Stereocyclops histrio, (Carvalho, 1954) (Carvalho, 1954)

Mira-Mendes, Caio Vinicius De, Neto, Edvaldo Moreira Da Silva, Dias, Iuri Ribeiro, Costa, Renan Nunes, Campos, Yhasmynn Pensee Pinheiro, Solé, Mirco & Weber, Luiz Norberto, 2025, The tadpole of Stereocyclops histrio (Carvalho, 1954) (Anura: Microhylidae), a poorly known amphibian from northeastern Brazil, Zootaxa 5636 (1), pp. 193-197 : 194-196

External morphology .

Morphological measurements are shown in Table 1 . Body triangular/depressed in lateral view (BH/BW = 0.72–0.75%) ( Fig. 1A ), short (BW/BL = 0.75–0.81%) and slightly elliptical in dorsal and ventral view ( Fig. 1 BC). Snout slightly sloping in lateral view and semicircular in dorsal view ( Figs. 1 AB). Eyes small (ED/BH = 0.17%), located laterally. Nostrils absent. Spiracle ventral, medial, intermediate size (SL/BL = 0.14–0.27%) and wide (SW/BH = 0.23–0.44%), located in posterior third of body, directed posteriorly, with wide opening ( Fig. 1D ). Vent tube medial, directed posteroventrally and concealed by the spiracle ( Fig. 1E ). Tail length corresponding to 0.55–0.63% of TL, of intermediate height (MTH/TAL = 0.38–0.46%), presenting the greatest height in its posterior third. Tail with a thick tissue from the base to almost mid-length in all specimens analyzed. Caudal musculature slightly robust (TMH/BH = 0.47–0.50%), of intermediate width (TMW/BW 0.29–0.34%). Dorsal fin originated at body-tail junction, ventral fin originated at level of vent tube. Fins with slightly convex margins, tapering in its final third. Dorsal fin of intermediate height (DFH/TAL = 0.14%); ventral fin higher (VFH/TAL = 0.17–0.19%; DFH/VFH = 0.73–0.84%). Oral apparatus anterior ( Fig. 1F ), of intermediate size (ODW/BW = 0.28–0.32%), without keratinized parts, marginal and submarginal papillae absent. A pair of smooth-edged semicircular flaps (folds) pending lateral to the oral opening, separated by a medial inverted U-shaped notch.

Coloration. In formalin, specimens present a general background color pattern of dark brown. In lateral view, dark brown irregular blotches, and small or large white irregular blotches; larger white blotches commonly from ventral middle third of body to body-tail junction ( Fig. 1A ). In dorsal view, body color pattern of dark brown ( Fig. 1B ). Venter light brown, with irregular spots close to the eye, in the anterior third of body; two lateromedial elliptical white blotches on the anterior third of the body, reaching larger white blotches in the posterior third ( Fig. 1C ). Musculature of the tail in lateral view shows a light beige background color, with dark brown strips on the upper and lower portions of the musculature up to the middle third of the tail. In dorsal view, dark brown tail musculature up to the middle third of the tail, and light beige from the middle third to the tip. Dorsal and ventral fins light brown background, with a translucent portion on the dorsal fin above the tail musculature, and irregular translucent spots on the ventral fin. Iris black with narrow translucent ring around pupil.

Lateral line system. Indistinct in life, and barely visible in preserved material ( Fig. 2A–C ). All lines present transversally oriented, elliptical or circular, whitish stitches. Supraorbital line short, beginning close to the snout tip and ending at the level of the eyes. Posterior supraorbital line in part evident from dorsal view, positioned behind the eye. Infraorbital line extending from the snout tip to the region below the eye. A short dorsal body line along the margin of the thick tissue of the tail, from the tail base to about mid-length of the tissue.

Variation. In lateral view, the end of the tail can be pointed (n = 12) or rounded (n = 8). In dorsal view, the body can vary from slightly elliptical (n = 15) to circular (n = 5). In lateral and ventral view, irregular dark brown spots and irregular white spots may vary in arrangement between individuals.

Tadpoles of Sterocyclops histrio share several features with congeneric species and with other gastrophrynine microhylids, including lateral eyes, upper lip with pending flaps, spiracle near the vent tube, vent tube free from the ventral fin, and nasolacrimal groove and lower lip papilla absent (Donelly et al. 1990). Stereocyclops tadpoles can be distinguished by size and tail coloration. At comparable stages, tadpoles of both S. incrassatus and S. parkeri are larger (ca. 28.5–48.3 mm and 38.2–41.7 mm respectively) than those of S. histrio , and whereas S. histrio larvae exhibit an overall beige uniform coloration, those of S. parkeri and S. incrassatus have respectively, a distinct white band along more than half the tail musculature length, and thin light band ( Dubeux et al. 2020; Wogel et al. 2000; Mira-Mendes pers. obs.). Lateral lines are visible in tadpoles of S. incrassatus (Mira-Mendes pers. obs.), but the lack of a detailed description prevents a direct comparison of this trait among species within the genus.

Figures

FIGURE 1. Stereocyclops histrio tadpole at Stage 38. (A) Lateral, (B) dorsal, (C) ventral view; scale bars = 10.0 mm; details of (D) spiracle (SP) in lateral view; (E) vent tube (VT) in ventral view; and (F) oral apparatus; scale bars = 1.0 mm.

FIGURE 2. Lateral line system of Stereocyclops histrio tadpole at Stage 36. (A) Dorsal and (B) lateral views of the body; (C) lateral view of the tail. Scale bars = 1 mm. SO Supraorbital line; PSO Posterior supraorbital line; IO Infraorbital line; D Dorsal body line.