Hyloscirtus arcanus, Rivera-Correa & Ron & Nunes & Araujo-Vieira & Pinheiro & Grant, 2024

Rivera-Correa, Mauricio, Ron, Santiago, Nunes, Ivan, Araujo-Vieira, Katyuscia, Pinheiro, Paulo D. P. & Grant, Taran, 2024, Forty years later: a new Andean stream treefrog of the genus Hyloscirtus (Anura: Hylidae) from Ecuador, with comments on arm hypertrophy in the H. larinopygion group, Zootaxa 5474 (2), pp. 101-126 : 104-112

Hyloscirtus arcanus sp. nov.

( Figs. 2–4 )

Hyla lindae, Duellman & Hillis (1990: 16 ; table 3–4; fig. 7; appendix I; part)

Hyloscirtus lindae, Wiens et al. (2006 : supp. mat. fig A2: 16; 2010: 877); Pyron & Wiens (2011: 570)

Hyloscirtus “ lindae ”, Almendáriz et al. (2014: 37; table 1; fig. 2; part)

Hyloscirtus sp. , Ron et al. (2018: 101; table 1; fig. 1)

Holotype. KU 202730 , adult male, Ecuador, Provincia de Morona-Santiago, 23.3 km WSW Plan de Milagro , (- 3.2075, -78.5450; 2350 m a.s.l), collected on 12 March 1984 by David M. Hillis and Patricia A. Burrowes.

Paratypes. KU 202731 , adult male, collected with holotype . KU 202728 , adult male ; KU 202729 , adult female, Ecuador, Provincia de Morona-Santiago, 21.6 km SW Plan de Milagro , (-3.1907, -78.5414; 2425 m a.s.l), collected on 4 March 1984 by William E. Duellman and David M. Hillis .

Etymology. The specific epithet is derived from the Latin adjective arcanus , meaning “hidden” or “secret,” in reference to the fact that this species remained concealed in a museum and literature under an erroneous taxonomic identity for forty years.

Definition. We assign the new species to Hyloscirtus and the H. larinopygion group based on its phylogenetic placement ( Wiens et al. 2006, 2010; Pyron & Wiens 2011; Almendáriz et al. 2014; Ron et al. 2018), corroborated by our own phylogenetic analysis ( Fig. 1 ). Hyloscirtus arcanus sp. nov. can be distinguished from other species of the H. larinopygion group by the following combination of characters: dorsum and venter pale to dark gray, concealed surfaces of limbs bluish gray (in life); digital discs orange-red to bright-red (in life); iris dull olive bronze (in life); large, curved, spine-shaped prepollex; strongly hypertrophied forelimbs in males; large, thick, supracloacal flap and supratympanic fold; nuptial pad absent; vomerine odontophore includes two small processes in contact with each other and carrying four teeth each; finger webbing formula: II 12/3 – 22/3 III 2 + –2 IV, toe webbing formula: I 11/2 –2 - II 1 - –2 III 11/2 – 21/2 IV 21/2 –1 - V, and spicules on snout, canthus rostralis, eyelid, supratympanic fold, and shank in adult males.

Diagnosis. In the Hyloscirtus larinopygion group, only four species possess highly hypertrophied forelimbs in adult males and enlarged, curved, spine-shaped prepollex. These species are H. tapichalaca ( Kizirian et al. 2003) , H. condor (Almendáriz et al. 2014) , H. diabolus ( Rivera-Correa et al. 2016) , and H. hillisi ( Ron et al. 2018) . The remaining species in the H. larinopygion group, including H. lindae , lack these characters, exhibiting, instead, a blade-shaped prepollex and non-hypertrophied forelimbs ( Kizirian et al. 2003; Rivera-Correa & Faivovich 2013; Rivera-Correa et al. 2016; Pinheiro et al. 2022; Reyes-Puig et al. 2022; Sánchez-Nivicela et al. 2023). Hyloscirtus arcanus sp. nov. resembles H. lindae in having red disks on fingers and toes. In addition to forearm morphology, males of both species differ in shank dorsal texture, being smooth in H. lindae and spiculate in H. arcanus sp. nov. ( Fig. 2 ). Hyloscirtus arcanus sp. nov. differs from species of the southern clade in the following characteristics described below.

