The glass frogs belong to the amphibianfamilyCentrolenidae (orderAnura), native to the Central American Rainforests. The general background coloration of most glass frogs is primarily lime green, the abdominal skin of some members of this family is transparent and translucent, giving the glass frog its common name. The internal viscera, including the heart, liver, and gastrointestinal tract, are visible through the skin. When active their blood makes them visible; when sleeping most of the blood is concealed in the liver, hiding them. Glass frogs are arboreal, living mainly in trees, feeding on small insects and only coming out for mating season. Their transparency conceals them very effectively when sleeping on a green leaf, as they habitually do.[1] However, climate change and habitat fragmentation has been threatening the survival rates of the family.
The first described species of Centrolenidae was the "giant" Centrolene geckoideum, named by Marcos Jiménez de la Espada in 1872, based on a specimen collected in northeastern Ecuador. Several species were described in subsequent years by different herpetologists (including G. A. Boulenger, G. K. Noble, and E. H. Taylor), but usually placed together with the tree frogs in the genera Hylella or Hyla.
The family Centrolenidae was proposed by Edward H. Taylor in 1945. Between the 1950s and 1970s, most species of glass frogs were known from Central America, particularly from Costa Rica and Panama, where Taylor, Julia F., and Jay M. Savage extensively worked, and just a few species were known to occur in South America. In 1973, John D. Lynch and William E. Duellman published a large revision of the glass frogs from Ecuador, showing the species richness of Centrolenidae was particularly concentrated in the Andes. Later contributions by authors such as Juan Rivero, Savage, William Duellman, John D. Lynch, Pedro Ruiz-Carranza, and José Ayarzagüena increased the number of described taxa, especially from Central America, Venezuela, Colombia, Ecuador, and Peru.
The evolutionary relationships, biogeography, and character evolution of centrolenidae were discussed by Guayasamin et al. (2008[2]) Glass frogs originated in South America and dispersed multiple times into Central America. Character evolution seems to be complex, with multiple gains and/or losses of humeral spines, reduced hand webbing, and complete ventral transparency. Researched by Santiago (2009)[3], evolution and speciation on glass frogs has shown that ecological gradient and isolation have a role in speciation and divergence in glass frogs by comparing glass frogs Mitochondrial DNA. Glass frogs have expanded from the Guiana shield to other rainforest and diversified further. They evolved to be able to survive and fit in with their environment.
The taxonomical classification of the glass frogs has been problematic. In 1991, after a major revision of the species and taxonomic characters, the herpetologists Pedro Ruiz-Carranza and John D. Lynch published a proposal for a taxonomic classification of the Centrolenidae based on cladistic principles and defining monophyletic groups.[4] That paper was the first of a series of contributions dealing with the glass frogs from Colombia that led them to describe almost 50 species of glass frogs. The genus Centrolene was proposed to include the species with a humeral spine in adult males, and the genus Hyalinobatrachium to include the species with a bulbous liver.[4] However, they left a heterogeneous group of species in the genus Cochranella, defined just by lacking a humeral spine and a bulbous liver.[4] Since the publication of the extensive revision of the Colombian glass frogs, several other publications have dealt with the glass frogs from Venezuela, Costa Rica, and Ecuador.
In 2006, the genus Nymphargus was erected[5] for the species with basal webbing among outer fingers (part of the previous Cochranella ocellata species group).
Four genera (Centrolene, Cochranella, Hyalinobatrachium, Nymphargus) have been shown to be poly- or paraphyletic[2] and recently a new taxonomy has been proposed (see below).
Classification
The family Centrolenidae is a clade of anurans. Previously, the family was considered closely related to the family Hylidae; however, recent phylogenetic studies[6] have placed them (and their sister taxon, the family Allophrynidae) closer to the family Leptodactylidae.
