Amphibians
Reticulated Glass Frog
Hyalinobatrachium valerioi · also called reticulated glass frog, La Palma glass frog
The reticulated glass frog is a grape-sized rainforest frog you can see through. Its back is ordinary leaf-green, but its underside is so transparent that from below you can watch its heart beat and its gut loop. And its father sits on the eggs.
Overview
Reticulated glass frogs live in humid rainforest along fast, clear streams from Costa Rica to Ecuador. They are tiny (~2 cm), nocturnal, and arboreal, hunting small insects on leaves in the streamside canopy. From above they are lime green with yellow spots — a good leaf mimic — but their belly and limb undersides are clear, showing the heart, liver, intestine, and eggs through the body wall.
Controlled transparency
The see-through underside blurs the frog’s outline against a backlit leaf, hiding it from predators looking up from below. Sleeping glass frogs go further: a 2022 study found they pack about 90% of their red blood cells into the liver, roughly doubling their transparency and removing the red tint, then reverse it on waking — a rare case of an animal actively tuning its own transparency.
The guarding father
The female lays 20–30 eggs on the underside of a leaf over a stream and leaves. The male stays, tending several clutches at once — keeping them moist, fighting off parasitic wasps — and egg survival is far higher with him present. His own transparent body over a clutch may help hide the eggs from above. Hatching tadpoles drop into the stream below.
Status
The reticulated glass frog is Least Concern, but glass frogs as a family depend entirely on clean forested streams and are among the first frogs to vanish when a catchment is degraded — good indicator species for tropical stream health.
Reticulated Glass Frog: common questions
Can you really see a glass frog's heart beating?
Yes. In the reticulated glass frog and its close relatives, the skin and underlying tissues of the belly and the undersides of the limbs are almost entirely lacking in pigment and are thin and translucent, so when you look at the frog from below — for example while it clings to a glass surface or a clear leaf — the internal organs are visible through the body wall. The beating heart, the white-sheathed liver, the coiled intestine, and in a gravid female the mass of developing eggs can all be seen. The frog's back, by contrast, is opaque green, so from above it just looks like a small leaf frog; the transparency is confined to the underside, which is the side a predator looking up from below would see.
What is the point of being transparent?
For a small green frog sitting on a leaf, the main threat is being spotted from below or the side by a bird, snake, or another frog, against the bright, backlit leaf. A solid-bodied frog casts a dark silhouette; a partly transparent one blurs into the leaf. Research published in 2022 showed that sleeping glass frogs go further — they actively concentrate about 90% of their red blood cells (the most opaque part of their body) into the liver, which is wrapped in a mirror-like membrane, roughly doubling their transparency and removing the reddish tint. They reverse this when they wake and become active. It is a rare example of an animal controlling its own transparency, and it works because the frog is still while it does it.
Do glass frogs really have caring fathers?
Yes, and it is unusual among frogs. The female reticulated glass frog lays a jelly clutch of around 20 to 30 eggs on the underside of a leaf hanging over a stream and then leaves. The male stays. He tends several clutches at once from different females, sitting among or over them by night and often by day, urinating on them or pressing his moist body against them to keep them hydrated, and attacking parasitic wasps and flies that try to lay in the eggs. Egg survival is much higher with an attending father than without one. Some work also suggests the male's transparent body, spread over a clutch, helps disguise the eggs from predators looking down.
Are glass frogs endangered?
The reticulated glass frog itself is currently listed as Least Concern and remains reasonably common along forested streams from Costa Rica to Ecuador. But glass frogs as a group (family Centrolenidae, over 150 species) are a conservation concern — they depend absolutely on clean, fast, forested streams for their stream-dwelling tadpoles, so they are among the first frogs to disappear when a catchment is logged, farmed, dammed, or polluted, and several species with tiny ranges in the Andes are Endangered or Critically Endangered. The chytrid fungus has also hit some populations. They are good indicator species for the health of tropical montane streams.