansions increasing posteriorly; ceratobranchial 1 with small process on anterior margin of mesial portion; ceratobranchial 3 with continuous laminar expansion on postero-lateral margin; ceratobranchial 5 toothed on posterodorsal surface, with 42 to 45(2) teeth aligned in one row. Four epibranchials with similar size; epibranchial 2 slightly larger than others, with small pointed process on laminar expansion of posterior margin; epibranchial 3 with mesially-curved uncinate process on laminar expansion of posterior margin. Two wide pharyngobranchials (3 and 4); pharyngobranchial 3 with small roughly triangular laminar expansion on posterior margin; rounded expansion in some specimens. Upper tooth plate roughly oval, 48 to 58(2) teeth aligned in two rows on posteroventral surface; rows closely aligned.

Lateral-line canal reaching cephalic laterosensory system through pterotic-extrascapular, branching twice before reaching sphenotic: pterotic branch, with single pore, preoperculomandibular branch conspicuously reduced, with single pore opening at postotic main canal; postotic main canal widens just posterior to pterotic branch. Sensory canal continuing through pterotic-extrascapula, reaching sphenotic as temporal canal, which splits into two branches: one branch giving rise to infraorbital canal, other branch connecting to frontal through supraorbital canal, both with single pore. Supraorbital canal branched, running through nasal bone. Epiphyseal branch conspicuously reduced; pore opening close to supraorbital main canal, directed towards frontal fontanel. Nasal canal with three openings, first on posterior edge, second on posterolateral portion and generally fused with first pore, and third on anterior edge. Infraorbital canal running through entire infraorbital 2, extending to infraorbital 1 and opening into two or three pores. Preoperculomandibular branch giving rise to preoperculo-mandibular canal, which runs through entire preopercle with three openings, leading to pores 3, 4, and 5, respectively.

Dorsal fin subtriangular, located just posterior to second or third dorsolateral body plate. Dorsal-fin rays II,8*(20), posterior margin of dorsal-fin spine with 20 to 24 ranging from strongly reduced to poorly-developed serrations; most serrations directed towards tip of spine; some serrations variably perpendicularly directed; serrations absent close to origin of spine; small odontodes on anterior and lateral surfaces of spine ( Fig. 2B ). Nuchal plate moderately developed, almost entirely exposed, with minute odontodes. Spinelet short; spine typically well developed, with adpressed distal tip surpassing posterior origin of dorsal-fin base. Pectoral fin roughly triangular, its origin just posterior to gill opening. Pectoral-fin rays I,6,i(1), I,7*(3), I,7,i(12), I,8(3) or I,9(1), posterior margin of pectoral spine with 25 to 28 poorly- to moderately-developed serrations along almost its entire length, absent close to origin of spine; most serrations directed towards tip of spine; some serrations perpendicularly directed; small odontodes on anterior, dorsal and ventral surfaces of spine ( Fig. 2C ). Anteroventral portion of cleithrum exposed; posterolateral portion of scapulocoracoid moderately developed, exposed, with anterior portion slightly expanded anteriorly, not in contact with anteroventral portion of cleithrum; exposed areas bearing small odontodes. Opening of axillary gland sensu Kiehl et al. (2006) located just posterior to pectoral-fin spine base. Pelvic fin oblong, located just below first or second ventrolateral body plate, and at vertical through dorsal-fin spine or first branched dorsal-fin ray. Pelvic-fin rays i,5*(20). Adipose fin roughly triangular, separated from base of last dorsal-fin ray by six or seven dorsolateral body plates. Anal fin subtriangular, located just posterior to 12 th or 13 th ventrolateral body plates, and at vertical through adipose-fin spine base or region of preadipose platelets. Anal-fin rays i,4,ii(1), ii,5(16), i,6*(2) or ii,5,i(1). Caudal fin bilobed, with dorsal and ventral lobes similar in size or dorsal lobe slightly larger than ventral lobe. Caudal-fin rays i,11,i(1) or i,12,i*(19), generally four or five dorsal and ventral procurrent rays.

