Glass frogs

(Centrolenidae)

Class Amphibia

Order Anura

Family Centrolenidae

Thumbnail description
Mostly diminutive green frogs with toe pads, large eyes directed forward, and transparent ventral skin

Size
0.7–3.2 in (18–81 mm)

Number of genera, species
3 genera; 133 species

Habitat
Humid tropical forest

Conservation status
Not threatened

Distribution
Mexico, Central America, and South America

Evolution and systematics

There are no known fossils of these small, fragile frogs; thus, all systematic studies have relied solely on extant species. Centrolenidae is a monophyletic lineage, and as early as 1973, Lynch considered the Centrolenidae to be phylogenetically close to the neobatrachian families of Pseudidae and Hylidae based on the presence of intercalary elements in the digits. In 1993 Linda Ford and David Cannatella noted that this feature is found in other families as well and that further study would be needed to determine if this condition arose independently in centrolenids. Molecular genetic data have yet to be brought to bear on the phylogenetic position of the Centrolenidae. No subfamilies are recognized.

Three genera are recognized within Centrolenidae. Centrolene, with 39 species, is characterized by the presence of a humeral spine in males. Hyalinobatrachium, literally translated as "glass frog," with 33 species, is characterized by the possession of a bulbous liver. Cochranella, with 61 species, lacks the diagnostic features of the other genera. It is thought that many new glass frogs will be discovered as poorly studied areas are explored and new molecular genetic techniques are used in the systematic study of these frogs.

Physical characteristics

The family is characterized by having the two elongate ankle bones (astragulus and calcaneum) fused, a medial process on the third metacarpal bone in the hand, T-shaped terminal phalanges, an intercalary element between the penultimate and terminal phalanges, and deposition of eggs on leaves or rocks above streams. Most centrolids are small, at just 0.7–1.2 in (18–30 mm) in length, but one species, Centrolene geckoideum, is a relative giant, reaching 3.2 in (81 mm). Aside from this exceptional species, glass frogs have slender, fragile bodies with long, thin limbs and webbed feet. The digits of these excellent climbers terminate in enlarged toe pads resembling those of tree frogs (Hylidae). In dorsal view, the head is round, with large, protruding eyes set more dorsally than in most frogs.

Most glass frogs are a shade of green, varying from pale lime green to dark forest green. A few species, such as Cochranella igonta, are brown or tan. Species may lack any pattern at all, but most possess pale yellow or white spots or even multicolored dots, termed ocelli. The ventral surfaces and frequently the peritoneum (membrane enclosing internal body cavities) of these frogs are transparent, so that the internal organs and

bones can easily be seen in the living frogs. It is through this transparent skin that one also can see that the bones of centrolenid frogs are either white or green. Unique pigment cells in the skin reflect infrared radiation, the same wavelength of light reflected by plants but invisible to the human eye. This pigmentation is thought to camouflage glass frogs when they sit on leaves, thus protecting them from pit vipers and birds.

Distribution

Glass frogs occur from southern Mexico to Bolivia, east to northeastern Argentina and southeastern Brazil. The genus

Hyalinobatrachium is most diverse in Central America, whereas Cochranella and Centrolene are more speciose in South America. Many species of glass frogs are endemic to the Andean valleys and thus have restricted distributions.

Habitat

Glass frogs are associated almost exclusively with vegetation along and above streams, predominantly in montane cloud forest. Tadpoles inhabit slow-moving portions of forested streams after hatching from eggs deposited on leafy vegetation above the water.

Behavior

Glass frogs are nocturnally active with diurnal retreats. Males of some species are known to engage in combat for calling or egg-laying sites by wrestling for prime breeding spots. Several species exhibit parental care in the form of egg attendance by one or the other parent, who sits near or directly on the clutch of eggs.

Feeding ecology and diet

Little is known of the diet of glass frogs. They eat primarily insects, although the large Centrolene geckoideum has been known to consume small vertebrates.

