Glassfrogs “hide” blood in their liver while sleeping to stay transparent

Tiny, translucent glassfrogs increase their transparency two- to threefold while sleeping by temporarily storing red blood cells in their liver, according to a new study. The findings – which provide insight into this unique adaptation in vertebrates  – could be used in understanding blood flow more broadly and in the development of new anticoagulants or other cardiovascular drugs. The glassfrog (Hyalinobatrachium fleischmanni) is a tropical, leaf-dwelling frog species with a transparent body and translucent skin – adaptations these frogs use as camouflage against would-be predators as they sleep on green leaves during the day. For many vertebrates, particularly terrestrial species, achieving a glassfrog’s level of transparency is challenging because of the numerous red blood cells (RBCs) that circulate continuously throughout the body, rendering even highly transparent tissues opaque. In this study, Carlos Taboada and colleagues investigated how glassfrogs seemingly overcome this physiological barrier. Using calibrated color photography to measure animal transparency and photoacoustic imaging to trace the movement of RBCs in living frogs, Taboada et al. found that glassfrogs become 34 to 61% more transparent, on average, while sleeping, suggesting that the animals actively maintain their dynamic transparency. According to the authors, they do this by removing roughly 89% of their RBCs from circulation and “hiding” them in the liver during sleep, without any detrimental vascular or metabolic effects. As the frogs wake up and become more active, the number of RBCs in circulation increases greatly, as does their opacity. Since such high local concentrations of RBCs in most vertebrates often results in vaso-occlusion, or clotting, these findings may offer insights into the mechanisms involved in preventing these and other vascular pathologies. “The mechanism that drives RBC re-distribution in glassfrogs is not understood. It is unclear if the glassfrog can actively manipulate the changes in RBC circulation – for example, in the presence of a predator,” write Nelly Cruz and Richard White in a related Perspective. “Another intriguing question is how sequestration of RBCs in the liver affects cellular respiration and whether the glassfrog has a special metabolism that adapts to the drastic changes in RBC circulation.