Glassfrogs conceal blood in their liver to maintain transparency

Transparency in animals is a complex form of camouflage involving mechanisms that reduce light scattering and absorption throughout the organism. In vertebrates, attaining transparency is difficult because their circulatory system is full of red blood cells (RBCs) that strongly attenuate light. Here, we document how glassfrogs overcome this challenge by concealing these cells from view. Using photoacoustic imaging to track RBCs in vivo, we show that resting glassfrogs increase transparency two- to threefold by removing ~89% of their RBCs from circulation and packing them within their liver. Vertebrate transparency thus requires both see-through tissues and active mechanisms that “clear” respiratory pigments from these tissues. Furthermore, glassfrogs’ ability to regulate the location, density, and packing of RBCs without clotting offers insight in metabolic, hemodynamic, and blood-clot research. Description Hiding in plain sight Transparency provides a form of camouflage that is particularly useful for increasing crypsis in motionless animals. Many taxa have evolved transparency, but it is particularly challenging for vertebrates because red blood cells attenuate light. Taboada et al. found that glass frogs are able to maintain a high level of transparency because a large proportion of their red blood cells are “hidden” in the liver (see the Perspective by Cruz and White). This strategy allows the frogs to attain transparency when they are the most vulnerable. Understanding this liver-packing process may inform our understanding of hemodynamics more broadly. —SNV Glass frogs actively maintain transparency by removing red blood cells from the circulation and concealing them in their livers.