Hyloscirtus tapichalaca : axillary and inguinal regions with pink-orange marks (absent in H. arcanus sp. nov.), thighs and shanks with brown transverse bands (absent in H. arcanus sp. nov.), concealed surfaces of limbs brown (concealed surfaces of limbs bluish gray in H. arcanus sp. nov.), lateral and lower margin of cloaca knee, elbow, and outer margin of forearm white (white absent on cloaca and limbs in H. arcanus sp. nov.), digital discs white (orange-red to bright-red digital discs in life or greyish-orange in preserved specimens of H. arcanus sp. nov.), iris yellow-gold with fine black reticulations (dull olive-bronze in H. arcanus sp. nov.), vomerine odontophore processes separate (contacting medially in H. arcanus sp. nov.), dorsal spicules absent (present in H. arcanus sp. nov.), gular and abdominal region dark grey (pale grey in H. arcanus sp. nov.), and calcar short and triangular (transverse fringe in H. arcanus sp. nov.).

Hyloscirtus condor : dorsum dark brown with contrasting yellow spots (gray without spots in H. arcanus sp. nov.), digital discs light brown, yellow-brown, or brown with abundant yellow blotches (orange-red to bright-red digital in H. arcanus sp. nov.), iris golden with fine tan reticulations (dull olive-bronze in H. arcanus sp. nov.), vomerine odontophore processes separate (contacting medially in H. arcanus sp. nov.), dorsal spicules in males absent (present in H. arcanus sp. nov.).

Hyloscirtus diabolus : dorsum dark brown with abundant small yellow marks or yellow blotches (gray without spots in H. arcanus sp. nov.), concealed surfaces of thighs and shanks dark brown (bluish-gray in H. arcanus sp. nov.), digital discs dark brown, with or without yellow spots (orange-red to bright-red digital discs of H. arcanus sp. nov.), iris dark red (dull olive-bronze in H. arcanus sp. nov.), vomerine odontophore processes separate (contacting medially in H. arcanus sp. nov.), and dorsal spicules in males absent (present in H. arcanus sp. nov.).

Hyloscirtus hillisi : dorsum dark brown with contrasting round orange marks (gray without spots in H. arcanus sp. nov.), digital discs dark brown (orange-red to bright-red in H. arcanus sp. nov.), iris bronze or yellowish with brown reticulations (dull olive-bronze in H. arcanus sp. nov.), fleshy calcar absent (present in H. arcanus sp. nov.).

Description of holotype. Adult male, 64.9 mm SVL ( Fig. 3 ). Body robust. Head 16% wider than long, as wide as body; head width 35% of SVL; head length 27% of SVL. Snout rounded in dorsal view, truncate in profile ( Fig. 4A–B ); canthus rostralis rounded, indistinct; loreal region concave; lips rounded, faintly flared; nostrils weakly protuberant, directed anterolaterad, positioned at the level of anterior margin of the lower jaw. Dorsal surface of internarial region concave. Interorbital distance slightly larger than upper eyelid. Eye prominent, diameter larger than eye-nostril distance. Tympanum and tympanic annulus inconspicuous. Supratympanic fold prominent, extending from behind eye and to posterior edge of arm insertion. Region between head and suprascapulae depressed. Vomerine teeth in two short and massive processes, in contact medially, located behind choanae; each series with four prominent teeth. Choanae small, ovoid, separated by more than 3.5 times their maximum diameter. Tongue cordiform, attached along entire length except narrow free area around postero-lateral margin. Vocal slits longitudinal, originating lateral to tongue, extending to the corner of the mouth. Vocal sac large, single, median, subgular, evident externally. Mental gland undetectable.

Upper arm and forearm hypertrophied; axillary membrane absent; thick ulnar fold present. Fingers short, thick, bearing large, ovoid discs with circumferential grooves clearly defined by the difference in size between disc and pad; width of finger III disc 53% of eye diameter. Relative lengths of fingers 1 <2 <4 <3. Fingers with dermal fringes, webbing thick, conspicuous; webbing formula II 2 - – 22/3 III 21/3 –2 IV ( Fig. 4C ). Distal subarticular tubercles large, single, rounded in ventral view, conical in profile. Palmar surfaces with deep folds. Supernumerary tubercle at the base of palm large, high, and round. Outer metacarpal tubercle well defined, thick, elliptical, flat. Inner metacarpal tubercle large, following outline of underlying enlarged, protruding spine-shaped, distal prepollex. Nuptial pad absent.

Hindlimbs robust; tibia length 48% of SVL; foot length 45% of SVL. Calcar forming transverse fringe; tarsal fold absent, but tubercles extending along the outer edge of tarsus; inner tarsal fold present. Inner metatarsal tubercle large, ovoid; outer metatarsal inconspicuous. Toes short, lacking lateral fringes, toe discs smaller than those of fingers, slightly wider than digit shaft. Relative length of toes 1 <2 <5 <3 <4; toe webbing formula: I 11/2 –2 II 11/2 –2 + III 11/2 – 21/2 IV 21/2 –2 - V ( Fig. 4D ). Subarticular tubercles large, rounded in ventral view, conical in profile; single rows of smaller supernumerary tubercles along axis of each toe. Cloacal opening directed posteriorly at upper level of thighs; supracloacal flap large, thick; edges of vent with numerous small folds; cloacal sheath short. Dorsal skin, gular region, pectoral region, and flanks smooth except for abundant spicules on the loreal region, eyelids, posterior dorsum, and tibia; belly and proximal portion of ventral surface of inner thigh granular.