Reproductive biology

In habitats without seasonal variations, glass frog breeding is continuous throughout the year, whereas in seasonal climates breeding is tied closely to the rainy season. Breeding occurs under the darkness of night either during rains or, in cloud forest species, during evening mists from clouds. Males call from selected sites on leaves overhanging forest streams. Most calls are a high-pitched series of whistles. Females are attracted to calling males, who have secured suitable sites for calling and egg deposition. Males of some species are known to defend these sites by aggressively posturing with stiffened limbs to challenge intruding males. In some species, such as Cochranella griffithsi, males physically battle with other males for these calling and egg-deposition sites. Such wrestling matches involve grappling until one individual looses his grip and falls. The humeral spines on males of the genus Centrolene are thought to be used as weapons in such territorial disputes, because males sometimes are found with scars on the head and body.

In nearly all species for which reproduction is known, axillary amplexus and egg deposition take place at or near these won breeding sites. The eggs are deposited on leaf surfaces, either above or below, depending on the species. One remarkable exception is the giant Centrolene geckoideum, which deposits masses of eggs on rock faces in splash zones of rapids and waterfalls. Adults, either male or female, are known to attend these eggs, often sitting directly on top of them. By night or day, this behavior is thought to protect eggs from invertebrate predators and desiccation. Occasionally, adult Centrolene geckoideum attend multiple clutches at varying degrees of developmental maturity at the same location. Larvae hatch and fall into the streams, which, for Andean species, may be rushing torrents. Tadpoles settle in the substrate of eddies or other slow-moving portions of the stream. Tadpoles in oxygen-poor microhabitats often are colored bright red as a result of blood flowing close to the surface of their unpigmented skin. There is no known parental care of the larvae.

Conservation status

There is no official conservation status for any species of glass frog, but many species have restricted distributions that make them vulnerable to extinction.

Significance to humans

Glass frogs have had little direct use by humans; however, they have aesthetic value and are potential indicators of overall ecosystem health, especially in tropical montane stream ecosystems.

Species accounts

List of Species

Pacific giant glass frog

Centrolene geckoideum

taxonomy

Centrolene geckoideum Jiménez de la Espada, 1872, las riberas del rio Napo en el Ecuador.

other common names

None known.

physical characteristics

In this species the males are larger than the females. Males grow to 2.8–3.2 in (70.2–80.7 mm) and females to 2.4–2.9 in (60.7–72.9 mm) in snout-vent length. This largest centrolenid has relatively small eyes, heavily webbed digits, and large, rectangular-shaped toe pads. Males have large, muscular fore-arms and a long, sharply pointed bony spine on the humerus. The dorsum is lime green to dark forest green. The skin is tuberculate, with some small, scattered white flecks; in males the tubercles are finely spiculate. The bones are green.

distribution

This species ranges through Andean Ecuador and Colombia at elevations of 5,740–9,840 ft (1,750–3,000 m).

habitat

The habitat of the Pacific giant glass frog is upper montane cloud forest along swiftly flowing, shaded streams with numerous waterfalls.

behavior

The Pacific giant glass frog is nocturnal and uses rock faces or leaves as diurnal retreats. At night, males call from splash zones behind waterfalls or on boulders in torrents. There have been no direct observations, but it is hypothesized that these frogs,

like some other centrolids, may be territorial, battling for prime calling and oviposition (egg-laying) sites. Adult males found in the field in Colombia had numerous scars on the face, head, and flanks, which may have been the result of battles between males using their sharp, bony humeral spines.

feeding ecology and diet

This large centrolid feeds on a variety of insects and also ingests frogs and fish.

reproductive biology

Male Pacific giant glass frogs call at night throughout the year within splash zones behind waterfalls or on boulders in torrents. The call is a loud, high-pitched, trilled whistle, 155–373 milliseconds in duration and produced at intervals of 1.48–5.05 min, with emphasized frequencies of 3,468–4,187 Hz. The calls lack consistent amplitude modulation; this may be related to the din of the rushing water, which would obliterate any subtle characteristics in the calls. Tadpoles are elongate and slender with low caudal fins and eyes positioned dorsally. The oral disc has thin jaw sheaths.

conservation status

Not threatened.

significance to humans

None known.

