Incilius spiculatus, (Mendelson, 1997) (Mendelson, 1997)

Arreortúa, Medardo, Flores, Carlos A., Simón-Salvador, Pablo Rogelio, Santiago-Dionicio, Hermes & González-Bernal, Edna, 2021, Description of the tadpole and natural history notes of Incilius spiculatus (Mendelson, 1997), an Endangered toad endemic to the Sierra Madre de Oaxaca, Mexico, Amphibian & Reptile Conservation (e 281) 15 (2), pp. 31-39 : 32-36

Results

Tadpole Description

Average measurements (mm) for Gosner stage 35: body length 13.28, tail length 21.68, tail muscle height 3.41, maximum tail height 6.09, total length 34.96, tail muscle width 3.31, internarial distance 1.66, interorbital distance 4.70; measurements for other Gosner stages are presented in Table 1 . The body is ovoid in dorsal view, widest at about the middle point and narrower near the tail; depressed in lateral view. Snout nearly semicircular in dorsal profile, rounded at the tip in lateral profile. Spiracle sinistral with inner wall free from body. External nares ovoid situated nearer to eyes than to snout. Eyes dorsal. Vent tube medial. Tail rounded at the tip. Caudal musculature highest at base, gradually tapering to a pointed tip; dorsal fin reticulated ( Fig. 1 ).

Oral disk small; labial tooth row formula 2(2)/3, A1 slightly longer than other rows; A2 gap narrow, approximately width of 3–8 teeth, P1 and P2 equal in length, P3 being the longest posterior row; labial papillae restricted to lateral portions of oral disc disposed in two interposed series ( Fig. 1A ). At stage 24, the larvae show an elongation of the oral apparatus that protrudes from the body ( Fig. 2D–E ).

In life, the color of the body is uniformly Cinnamon Brown (color 43 in Köhler 2012), except for the ventral part of the mouth where the color becomes paler. The venter is slightly transparent, with a counter-clockwise coiled intestine visible. Around Gosner stage 35, small Cream Yellow (82) dots appear throughout the body and the iris ( Fig. 2G ). The tail fins are transparent with large Cinnamon Brown (43) pigment granules forming a reticulate pattern on the dorsal fin. Around Gosner stage 37, the ventral parts of the limbs are pale Cinnamon Brown (43) with Cream Yellow (82) dots and dark brown bars dorsally. In preservative, the tadpole body and the tail musculature are Natal Brown (49), while the ventral part of the body is slightly translucent.

Tadpole development required approximately 35 days to complete metamorphosis under laboratory conditions ( Fig. 2H ). Three days after collection ( 8 February 2019), embryonic development reached Gosner stage 12. Three days later, the embryos had reached stage 18, and four days after that had developed into stage 25 tadpoles.

Adult Breeding Behavior

In February 2019, an amplecting pair of Incilius spiculatus was observed at 640 m asl in the shallower margins of a river (Río Coyul), where the water current was slowed by the presence of rocks and aquatic vegetation. The Río Coyul is a permanent river with an average width of 8.19 m at the site where the amplexus was observed ( Fig. 2F ).

The toads used vegetation and material on the bottom of the river to maintain their position in the water during the amplexus. Mating was observed during the day and the amplexus was axillary ( Fig. 2A–B ). At the time of the observations (1250 h GST), the water temperature was 19.6 °C. The species exhibited an ovipositional string mode with a double row of eggs arrangement ( Altig and McDiarmid 2007). The estimated clutch size was 4,500 eggs, and the clutch was attached to aquatic vegetation on rocks at the river margins at about 35 cm depth ( Fig. 2C ). Amplectant behavior was observed from the time of encounter until the toads separated (approximately 2.67 h). The female body temperature was 21.0 °C, SVL 85.5 mm, and the clutch temperature was 21.0 °C. In addition to the amplectant pair, a second male toad was observed in the water at a distance of 1.0 m. A second egg clutch was observed in another pool located 3.70 m away; but the toads that laid this clutch were not observed.

Adult Diet

Fecal samples indicated that Hymenoptera were the dominant prey (15 individuals, 48.3%), followed by Coleoptera (7, 22.5%), Scorpiones (6, 19.3%), Orthoptera (2, 6.4%), and Blattodea (1, 3.2%).

Extension of Elevational Range

Previously , the elevational range reported for I. spiculatus was from 800–1,689 m asl ( Mendelson 1997). During this survey, two juvenile individuals were observed at Santa Cruz Tepetotutla, Oaxaca, Mexico, in a patch of primary cloud forest vegetation at an elevation of 1,758 m asl ( 17.71862°N, - 96.55911°W) datum WGS84, in addition to three individuals in San Pedro Yolox at 682 m asl ( 17.63622°N, - 96.42735°W), 643 m asl ( 17.64001 °N, - 96.43061°W), and 642 m asl ( 17.64013°N, - 96.43056°W). With these records, the altitudinal range of this species is now extended as including from 642 to 1,758 m asl.

Key to the tadpoles of the forest toads of Mexico and Central America

1a. A2 Gap present.........................................................……………….………………………………………...........4

1b. A2 Gap absent………………...………………………………….................……...................................................2

2a. Tail fin coloration variable; known from Mexico...………......………..............................................................3

2b. Tail fins light brown with widely dispersed dark brown dots; known from western Costa Rica and Panama ………… ……………………………………….….....................................................................……............................ I. aucoinae

3a. Known from Sierra Madre Oriental of Veracruz and Puebla, Mexico ………………… I. cristatus

3b. Known only from Sierra de los Tuxtlas Veracruz, Mexico ………….…………............... I. cavifrons

4a. Tail fins transparent.............................................................................…….........................................................5

4b. Tail fins uniformly dark brown..............................................…………........................................ I. tutelarius

5a. Tail musculature black..........................................................……......................................................................6

5b. Tail musculature brown......................................................…….........................................................................7

6a. Tail musculature partially black with scattered pale areas. Known from southern Mexico to western Guatemala.........................................………………………………………………........... I. macrocristatus ; I. aurarius

6b. Tail musculature black. Tail fins reticulated and flecked with black. A-2 gap width about 2 labial teeth. Known only from north-eastern Honduras.…………………………………………………………………………...... I. leucomyos

7a. Dorsal fin has large Cinnamon Brown (43) granules forming a reticulation. A-2 gap width equal to

3–8 labial teeth. Known only from Sierra Madre de Oaxaca.........……………..................... I. spiculatus

7b. Dorsal and ventral fins with yellow reticulation. A-2 gap wide, width equal to 10–15 labial teeth. Widely distributed across southern Mexico and Central America........…………...................... I. valliceps

Figures

Fig. 1. Tadpole of Incilius spiculatus. (A) Oral disc at Gosner stage 37, (B) lateral view at Gosner stage 35, and (C) dorsal view at Gosner stage 35.

Fig. 2. Breeding behavior of Incilius spiculatus. (A–B) Amplexus (axillary type) and oviposition, (C) egg string staggered in unilayered tube, (D–E) dorsal and ventral views of the tadpole head at Gosner stage 24 showing the “elongated mouth,” (F) Rio Coyul, San Pedro Yolox; (G) lateral view of tadpole at Gosner stage 39, and (H) lateral view of metamorphic individual.