Transparency in animals is a complex form of camouflage involving mechanisms that reduce the scattering and absorption of light throughout the organism. Many species have evolved transparency, but it is particularly challenging for vertebrates because red blood cells attenuate light. In a new study, researchers from Duke University and elsewhere found that glassfrogs are able to maintain a high level of transparency because a large proportion of their red blood cells are ‘hidden’ in the liver.
The glassfrogs, or glass frogs, are members of the amphibian family Centrolenidae, with more than 150 species.
These frogs are found in the Neotropics, from southern Mexico, through Central America, into the northern half of South America, and along the Atlantic Forest of southeastern Brazil and the extreme northeastern margin of Argentina.
Their diversity is concentrated in the northern Andes, which hosts more than half of the species in the family.
The glassfrogs are generally small, ranging from 2 to 7.5 cm (0.8 to 3 inches) in length.
They are nocturnal and spend their days sleeping upside down on translucent leaves that match the color of their backs.
Their tummies, however, show something surprising: translucent skin and muscle that allows their bones and organs to be visible.
Recent research has proposed that this adaptation masks the frogs’ outlines on their leafy perches, making them harder for predators to spot.
In vertebrates, attaining transparency is difficult because their circulatory system is full of red blood cells that interact with light.
Studies have shown that ice fish and larval eels achieve transparency by not producing hemoglobin and red blood cells. But glassfrogs use an alternative strategy, according to the new study.
“Glassfrogs overcome this challenge by essentially hiding red blood cells from view,” said study’s first author Dr. Carlos Taboada, a researcher in the Biology Department and the Department of Biomedical Engineering at Duke University.
“They almost pause their respiratory system during the day, even at high temperatures.”
Dr. Taboada and his colleagues used a technique called photoacoustic imaging, which uses light to induce sound-wave propagation from red blood cells.
This allowed the researchers to map the location of the cells within sleeping frogs without restraint, contrast agents, sacrifice, or surgical manipulation — particularly important to this study because glassfrog transparency is disrupted by activity, stress, anesthesia, and death.
They focused on one particular species of glassfrog: Hyalinobatrachium fleischmanni.
They found that resting glassfrogs increase transparency two- to threefold by removing nearly 90% of their red blood cells from circulation and packing them within their liver, which contains reflective guanine crystals.
Whenever the frogs need to become active again, they bring the red blood cells back into the blood, which gives the frogs the ability to move around — at which point, light absorption from these cells breaks transparency.
In most vertebrates, aggregating red blood cells can lead to potentially dangerous blood clots in veins and arteries.
But glassfrogs don’t experience clotting, which raises a set of significant questions for biological and medical researchers.
“This is the first of a series of studies documenting the physiology of vertebrate transparency, and it will hopefully stimulate biomedical work to translate these frogs’ extreme physiology into novel targets for human health and medicine,” said study’s co-first author Dr. Jesse Delia, a reseacher with the Department of Biomedical Engineering at Duke University, the Department of Biology at Stanford University, and the Division of Vertebrate Zoology and Richard Gilder Graduate School at the American Museum of Natural History.
The findings appear in the journal Science.
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Carlos Taboada et al. 2022. Glassfrogs conceal blood in their liver to maintain transparency. Science 378 (6626): 1315-1320; doi: 10.1126/science.abl6620
This article was first published by Sci-News on 28 December 2023. Lead Image: The Yaku glassfrog (Hyalinobatrachium yaku) in life. Top row: adult male, holotype, in dorsal and ventral view. Bottom row: adult male, paratype. Image credit: Guayasamin et al, doi: 10.3897/zookeys.673.12108.
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