The monophyly of Centrolenidae is supported by morphological and behavioral characters, including: 1) presence of a dilated process on the medial side of the third metacarpal (an apparently unique synapomorphy); 2) ventral origin of the musculus flexor teres digiti III relative to the musculus transversi metacarpi I; 3) terminal phalanges T-shaped; 4) exotroph, lotic, burrower/fossorial tadpoles with a vermiform body and dorsal C-shaped eyes, that live buried within leaf packs in still or flowing water systems; and 5) eggs clutches deposited outside of water on vegetation or rocks above still or flowing water systems. Several molecular synapomorphies also support the monophyly of the clade.[6]
The taxonomic classification of the Centrolenidae was recently modified. The family now contains two subfamilies and 12 genera.[7]
The evolutionary advantage of a partly clear skin and an opaque back was a mystery, as it did not seem to be effective as camouflage. It was found that the colour of the frog's body changed little against darker or lighter foliage, but the legs were more translucent and consequently changed in brightness. By resting with the translucent legs surrounding the body, the frog's edge appears softer, with less brightness gradient from the leaf to the legs and from the legs to the body, making the outline less noticeable. This camouflage phenomenon, in which the frog's edges are softened to match the relative brightness of its surroundings, is referred to as edge diffusion.[8] Herpetology researchers study the pros and cons of transparency in glass frogs, it was established that the transparency offers more than regular color changes in the skin itself through limited pigments. Experiments with computer-generated images and gelatine models of opaque and translucent frogs found that the translucent frogs were less visible, and were attacked by birds significantly less often.[9] Photographs of the frogs were taken both at nighttime and during the day, results showed little to no visibility of the frogs on any leaves in the daytime or nighttime. It was found in 2022 that these frogs have the ability to conceal red blood cells concentrated inside their livers, increasing transparency when they are vulnerable. While this would cause massive clotting in most animals (including humans), glass frogs are able to regulate the location, density, and packing of red cells without clotting. The findings could advance medical understanding of dangerous blood clotting.[10][1]
Characteristics
Glass frogs are generally small, ranging from 3–7.5cm (1.2–3.0in) in length. They appear light green in color over most of their bodies, except for the skin along the lower surface of the body and legs, which are transparent[11] or translucent.[9] The glass frog's transparent skin allows an external view of the viscera—the internal organs present in the body's main cavity—making it so observers can witness the frog's internal processes, such as the heart beating and pumping blood through its arteries. Patterning of glass frogs is varied amongst different species, while some appear as a uniform green color, others display spots that range from yellow to white, mimicking the coloration of their eggs. [8]
Their digit tips are expanded, allowing them to climb, thus allowing most to live in elevated areas along forest streams, such as trees and shrubs. [8]
Glass frogs are similar in appearance to some green frogs of the genus Eleutherodactylus and to some tree frogs of the family Hylidae. However, hylid tree frogs have eyes that face to the side, whilst those of glass frogs face forward.
Two members of the glass-frog family Centrolenidae: Centrolenella fleischmanni, now called Hyalinobatrachium fleischmanni, and C. prosoblepon, and of the hylid subfamily Phyllomedusinae: Agalychnis moreletii and Pachymedusa dacnicolor, reflect near-infrared light (700 to 900 nanometers) when examined by infrared color photography. Infrared reflectance may confer adaptive advantage to these arboreal frogs both in thermoregulation and infrared cryptic coloration.[12] An endangered species of glass frog found in Peru was compared to the N. mixomaculatus species and the following results were recorded: no humeral spine, no webbed fingers between II and III, finger I shorter than II, no vomerine teeth, no ulnar and tarsal tubercles or folds, no white pigment in the visceral or hepatic peritonea, and differing coloration and spots.
Lifecycle
Mating
Mating begins by the call of a male tree frog, who is perched either on the underside or top of a leaf over a lake edge or a stream. Once a female has responded to the male's call, mating begins on the leaf in the amplexus physical position, in which the male wraps his arms around the female and attaches himself to her back. Once the physical mating process has concluded, the female produces her eggs onto the leaf before departing, leaving the male to defend the newly-laid eggs against predators. [8]
Centrolenidae is a species that has long-term parental care, males guard the clutch for various days after the eggs are laid. Environmental aspects also play into the amount of time the male glass frog tends to the young, such as rainfall or wind. Female post-oviposition care is most often based on body conditions, whether or not she is able to fend for herself will tell how long after her eggs are laid that she will remain by the clutch. Males will occasionally call for and mate with other females on the same leaf, establishing a multitude of different developmentally-staged egg clutches to guard.
Tadpoles
Once the tadpoles, the frog aquatic larval stage, have been hatched, they fall from their original position on the leaf into the water below. When living in the water the tadpoles feed on the leaf litter and streamside detritus until undergoing metamorphosis to become a froglet. [8]
Conservation
Predators
A main predator on the glass frog in its tadpole stage are "frog flies", which lay their eggs within the frog eggs; after hatching the maggots feed on the embryos of the glass frogs.[8]
Glass frog behaviors to avoid predation vary from species to species as well as circumstances. Hyalinobatrachium iaspidiense was observed having a flattened body posture to avoid predation, after disturbing the frog it propped up into a sitting position. Another male H. iaspidiense was observed protecting an egg clutch with a body positioning of extending all limbs and lifting its body from the leaf.
The biggest threats they have are deforestation, invasive species, pollution, habitat loss and illegal pet trade. These many threats have led to a decline in the population of this species.
