Glassfrogs, or centrolenids, are wide-skulled, long-limbed arboreal little frogs (SVL 20-60 mm), unique to the Central and South American cloud and rain forests. Not until 1951 did this group get recognised as a distinct and nameable entity: prior to this, species within the group (known to science since 1872) had been classified as part of Rhacophoridae, the Old World 'bush frogs' or 'shrub frogs'. Most glassfrogs lay their eggs on vegetation overhanging water or on rocks above the water surface and their tadpoles live in nearby streams. Many texts refer to them as ‘glass frogs’; I here follow several recent publications (and the trend that’s occurring in biological nomenclatural in general) in referring to them as ‘glassfrogs’. [Adjacent photo from here.]

Glassfrog eyes are set on the tops of their heads, they have adhesive disks on their digit tips, and – while they are generally greenish on their dorsal surface – they derive their common name from the fact that they lack pigment on their ventral surface, meaning that their undersides are essentially transparent. I have no idea why this is, and I’m not sure that anyone else does. Even stranger, many species have green bones (not all do: Ikakogi and the Hyalinobatrachium species have white ones).

The terminal phalanges of glassfrog digits are T-shaped (this is also the case in a few other neobatrachian groups, like poison-arrow frogs), the males of some species possess spines on their upper arms (these are used in territorial combat), and the two uniquely elongate ankle bones that characterise anurans (the tibiale and fibulare) are fused into a single element.

Centrolenidae is not a small or insignificant group: there are currently about 150 named species (a number that has increased substantial since the late 1980s; back then, there were about 65 recognised species), with numerous additional ones known but awaiting description (Cisneros-Heredia & McDiarmid 2006, pp. 12-13).

A revised taxonomy

Until recently, all glassfrogs were grouped into just three genera: Centrolene (distinctive due to its humeral spines), Hyalinobatrachium (distinctive due to its prominent, white liver), and Cochranella (which lacks both of these features). A fourth ‘genus’, Nymphargus, was named in 2007 for species previously included in Cochranella but lacking hand webbing (Cisneros-Heredia & McDiarmid 2007).

However, a phylogenetic analysis of molecular characters found the first of those three genera to be polyphyletic and Nymphargus to be paraphyletic to Centrolene (Guayasamin et al. 2008). A taxonomy created to reflect this phylogeny resurrected or coined seven additional genera: Chimerella, Espadarana, Rulyrana*, Sachatamia, Teratohyla, Vitreorana and Celsiella (Guayasamin et al. 2009). Centrolene in the strict sense (the clade that includes C. geckoideum – the very first glassfrog species to be named – and its close relatives) was found to be the sister-group to Nymphargus. Vitreorana, Teratohyla, Rulyrana, Sachatamia, Cochranella sensu stricto and Espadarana grouped together as a clade which formed the sister-group to a Centrolene + Nymphargus clade; this former clade was named Cochranellini. The few Celsiella species (previously lumped into Cochranella) formed a clade with Hyalinobatrachium. [Teratohyla image below by Bgv23.]

* This name was invented by combining the first letters of the names Ruiz-Carranza and Lynch with ‘rana’ (meaning frog). Pedro Ruiz-Carranza and John D. Lynch are amphibian specialists who have “contributed enormously to the understanding of centrolenid diversity, biology, and evolution” (Guayasamin et al. 2009, p. 36).

It seems that a divergence into a ((Centrolene + Nymphargus) + Cochranellini) clade and Celsiella + Hyalinobatrachium clade occurred early in glassfrog evolution: Guayasamin et al. (2009) named the first clade Centroleninae and the second Hyalinobatrachinae. While molecular and morphological characters have been recognised for both clades, Guayasamin et al. (2009) drew attention to the very different fighting behaviours of these frogs. Male hyalinobatrachines battle on top of leaves and wrestle, with their fighting poses often resembling amplexus. In contrast, male centrolenines hang from leaf edges by their hindfeet and wrestle while dangling in the air! Hyalinobatrachines lack the large and often formidable humeral spines present in some centrolenines. [Image below by Mauricio Rivera Correa and from the CalPhotos database.]

A taxon previously included in Centrolene (C. tayrona) but found in Guayasamin et al.'s (2009) phylogeny to group outside the Hyalinobatrachinae + Centroleninae clade was given its own ‘genus’ – Ikakogi – by these authors. Accordingly, they hypothesised that Ikakogi represented a lineage at least as old as both hyalinobatrachines and centrolenines (incidentally, fossil centrolenids are unknown, and hypotheses about when the divergence events might have occurred depend on distribution and biogeography). However, Pyron & Wiens (2011) found Ikakogi to be part of Centroleninae (specifically, the sister-taxon to the rest of the clade). Whereas other glassfrog species are consistent in whether they prefer the upper or lower surfaces of leaves as egg-deposition sites, I. tayrona is unusual in that it will deposit eggs on either surface. It’s further unusual in that females guard the egg clutches. There are other glassfrog species where egg-guarding occurs, but (so far as we know), it’s always males that do the guarding.