Pichincha glass frog

Centrolene heloderma

taxonomy

Centrolenella heloderma Duellman, 1981, Quebrada Zapadores, 3.1 mi (5 km) east-southeast of Chiriboga, Provincia de Pichincha, Ecuador.

other common names

None known.

physical characteristics

Males are 1.1–1.2 in (26.8–31.5 mm), and females are 1.3 in (32.3 mm) in snout-vent length. This moderately large centrolenid has small eyes. Males have a blunt humeral spine. The toes are about four-fifths webbed, and the digits have expanded toe pads. This species has unique tuberculate skin on the dorsum. The dorsum is dark forest green with bluish white tubercles and a yellow margin on the lip. The bones are green.

distribution

This species inhabits cloud forest on the Pacific slopes of the Andes in Colombia and Ecuador at elevations of 6,430–7,870 ft (1,960–2,400 m).

habitat

The Pichincha glass frog inhabits streams in the upper limits of montane cloud forest.

behavior

Not known.

feeding ecology and diet

Not known.

reproductive biology

Little is known of the reproductive biology of this frog. The call is a harsh peep made from the upper surfaces of leaves and ferns 3.3–13.1 ft (1–4 m) above streams on cliff faces below seepages.

conservation status

Not threatened.

significance to humans

None known.

Nicaragua glass frog

Centrolene prosoblepon

taxonomy

Hyla prosoblepon Boettger, 1892, Plantage Cairo (La Junta) near Limon, Costa Rica.

other common names

None known.

physical characteristics

Males are 0.9–1.1 in (21.7–28.1 mm), and females are 1.0–1.1 in (25.4–27.8 mm) in snout-vent length. The dorsum is green with or without black dots. The tips of the digits are pale yellow, and the chest is white. The skin is shagreen on the dorsum, and the belly and thighs are granular. Males possess a pointed humeral spine. The bones are green.

distribution

This species occurs in Nicaragua, Costa Rica, Panama, and the Pacific slopes of Colombia and Ecuador at elevations of 328–4,921 ft (100–1,500 m).

habitat

The Nicaragua glass frog inhabits vegetation associated with cascading streams. Tadpoles occupy the bottom of silt-bottomed pools in streams.

behavior

Aggressive behavior takes place between calling males. One or both frogs dangle upside down while holding vegetation with their hind legs. The males grapple with each other until one drops off or flattens his body against the leaf.

feeding ecology and diet

Not known.

reproductive biology

Breeding is coincident with significant rainfall. The call consists of three short beeps with a pitch of 5,300–6,000 Hz at a frequency of one to 43 calls per hour. Calls are made from the tops of leaves over streams. Males are not territorial and initiate amplexus with the female. Egg deposition can occur some distance from the calling site at heights of 0–10 ft (0–3 m) above the ground, usually on the top side of leaves but also on moss-covered rocks and branches. The average clutch of 20 black eggs is attended during the first night by the female, who lies motionless on top of the clutch. Males call vigorously during amplexus and immediately after egg deposition. Tadpoles are elongate and slender, with low caudal fins and eyes positioned dorsally. The oral disc has thin jaw sheaths and a labial tooth row formula of 2 (1)/3.

conservation status

Not threatened.

significance to humans

None known.

Ecuador Cochran frog

Cochranella griffithsi

taxonomy

Cochranella griffithsi Goin, 1961, Río Saloya, Ecuador.

other common names

None known.

physical characteristics

Males are 0.8–1.9 in (19.7–26.1 mm), and females are 0.8–1.0 in (20.0–24.8 mm) in snout-vent length. The dorsum is pale yellowish green, with or without dark green flecks. The tips of the digits are pale yellow, and the chest is white. The skin is shargreen on the dorsal surfaces, granular on the belly and posterior surfaces of the thighs, and smooth on other areas. The bones are pale green.

distribution

The Ecuador Cochran frog occurs on the Pacific slopes of the Andes in southern Colombia and adjacent Ecuador at elevations of 3,940–8,700 ft (1,200–2,650 m).

habitat

The species inhabits cloud forest.

behavior

Males call from leaves of herbs and bushes over cascading streams by night and seek out retreats in such places as the axils of elephant ear (Colocasia esculenta) plants by day. Aggressive behavior among males is associated with breeding and territoriality. Competing males grasp each other in a belly-to-belly fashion. While hanging from vegetation by the hind limbs, the combatants wrap their forelimbs about each other's neck. In this position the frogs repeatedly flex and extend their outstretched hind limbs so as to move their bodies up and down while swinging laterally.

feeding ecology and diet

Not known.

reproductive biology

Males make calls from vegetation over streams. Eggs are laid on the tips of leaves overhanging streams, into which hatchling tadpoles drop and complete their development.

conservation status

Not threatened.

significance to humans

None known.