Color of holotype in life. Dorsal surfaces and flanks dark gray, ventral surfaces pale gray. Concealed surfaces of limbs bluish gray; digital discs orange-red. Iris dull olive-bronze.

Color of holotype in preservative. Dorsum and flanks grayish purple, ventral surfaces pale gray. Concealed surfaces of limbs gray; toe webbing cream gray, digital discs orange pink.

Measurements of the holotype (mm). SVL 64.9; HL 17.7; HW 21.0; ED 7.4; EN 4.0; NSD 3.0; IND 4.7; AMD 10.5; FAL 10.7; FAW 9.6; HAL 22.4; THL 31.0; TL 31.4; TAL 16.7; FL 29.4; TFD 4.0; FTD 3.6.

Variation and sexual dimorphism. The female (KU 202729) is slightly larger than the males, and although her forelimbs are robust, they do not present the remarkable hypertrophy observed in the three males (FAW/FAL 0.45 in female, 0.78–0.88 in the three males; Fig. 5 ). The prepollex is an osseous spine-shaped element in both sexes, although it is slightly smaller in the female (inner metacarpal tubercle length/FAL 0.59 in female, 0.70–0.72 in the three males). The tympanum is partially visible in the female, being covered dorsally by the supra-tympanic fold, and is rounded and inclined medially towards the transversal body axis such that it is almost visible from above; its diameter is 44% of eye diameter; in males, the tympanum is inconspicuous. In life, the dorsum of males was dark gray and dull grayish brown in female. The belly of the female was dark gray, light gray in males. The color on the tips of the digits varied from reddish-orange to bright red (tomato red), covering almost the entire disc (see male KU 202731; Fig. 6 ). The males have keratinized spicules on the snout, canthus rostralis, eyelid, supratympanic fold, dorsum, and near the cloaca and shanks; spicules are absent in the only female. The males have subtly more extensive webbing than the female; the webbing formula varies as follows: II (2–2 -)–(21/2 – 22/3) III (21/3 – 21/2)–(2 + –2 -) IV / I (2–2 +)–(2 + –2 -) II 11/2 –(2 + –2 -) III (11/2 –2 -)–(2– 21/2) IV (2– 21/2)–(11/2 –2 -) V. Other secondary sexual characters (i.e., vocal slits, vocal sac) are absent in the female. Morphometric measurements of the paratypes are given in Table 1 .

Prepollex anatomy. The prepollex of adult male KU 202728 is entirely osseous and consists of two elements, a short, cuboid, proximal element and a distal, enlarged, spine-shaped element ( Fig. 7A ). The distal element has a small dorsomedial crest that anchors one of the insertions of the m. abductor pollicis longus and lacks a postarticular process. Metacarpal II articulates with both proximal and distal elements of the prepollex without a medial expansion of its proximal epiphysis.

Ventrally, the prepollex serves as point of insertion for three muscles: the m. adductor pollicis, m. flexor indicis brevis profundus, and m. pronator quadratus. The m. adductor pollicis originates on the ventral surface of distal carpal 3-4-5 and inserts ventrally via a fleshy attachment on the distal prepollex. The m. flexor indicis brevis profundus also originates on distal carpal 3-4-5, with most fibers inserted ventromedially on metacarpal II and some fibers inserting laterally on the distal prepollex. The m. pronator quadratus has two origins, one on the ulnar side of the radioulna, distally and ventrally, and the other on the ulnare. Some of the fibers from both origins converge to insert ventrally via a tendon on the proximal portion of metacarpal II. The remaining radioulna fibers insert directly (ventrally and proximally) on the prepollical elements and deeply on element Y; the ulnare fibers insert proximally on the proximal portion of the prepollex. Thick, white connective tissue binds both the m. adductor pollicis and m. pronator quadratus together ventrally near the prepollex.