I. tayrona also possesses those humeral spines. If Ikakogi is hypothesised to be the sister-taxon to all other centrolenines, it could be that the spines are a primitive character for glassfrogs lost early on in hyalinobatrachines and also in several centrolenine lineages. Notably, the humeral spines present in the various glassfrog lineages differ in where they’re placed on the edge of the humerus and often seem to be formed from different parts of the bone. This could mean that the spines have actually evolved independently on two, three or more separate occasions – that might seem remarkable but similar things are known to have happened elsewhere (e.g., ear asymmetry in owls has evolved on at least seven separate occasions). With Ikakogi interpreted as a centrolenine (Pyron & Wiens 2011), things are different, since humeral spines are then unique to this lineage. Still – did they evolve once, or several times within Centroleninae?

Where in the anuran tree?

It is uncontroversially accepted that glassfrogs are part of the major neobatrachian clade termed either Hyloides, Hyloidea or Bufonoidea: that is, they’re part of the same major group as treefrogs, toads, horned frogs and poison-dart frogs. Glassfrogs used to be considered especially close to hylids (treefrogs), but data from mitochondrial DNA indicates that they're more closely related to toads, leptodactylids and kin (Darst & Cannatella 2004, Frost et al. 2006, Pyron & Wiens 2011) and are especially close to leptodactylids. Frost et al. (2006) proposed the name Diphyabatrachia for the centrolenid + leptodactylid clade.

It has also been suggested that glassfrogs are the sister-taxon of Allophryne ruthveni (e.g., Austin et al. 2002, Guayasamin et al. 2008, 2009), a controversial and problematical toothless hyloid that has often been given its own ‘family’, Allophrynidae (a second species of Allophryne has recently been discovered, but I don’t think it’s been published yet. Please let me know if you know otherwise [UPDATE: it’s A. resplendens Castroviejo-Fisher et al., 2012 (Castroviejo-Fisher et al. 2012). Thanks to Diego Cisneros-Heredia for this.]). Frost et al. (2006) even subsumed Allophryne into Centrolenidae and changed the taxonomy such that their version of Centrolenidae included an Allophryninae and a Centroleninae. Guayasamin et al. (2009) decided that the best course of action was to keep Allophryne outside of Centrolenidae but to use the name Allocentroleniae for the Allophryne + Centrolenidae clade. The simplified cladogram shown here doesn’t include the names Diphyabatrachia or Allocentroleniae (nor is Allophryne featured), but hopefully you get the point.

Lest we forget: the Global Amphibian Crisis

This article is a re-vamped version of a section of text that appeared on ver 2 in 2007. As you will know, I’m sure, frogs all around the world are currently beleaguered by various issues, one of the most worrying of which is the spread of the aquatic fungal pathogen Batrachochytrium dendrobatidis (Bd for short).

A huge amount of research on Bd has been done since it was first recognised as a cause of amphibian decline. We now know that several anuran species are immune (or largely immune) to the disease caused by the pathogen and that they act as reservoirs for the pathogen, passing it to other, more suspectible species by sharing the same habitats. Ironically, the African clawed frog Xenopus laevis and American bullfrog Lithobates catesbeianus (or Rana catesbeiana) – the two anuran species that people have taken with them all around the world – are among those Bd-resistant species. The spread of Bd has also been linked with climate change. This will all be very familiar stuff if you know anything already about anurans or conservation, but it remains an area of major concern and certainly hasn't gone away. For news on anuran conservation, the spread of Bd and other, related issues, keep an eye on ZSL's Frog Blog, Amphibian Ark and Frog Matters.

For previous Tet Zoo articles on anurans, see...

Refs - -

Castroviejo-Fisher, S., Pérez-Peña, P. E., Padial, J. M. & Guayasamin, J. M. 2012. A second species of the family Allophrynidae (Amphibia: Anura). American Museum Novitates 3739, 1-17.

Cisneros-Heredia, D. F. & McDiarmid, R. W. 2006. A new species of the genus Centrolene (Amphibia: Anura: Centrolenidae) from Ecuador with comments on the taxonomy and biogeography of Glassfrogs. Zootaxa 1244, 1-32.

- . & McDiarmid, R. W. 2007. Revision of the characters of Centrolenidae (Amphibia: Anura: Athesphatanura), with comments on its taxonomy and the description of new taxa of glassfrogs. Zootaxa 1572, 1-82.

Darst, C. R. & Cannatella, D. C. 2004. Novel relationships among hyloid frogs inferred from 12S and 16S mitochondrial DNA sequences. Molecular Phylogenetics and Evolution 31, 462-475.

Frost, D. R., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. 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, 1-370.

Guayasamin, J. M., S. Castroviejo-Fisher, J. Ayarzaguena, L. Trueb y C. Vilá. 2008. Phylogenetic relationships of glass frogs (Centrolenidae) based on mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution 48, 574-595.

- ., S. Castroviejo-Fisher, L. Trueb, J. Ayarzagüena, M. Rada, C. Vilá. 2009. Phylogenetic systematics of Glassfrogs (Amphibia: Centrolenidae) and their sister taxon Allophryne ruthveni. Zootaxa 2100, 1-97.

Pyron, R. A. & Wiens, J. J. 2011 A large-scale phylogeny of Amphibia including over 2,800 species, and a revised classification of extant frogs, salamanders, and caecilians. Molecular Phylogenetics and Evolution 61, 543-583.