Lynch's Cochran frog

Cochranella ignota

taxonomy

Centrolenella ignota Lynch, 1990, Peñas Blancas, Farallones de Cali, 3.7 mi (6 km) by the road southwest of Pichindé, Valle de Cauca, Colombia.

other common names

None known.

physical characteristics

Males are 0.9–1.0 in (22.3–25.4 mm), and females are 1 in (24.2–24.4 mm) in snout-vent length. The dorsum is tan-brown to pale olive, with black ocelli with orange or yellow centers. The skin is smooth and has elevated flat, white warts. The head is rounded in dorsal view, and the protruding eyes are directed anteriorly. The toes are about one-half webbed, with enlarged toe pads. The bones are pale green.

distribution

This species occurs in the western Andes of Colombia at elevations of 6,230–6,430 ft (1,900–1,960 m).

habitat

Lynch's Cochran frog inhabits montane cloud forest streams.

behavior

Not known.

feeding ecology and diet

Not known.

reproductive biology

The advertisement call is a series of chirps, which males make from vegetation over streams.

conservation status

Not threatened.

significance to humans

None known.

Spotted Cochran frog

Cochranella ocellata

taxonomy

Hylella ocellata Boulenger, 1918, Huancabamba, eastern Peru.

other common names

None known.

physical characteristics

Males are 0.8–1.0 in (21.0–25.1 mm), and females are 1.1 in (29 mm) in snout-vent length. The dorsum is dark green with large, dark-edged, pale bluish white ocelli. The dorsal skin is shagreen. The bones are green.

distribution

This species lives on the Amazonian slopes of the Andes in Peru at elevations of 5,350–5,580 ft (1,630–1,700 m).

habitat

The spotted Cochran frog inhabits cloud forest.

behavior

Not known.

feeding ecology and diet

Not known.

reproductive biology

Not known.

conservation status

Not threatened.

significance to humans

None known.

Atrato glass frog

Hyalinobatrachium aureoguttatum

taxonomy

Centrolenella aureoguttatum Barrera-Rodrigues and Ruiz-Carranza, 1989, Chocó, Colombia, 14 mi (23 km) carretera El Carmen-Quibdo.

other common names

None known.

physical characteristics

Males are 0.8–0.9 in (20.4–23.3 mm), and females are 0.9 in (22.9–23.9 mm) in snout-vent length. The dorsum is yellow-green with scattered large brown chromatophores (pigment cells). Two to five large yellow spots free of the brown chromatophores are prominent dorsally. The bones are white.

distribution

This species ranges across the western slopes of the Andes in Colombia at elevations of 150–5,120 ft (45–1,560 m).

habitat

These frogs are active at night on vegetation 3.3–22.9 ft (1–7m) above rapidly flowing streams with abundant canopy and high local humidity.

behavior

Not known.

feeding ecology and diet

Not known.

reproductive biology

The call is unknown, but these frogs engage in axillary amplexus. Clutches of 25–35 clear green eggs are deposited in a translucent gelatinous mass on the undersides of leaves, usually Heliconia. Parental care is provided in the form of egg attendance within 2 in (5 cm) of the clutch or directly upon it.

conservation status

Not threatened.

significance to humans

None known.

Fleischmann's glass frog

Hyalinobatrachium fleischmanni

taxonomy

Hylella fleischmanni Boettger, 1893, San José, Costa Rica.

other common names

English: Northern glass frog; Spanish: Ranita de vientre transparente.

physical characteristics

Males are 0.8–1.0 in (19.2–25.5 mm) in snout-vent length. The dorsum is pale green with pale yellow or yellowish green spots and darker green reticulations. The belly is white, and the tips of the digits are yellow. The bones are white.

distribution

This is the most widespread species of glass frog; it ranges from Guerrero and Veracruz, Mexico, through Central America to Colombia, Venezuela, Guyana, and Surinam at elevations of 200–4,790 ft (60–1,460 m).

habitat

This species inhabits vegetation near moderate to fast-flowing streams at low elevations.

behavior

Males exhibit territorial behavior by aggressively defending calling and oviposition sites. Both males and females are known to attend the developing egg clutches by sitting on the eggs during the night and sleeping near but not directly on the eggs during the day.