Dorsally, the prepollex also interacts with three muscles: the m. abductor pollicis longus, m. extensor indicis brevis medius, and m. abductor indicis brevis dorsalis. The m. abductor pollicis longus has three slips, one with a broad, fleshy origin from across almost the entire length of radioulna, another, narrower, originating distally from the distal epicondyle of the humerus via a broad but thin tendon, and a third robust slip with a fleshy origin from the lateral crest of the humerus, proximal to the second slip’s origin. The slip from the radioulna inserts on both metacarpal II, at its midlength dorsomedially, and dorsally on the portion of the prepollex; both slips insert via a common, broad tendon. The two slips from the humerus join the same tendon of insertion on the portion that attaches to the prepollex ( Fig. 7A ). The m. extensor indicis brevis medius originates on the radiale and inserts dorsomedially on metacarpal II, proximal to the m. abductor pollicis longus insertion, dorsolaterally on the distal prepollex. The m. abductor indicis brevis dorsalis originates on element Y and inserts dorsally on the distal prepollex via a fleshy attachment between the insertions of the m. abductor pollicis longus and m. extensor indicis brevis medius. The m. dorsometacarpalis indicis proximalis lacks the slip originating from the distal prepollex.

Distribution and natural history. Hyloscirtus arcanus sp. nov. is known from two nearby localities on the eastern Andean slopes of southern Ecuador (Provincia Morona Santiago; Fig. 8 ; airline distance between localities <2 km). The elevation is 2,350 –2,425 m a.s.l. Specimens KU 202728–29 were found at night on 4 March 1984 on tree branches in the spray zone of a waterfall. The male was calling from a cavity beneath moss; the call was a whistle with a three-note pattern and a low-pitched. The female was on the branch of a bush. An additional calling male (not collected) was observed higher up the same waterfall. The vegetation type (according to the classification of Sierra et al. 1999) is Cloud Montane Forest of the eastern Andes. The tadpole is unknown. The new species occurs less than 10 km from populations of H. pacha ( Duellman and Hillis, 1990) , a species of the northern clade ( Duellman & Hillis 1990).

The skin on the tip of the prepollex presents scars in the males but not in the female, suggesting its effective usage only by males. Probably, during combats and/or amplexus the prepollex pierces the encapsulating skin that covers it. However, as observed in the males KU 202728 and KU 202731, the female KU 202729 also has scars on the dorsum, suggesting either combat or a complex breeding behavior. The oviducts of the female are full of large, unpigmented oocytes (oocyte diameter 2.28–2.97 mm; n = 3; Fig. 7B ).

Figures

FIGURE 1. Phylogenetic relationships of Hyloscirtus as recovered in one of the 92 most parsimonious trees obtained from the analysis of 12S, tRNAVal, and 16S rRNA gene fragments with gaps as fifth state, equal weights for all transformations, and zero-length branches collapsed. Black circles indicate nodes that collapse in the strict consensus. Numbers above nodes are parsimony jackknife support values. An asterisk (*) indicates groups with 100% for parsimony jackknife frequencies. Nodes lacking values have <50% jackknife support; all are retained in the strict consensus.

FIGURE 2. Dorsal view of right leg of (A) holotype of Hyloscirtus arcanus sp. nov. (KU 202730) and (B) paratopotype of H. lindae (KU 155475), both males. Notice the presence of scattered spiculae on the shank in (A), absent in (B). Scale bars = 5 mm.

FIGURE 3. Dorsal and ventral view of the holotype of Hyloscirtus arcanus sp. nov. (KU 202730; adult male, SVL 64.9 mm).

FIGURE 4. Drawings of Hyloscirtus arcanus sp. nov. (KU 202730, holotype). (A) Head in lateral view (B) head in dorsal view; (C) left hand in ventral view; (D) left foot in ventral view. Scale bars = 10 mm.

FIGURE 5. Paratype series of Hyloscirtus arcanus sp. nov.. On top dorsal view, on the bottom ventral view. From left to right: female KU 202729, males KU 202728 and KU 202731. Scale bars = 20 mm.

FIGURE 6. Hyloscirtus arcanus sp. nov. in life. (KU 202731, paratype, adult male, SVL 67.8 mm). Photo: D. Hillis (1984).

FIGURE 7. A. Dorsal view of left forearm of Hyloscirtus arcanus sp. nov. adult male KU 202728 with the three slips of the m. abductor pollicis longus highlighted. White highlights tendons; maroon highlights the slip originating via fleshy attachments on the radioulna; teal highlights the slip originating via a tendon on the distal epicondyle of humerus; olive highlights the slip originating via a fleshy attachment on the cristal lateralis of humerus. The following muscles were previously removed: m. extensor communis, m. extensor carpi ulnaris, m. epicondylo-cubitalis, and m. epitrochleo-cubitalis. Scale bar = 3 mm. B. Ventral cut on abdomen of Hyloscirtus arcanus sp. nov. female specimen (KU 202729) revealing the large and mature oviducal oocytes, with unpigmented animal pole. Scale bar = 2 mm.

FIGURE 8. Map showing currently known localities for species of the southern clade of the Hyloscirtus larinopygion group, including Hyloscirtus arcanus sp. nov. (the species with the northernmost distribution of the clade).