Biology
Glass frogs are mostly arboreal. They live along rivers and streams during the breeding season, and are particularly diverse in montane cloud forests of Central and South America, although some species occur also in Amazon and Chocóanrainforest and semideciduous forests.
Hyalinobatrachium valerioi glass frogs are carnivores, their diet mainly including small insects like crickets, moths, flies, spiders, and other smaller frogs.[14]
The eggs are usually deposited on the leaves of trees or shrubs hanging over the running water of mountain streams, creeks, and small rivers. One species leaves its eggs over stones close to waterfalls. The method of egg-laying on the leaf varies between species. The males usually call from leaves close to their egg clutches. These eggs are less vulnerable to predators than those laid within water, but are affected by the parasitic maggots of some fly species.[15] Some glass frogs show parental care: in many species, glass frog females brood their eggs during the night the eggs are fertilized, which improves the survival of the eggs, while in almost a third of species, glass frog males stay on guard for much longer periods. After they hatch, the tadpoles fall into the waters below.[16] The tadpoles are elongated, with powerful tails and low fins, suited for fast-flowing water.[15] Outside of the breeding season, some species live in the canopy.
The majority of amphibians use cutaneous respiration, or the process of breathing through the skin. Due to the importance of the skin, amphibians are very sensitive to what goes through their permeable skin, the stratum corneum is the main skin barrier which is much thinner than other classes such as mammals or birds. Chemicals and high amounts of elements in water or rainfall may disturb frogs’ health and possibly lives.
Related Research Articles
Centrolene is a genus of glass frogs in the family Centrolenidae. The adult males are characterized by having a humeral spine, as most members of this family. The delimitation of this genus versus Cochranella is not fully resolved, and some species formerly in Centrolenella — which is nowadays synonymized with Centrolene — are now in Hyalinobatrachium.
Cochranella is a genus of glass frogs, family Centrolenidae. They are found in Central America from Honduras southward to the Amazonian and Andean cloud forests of Colombia, Ecuador, Peru, and Bolivia.
Hyalinobatrachium is a genus of glass frogs, family Centrolenidae. They are widely distributed in the Americas, from tropical Mexico to southeastern Brazil and Argentina.
"Centrolene" medemi is a species of frog in the family Centrolenidae. The species occurs in the Cordillera Oriental in the Tolima, Caquetá, and Putumayo Departments in Colombia and adjacent Napo in Ecuador. The generic placement of this species within the subfamily Centroleninae is uncertain. The specific name medemi honors Fred Medem, collector of the holotype. Common name Medem giant glass frog has been coined for it.
Centrolene notosticta is a species of frog in the family Centrolenidae. It is found on the Cordillera Oriental in Colombia and on its extension to north, Serranía del Perijá, in the Zulia state in Venezuela.
"Cochranella" duidaeana, commonly known as the Duida Cochran frog, is a species of frog in the family Centrolenidae. It is endemic to Cerro Duida, Venezuela. The generic placement of this species within the subfamily Centroleninae is uncertain.
"Cochranella" riveroi is a species of frog in the family Centrolenidae. It is endemic to Cerro Aracamuni, Venezuela. The generic placement of this species within the subfamily Centroleninae is uncertain.
"Cochranella" xanthocheridia is a species of frog in the family Centrolenidae. It has an uncertain generic placement within subfamily Centroleninae; molecular data are not available and morphological and behavioural characters do not unambiguously place it in any specific genus.
Hyalinobatrachium aureoguttatum, also known as the Atrato Glass Frog and Sun Glassfrog, is a species of frog in the family Centrolenidae. It is found in northern Ecuador, Pacific lowlands and western slopes of the Cordillera Occidental in Colombia, and eastern Panama. It occurs from near sea level to 1,560 m (5,120 ft) asl.
Nymphargus is a genus of glass frogs in the subfamily Centroleninae, which was established in 2007. They are distributed in the Andean slopes of Colombia, Ecuador, Peru, and Bolivia. They are characterized by lacking webbing among the outer fingers, lacking humeral spines in adult males, and having a lobed liver covered by a transparent hepatic peritoneum. They can be more specifically characterized as having a head that is darker green than the body, there being yellow spots surrounded by black on head and body, upper eyelids are dark lavender. The conservation status of the Nymphargus frogs was largely believed to be critically endangered due to the minimal research done on this genus. Once the scope of the research was broadened the conservation status was able to be determined as being vulnerable. More frogs of different variations were found increasing the genus’ population.
Centroleninae is one of two subfamilies of the family Centrolenidae. It has nine genera distributed in Central America from Honduras south and east to northern and central South America. As of mid 2015, it contains 117 species.