feeding ecology and diet

Not known.

reproductive biology

The advertisement call of males, made from either the upper or lower surfaces of leaves overhanging streams, is a single untrilled "wheet" that is repeated after a short pause. Males aggressively defend calling and oviposition sites, and successful males may engage in many matings. Females choose a mate and initiate amplexus. Clutches of 18–30 eggs are deposited on the undersides of leaves directly over streams. Sometimes females, but usually males, attend clutches by sitting on eggs at night and near them but not on them by day. Fruit flies (Drosophila melanogaster) deposit eggs on clutches of Fleischmann's glass frog, and the maggots of the fruit fly develop in the clutches and consume the eggs and embryos, resulting in extremely high mortality rates. As many as 80% of clutches may be destroyed by biotic or abiotic factors. Tadpoles are elongate and slender, with low caudal fins and eyes positioned dorsally. The oral disc has thin jaw sheaths and a labial tooth row formula of 2 (1)/3. The tadpoles appear bright red as a result of blood flowing beneath the skin.

conservation status

Not threatened.

significance to humans

None known.

La Palma glass frog

Hyalinobatrachium valerioi

taxonomy

Centrolene valerioi Dunn, 1931, La Palma, Costa Rica.

other common names

None known.

physical characteristics

Males are 0.8–1.0 in (20.8–26.0 mm) in snout-vent length. The yellowish background with a bold reticulated pattern of green and dark flecks gives the appearance of prominent large yellow spots on the dorsum of this frog. The texture of the dorsum is smooth and that of the belly and thighs is rugose (wrinkled). The bones are white.

distribution

This species ranges across central Costa Rica to the Pacific slopes of Ecuador.

habitat

Not known.

behavior

Little is known aside from its reproductive biology.

feeding ecology and diet

Not known.

reproductive biology

The advertisement call is a single short "seet" that is repeated after a pause. Males of this species provide 24-hour parental care to clutches on leaves, more than that known for any other glass frog. Although diurnal attendance increases survivorship of eggs and unhatched larvae, it exposes the males to predation. The uncanny resemblance between the color pattern of the adult male frog and the appearance of an egg clutch on a leaf led to the suggestion that the patterning is a co-evolutionary adaptation to this increased diurnal predation risk.

conservation status

Not threatened.

significance to humans

None known.

Resources

Books

McDiarmid, Roy W. "Evolution of Parental Care in Frogs." In The Development of Behavior: Comparative and Evolutionary Aspects, edited by G. M. Burghardt and M. Bekoff. New York: Garland Press, 1978.

Periodicals

Grant, Taran, Wilmar Bolivar, and Fernando Castro. "The Advertisement Call of Centrolene geckoideum." Journal of Herpetology 32, no. 3 (1998): 452–455.

Greer, Beverly J., and Kentwood D. Wells. "Territorial and Reproductive Behavior of the Tropical American Frog Centrolenella fleischmanni." Herpetologica 36, no. 4 (1980): 318–326.

Jacobson, Susan K. "Reproductive Behavior and Male Mating Success in Two Species of Glass Frogs (Centrolenidae)." Herpetologica 41, no. 4 (1985): 396–404.

Lynch, John D., and William E. Duellman. "A Review of the Centrolenid Frogs of Ecuador, with Descriptions of New Species." Occasional Papers of the Museum of Natural History, University of Kansas 16 (1973): 1–66.

Rueda-Almonacid, José Vicente. "Estudio Anatomico y Relaciones Sistematicas de Centrolene geckoideum (Salienta: Anura: Centrolenidae)." Trianea 5 (1994): 133–187.

Ruiz-Carranza, Pedro M., and John D. Lynch. "Ranas Centrolenidae de Colombia. I. Propuesta de Una Nueva Clasificación Genérica." Lozania 57 (1991): 1–30.

——. "Ranas Centrolenidae de Colombia. IX. Dos Nuevas Especies del Suroeste de Colombia." Lozania 68 (1996): 1–11.

Villa, Jaime. "Biology of a Neotropical Glass Frog Centrolenella fleischmanni (Boettger), with Special Reference to Its Frogfly Associates." Milwaukee Public Museum Contributions in Biology and Geology 55 (1984): 1–60.

Erik R. Wild, PhD