Two laterosensory canals on trunk; first ossicle tubular, second ossicle laminar, both bearing small odontodes. Body plates with minute odontodes scattered over exposed area, with conspicuous line of odontodes confined to posterior margins. Dorsolateral body plates 22(1) or 23*(19). Ventrolateral body plates 20*(17) or 21(3). Dorsolateral body plates along dorsal-fin base 6*(20). Dorsolateral body plates between adipose- and caudal-fin 6(1), 7*(16) or 8(3). Preadipose platelets 2(9) or 3*(11). Ventral surface of trunk between posteroventral margin of cleithrum and pelvic-fin origin laterally delimited only by first ventrolateral body plate; ventral portion of first ventrolateral body plate ranging from slightly to moderately expanded anteriorly. Small platelets covering base of caudal-fin rays. Small platelets disposed dorsally and ventrally between junctions of lateral plates on posterior portion of caudal peduncle. Anterior margin of orbit, above region of junction between frontal and lateral ethmoid, region around nasal capsule, on region above lateral ethmoid, and dorsal, lateral and variably ventrolateral portions of snout with small- to relatively large-sized platelets bearing odontodes; platelets on snout conspicuously more concentrated above mesethmoid. Ventral surface of trunk with numerous small- to relatively large-sized irregular platelets bearing odontodes; region around pectoral-fin origin typically with larger platelets.

Vertebral count 21(2); ribs 5(2); first pair conspicuously large, its middle portion closely connected to first ventrolateral body plate; its tip connected to anterior external process of basipterygium. Parapophysis of complex vertebra well developed.

Color in alcohol. Overall color of body in Fig. 1 . Ground color of body paleto brownish yellow or beige. Top of head dark brown; dark area typically extending mesially towards posterior tip of parieto-supraoccipital as thin longitudinal line. Dorsal and lateral surface of head with conspicuous concentrations of dark brown or black chromatophores, forming rounded, irregular or striated, diffuse blotches in some specimens. Lateral surface of cleithrum with conspicuous concentrations of dark brown or black chromatophores, forming rounded, irregular or vermiculated, blotches in some specimens; blotches, when present, diffuse or slightly more marked than blotches on head. Dorsolateral body plates with conspicuous concentration of dark brown or black chromatophores, typically forming rounded, irregular or vertically elongated conspicuous dark blotches; diffuse in some specimens. Ventrolateral body plates, except for region around pelvic-fin origin, with conspicuous concentration of dark brown or black chromatophores, typically forming rounded, irregular or vertically elongated conspicuous dark blotches; diffuse in some specimens; region around pelvic-fin origin with dark brown or black chromatophores but typically not forming any conspicuous pattern; ventral portion of ventrolateral body plates between pelvic-fin origin to area just anterior to anal-fin origin variably with sparse dark brown or black chromatophores. Flank midline variably with distinct longitudinal series of dark brown or black blotches, generally more evident on posterior half of flank. Blotches on flanks ranging from small to moderate in size. Posterior margin of body plates typically with dark brown or black chromatophores, variably forming thin dark lines along border of plates. Dorsal-fin with conspicuous concentrations of dark brown or black chromatophores, forming small dark blotches; blotches roughly longitudinally or obliquely aligned in some specimens, variably fused with each other and forming stripes; some specimens with less evident blotches on posterior portion of fin. Pectoral fin with conspicuous concentrations of dark brown or black chromatophores, forming small, irregular dark spots; spots roughly transversally aligned in some specimens; variably diffuse. Pelvic fin with sparse dark brown or black chromatophores, not forming spots. Adipose fin with dark brown or black chromatophores, generally more numerous and concentrated on spine and on region of membrane close to spine, typically forming single dark patch. Anal fin with conspicuous concentrations of dark brown or black chromatophores, generally more evident along its base, middle portion, and/or posterior portion, forming dark blotches roughly aligned transversally; blotches diffuse and/or not aligned in some specimens. Caudal fin with conspicuous concentrations of dark brown or black chromatophores, mainly on rays, forming dark blotches roughly aligned transversally in three to six slender bars; some specimens with less evident blotches on posterior portion of fin.

Color in life. Similar to color pattern of preserved specimens, but with light ground color of body, and with greenish yellow iridescent coloration ( Figs. 3–4 ).

