Polka-dot tree frog

Fluorescent polka-dot tree frog under UV-light

In 2017, several polka-dot tree frogs collected in the Santa Fe Province in Argentina were the first amphibian, amongst more than 6,000 species of frog, identified as naturally fluorescent.[3][4][5] Among land-living vertebrates, it had only been confirmed earlier in certain parrots (as opposed to fish and invertebrates where relatively widespread).[3] A few months later, fluorescence was discovered in the closely related Hypsiboas atlanticus,[6] and in 2019 it was discovered in the distantly related pumpkin toadlet (Brachycephalus ephippium) and red pumpkin toadlet (B. pitanga).[7] In 2020, it was confirmed that fluorescence is relatively widespread among frogs and other amphibians.[8]

When exposed to ultra-violet light, the skin of the polka-dot tree frog emits a bright green fluorescent glow. The discovery was made accidentally when the researchers checked the skin secretions of the frog under UV light. They expected only a faint red fluorescence, because the frog skin contains biliverdin, a green tetrapyrrolic bile pigment responsible for its dull mottled browny-green colour.[9] The surprising green fluorescence is attributed to hyloin compounds, mainly hyloin-L1, hyloin-L2 and hyloin-G1.[3][4] These fluorescent molecules, in the dihydroisoquinolinone family and derived from the isoquinoline (a benzene ring fused to a pyridine ring), in an alkaloid protecting mucous membrane, have been identified in the lymph tissue, skin and glandular secretions of the frog.[3] Because it is linked to secretions from skin glands, they can also leave fluorescent markings on surfaces where they have been.[6]

The fluorescence intensity represents about 18−29% of the luminosity under twilight conditions and is suspected to play a possible role in the communication, camouflage and mating of the frog.[3]