Vitreorana is a genus of glass frogs that are native to South America, from the Atlantic Forest of Brazil and Argentina to the Amazon rainforest of Colombia and Ecuador and to the Venezuelan Coastal Range and the Guianas. One way one can tell this type of glass frog from others is through their green bones, lavender-colored dorsal, and white highlighted pigment. This genus has also started to become endangered, especially in Brazil, where many Vitreorana, such as V. eurygnatha due to habitat loss.
Hyalinobatrachinae is a subfamily of glass frogs that was established in 2009. They are found in the Americas from Mexico south to southeastern Brazil and Argentina.
Celsiella is a small genus of glass frogs endemic to Venezuela. It was established in 2009 and named in honour of Josefa Celsa Señaris, nicknamed "Celsi", a Venezuelan herpetologist who had worked with glass frogs.
Chimerella is a small genus of glass frogs, family Centrolenidae. They are found on the Amazonian slopes of the Andes in Ecuador and Peru, possibly extending into Colombia.
Espadarana is a genus of glass frogs. They are found in Central America and northern South America.
Sachatamia is a small genus of glass frogs. They are found in Central America and northern South America at altitudes below 1,500 m (4,900 ft) above sea level.
Rulyrana is a small genus of glass frogs. They are found in South America, on the Amazonian slopes of the Andes in Ecuador, Peru, and possibly Bolivia, as well as on the eastern slopes of the Cordillera Central and the western slopes of the Cordillera Oriental in Colombia.
Juan Manuel Guayasamin is an Ecuadorian biologist. He earned his Ph.D. in 2007 from University of Kansas, Department of Ecology and Evolutionary Biology and as of 2017 he is working as professor at Universidad San Francisco de Quito in Ecuador. His research interests include the evolution of glass frogs (Centrolenidae) and direct-developing anurans. His main contributions have been: phylogenetic taxonomy of glassfrogs, description of the variation of skin texture in frogs, description of numerous species of amphibians and reptiles, and a monographic review of all Ecuadorian glassfrogs. A team led by Juan M. Guayasamin discovered Hyalinobatrachium yaku in May 2017, a glassfrog with transparent venter. To date (2020), he has described a total of 6 amphibian genera, 55 species of amphibians, and 11 reptiles, including two geckos from the Galápagos Islands.
Ikakogi is a genus of frogs in the family Centrolenidae. It has been tentatively placed in the subfamily Centroleninae, although more recent analyses suggest that it is the sister group of the clade Centroleninae+Hyalinobatrachiinae.
1 2 Guayasamin, J. M.; Castroviejo-Fisher, S.; Ayarzaguena, J.; Trueb, L.; Vilá, C. (2008). "Phylogenetic relationships of glass frogs (Centrolenidae) based on mitochondrial and nuclear genes". Molecular Phylogenetics and Evolution. 48 (2): 574–595. doi:10.1016/j.ympev.2008.04.012. PMID18515151.
↑ Castroviejo-Fisher, S., (2009). Species Limits, and Evolutionary History Of Glass frogs. Digital Comprehensive Summaries of Uppsala Dissertations from the faculty of Science and Technology 642. 55 pp.
1 2 3 Ruíz-Carranza, P.M. and J. D. Lynch. 1991. Ranas Centrolenidae de Colombia I: propuesta de una nueva clasificación genérica. Lozania, 57, 1–30.
1 2 Frost, D. R.; Grant, T.; Faivovich, J. N.; Bain, R. H.; Haas, A.; Haddad, C. L. F. B.; De Sá, R. O.; Channing, A.; Wilkinson, M.; Donnellan, S. C.; Raxworthy, C. J.; Campbell, J. A.; Blotto, B. L.; Moler, P.; Drewes, R. C.; Nussbaum, R. A.; Lynch, J. D.; Green, D. M.; Wheeler, W. C. (2006). "The amphibian tree of life". Bulletin of the American Museum of Natural History. 297: 1–291. doi:10.1206/0003-0090(2006)297[0001:TATOL]2.0.CO;2. hdl:2246/5781. S2CID86140137.
↑ Zweifel, Robert G. (1998). Cogger, H.G.; Zweifel, R.G. (eds.). Encyclopedia of Reptiles and Amphibians. San Diego: Academic Press. pp.94–95. ISBN0-12-178560-2.
1 2 Zweifel, Robert G. (1998). Cogger, H.G.; Zweifel, R.G. (eds.). Encyclopedia of Reptiles and Amphibians. San Diego: Academic Press. pp.94–95. ISBN0-12-178560-2.
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