Glassfrogs (or centrolenids) are a really interesting but comparatively little known group of anurans, you might have heard of them. Ha ha, just kidding – you know them well already since they were recently covered at reasonable length here at Tet Zoo. Since that article went live, I’ve been talking with glassfrog expert Juan Manuel Guayasamin, author of Guayasamin et al. (2008, 2009) and so many other published articles. Juan was kind enough to share the several excellent glassfrog photos you see here. They give me an excuse to talk more about these amazing frogs, and also to better illustrate some of the aspects of glassfrog biology and diversity that I mentioned last time.

We start with a brilliant close-up shot of the ventral surface of Hyalinobatrachium aureoguttatum (photo by Martin Bustamante). The Hyalinobatrachium species exhibit transparency across much of their ventral surface (note the transparent/translucent tissues of the limbs), whereas some other glassfrogs are only transparent across part of their undersides (their hands, feet, arms and legs have properly pigmented skin). The peritoneal tissue covering the liver and guts can clearly be seen as bright white in this animal.

If you’re wondering which organs we’re looking at here, the heart is obvious while the white mass located further posteriorly is the intestines. That round yellow organ on the animal’s left side is the gall bladder. The white area between the heart and gall bladder should be the liver, but it’s hard to be sure about this since – in this photo – it’s essentially indistinguishable from the intestines. We tend to think of anurans as simple animals with a very basic anatomy; that’s partly true, but remember that part of the reason for this is that they are heavily and weirdly modified relative to the ancestral tetrapod condition. As I’ve said before on Tet Zoo, a case can be made that they are, in fact, among the most profoundly modified of all tetrapods.

Something I also wanted to mention last time concerns the detailed anatomy of glassfrog hands. Anurans have four fingers. The homology of these has been much-debated. It seems that the true digit I is absent, with the first digit actually being digit II (Fabrezi & Alberch 1996). There’s webbing between all four digits in Hyalinobatrachium, but other glassfrogs only have webbing between the third and fourth digits.

Some glassfrogs have the additional structure present in various anuran hands known as the prepollex, sometimes called the pseudothumb.

There’s been substantial interest in the idea that the prepollex might represent the true digit I – if so, this would mean that at least some anurans still have five digits. Recent embryological studies seem to show that this isn’t the case, and that the prepollex is a wholly novel structure. The anatomy of the prepollex is tremendously variable across anurans (Fabrezi 2001). It’s typically sexually dimorphic, sometimes being enlarged in males, sometimes being spade-like, and sometimes being spiny. We know that prepollices are used in male-male combat across anurans, and some frog species might use them in fending off snakes. You know, there’s a lot to say about anuran hands and digits… maybe I should come back to this subject some time.

Moving on... you'll recall, I'm sure, the amazing humeral spines present in many glassfrog species. Here’s a calling male Centrolene heloderma, its relatively small humeral spine being visible. He's calling while clinging to the upper surface of a leaf, a habit typical for Centrolene species. Should he succeed in attracting and mating with a female, she'll lay her eggs on the leaf underside. The photo is by Jaime Garcia.

Now check out the enormous spines visible in this front view of a male Centrolene geckoideum (below). The photo is by William E. Duellman. I'd really love to know more about the way in which these spines are used in combat, and also about the sorts of injuries they cause: this area of discussion will be familiar to you if you've listened to ep 1 of the Tet Zoo podcast. So far as I know, injuries caused by glassfrogs to other glassfrogs have yet to be reported or described (please say if you know otherwise). C. geckoideum was named by the brilliant Spanish biologist Marcos Jiménez de la Espada in 1872 and is the very first glassfrog species to be recognised by science. Jiménez de la Espada probably created the generic name Centrolene by combining the Greek words 'kentron' (meaning spike) and 'olene' (meaning elbow). The glassfrog Espadarana is named after him (Guayasamin et al. 2009), but it's also fitting that espada is Spanish for sword.

Those humeral spines are remarkable enough, but there’s also the fact that members of several glassfrog lineages fight by hanging upside-down from the edges of leaves or branches. So far as we know, this behaviour is unique to members of the large glassfrog clade Centroleninae (in Guayasamin et al.’s (2009) phylogeny, this is the group that includes Centrolene, Cochranella and all descendants of their most recent common ancestor: in other words, most glassfrogs except Celsiella and Hyalinobatrachium). Here, we see two individuals of Centrolene lynchi engaging in this behaviour. The photo is by Henry Imba.

As per usual, I ended up saying far more here than I intended to. However, there's tons more that I could have said. More anuran articles are due to appear here in the near future. My sincere thanks to Juan Manuel Guayasamin for sharing the photos used here, and to the people who took the photos in the first place.

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

Refs - -

Fabrezi, M. 2001. A survey of prepollex and prehallux variation in anuran limbs. Zoological Journal of the Linnean Society 131, 227-248.

- . & Alberch, P. 1996. The carpal elements of anurans. Herpetologica, 52, 188-204.

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.