Sexual dimorphism. As well-documented in Corydoradinae (see Nijssen, Isbrücker, 1980b; Britto, 2003; Spadella et al., 2017), male specimens of C. thanatos present a genital papilla, which is somewhat tubular in shape. Aquarium male specimens eventually display a conspicuous elongation of the first and second dorsal-fin branched rays ( Fig. 4B ).

Geographical distribution. Corydoras thanatos is currently known from the main channel of the rio Jamanxim and two of its tributaries, both with unknown names, rio

Tapajós basin, Novo Progresso Municipality, Pará State, Brazil ( Fig. 5 ).

Ecological notes. The new species was observed to inhabit from main channel of the rio Jamanxim to its small tributary streams ( Fig. 6 ). In the rio Jamanxim, it was mostly associated with the margins of the river, where juvenile specimens were observed shoaling together with juvenile and adult specimens of C. hypnos in the sandy shores, both species in relatively high abundance. In the small streams, adult and juvenile specimens were found shoaling together with juvenile and adult specimens of C. hypnos , generally in sites with submerged branches, leaf litter, and sandy and/or fine gravel substrate. In these streams, C. thanatos was found in higher numbers than C. hypnos . Although not shoaling together, the new species was found in syntopy with Corydoras sp. CW 171 in these streams. In all sampled sites, the new species was mostly observed and captured at night.

Etymology. The specific epithet “ thanatos ” refers to Thanatos (from the Greek θᾸ́νᾸΤος, thánatos, which means “death”), the Greek god or personification of death, and the twin brother of Hypnos, the god/personification of sleep. The name makes an allusion to the fact that even though both C. thanatos and C. hypnos may present some similarities (in color pattern), they are completely different in other aspects (general morphological pattern). A noun in apposition.

Conservation status. Corydoras thanatos is currently known from its type locality and two additional records in its surroundings. However , these records are in the border of a conservation unit, the Floresta Nacional do Jamanxim , and no threat to the species as a whole is currently suspected. According to the International Union for Conservation of Nature ( IUCN) categories and criteria (IUCN Standards and Petitions Subcommittee, 2019), Corydoras thanatos can be classified as Least Concern (LC).

Figures

FIGURE 1 | Corydoras thanatos, holotype, MNRJ 53287, 33.2 mm SL, Novo Progresso Municipality, Pará State, Brazil, stream with unknown name tributary to the rio Jamanxim, rio Tapajós basin.

FIGURE 2 | Lateral view of the head (A), lateral view of the dorsal-fin spine (B) and dorsal view of the left pectoral-fin spine (C) in cs paratype of Corydoras thanatos, CITL 383, 33.8 mm SL. Abbreviations: f: frontal, io1–2: infraorbital 1 and 2, iop: interopercle, n: nasal, op: opercle, pes: pterotic-extrascapular, pop: preopercle, prh: posterodorsal ridge of hyomandibula, pso: parieto-supraoccipital, sph: sphenotic. Scale bars = 1 mm.

FIGURE 3 | Paratypes of Corydoras thanatos photographed alive, showing general color pattern and morphology in lateral view in (A) INPA 59776, 14.2 mm SL, (B) INPA 59778, 21.9 mm SL, (C) INPA 59777, 29.0 mm SL, (D) INPA 59777, 30.0 mm SL, and (E) INPA 59777, 33.3 mm SL, all from tributaries to the rio Jamanxim, rio Tapajós basin.

FIGURE 4 | Uncatalogued aquarium specimens of Corydoras thanatos photographed alive, showing general color pattern and morphology of (A) a female specimen, and (B) a dimorphic male specimen. Photos by Hans Evers.

FIGURE 5 | Map of the rio Tapajós and adjoining areas, showing the geographical distribution of Corydoras thanatos (black circle), C. hypnos (white star), and C. psamathos (red squares). Each symbol may represent more than one locality.

FIGURE 6 | Natural habitat of Corydoras thanatos and C. hypnos, showing (A) the rio Jamanxim, type locality of C. hypnos, and one of its tributaries (B), type locality of C. thanatos, rio Tapajós basin in Pará State, Brazil.