Active mode of excretion across digestive tissues predates the origin of excretory organs

Abstract

Most bilaterian animals excrete toxic metabolites through specialized organs, such as nephridia and kidneys, which share morphological and functional correspondences. In contrast, the excretory mechanisms in non-nephrozoans are largely unknown, and therefore the reconstruction of ancestral excretory mechanisms is problematic. Here, we investigated the excretory mode of members of the Xenacoelomorpha, the sister group to Nephrozoa, and Cnidaria, the sister group to Bilateria. By combining gene expression, inhibitor experiments and exposure to varying environmental ammonia conditions we show that both, Xenacoelomorpha and Cnidaria, are able to excrete across digestive-associated tissues. Based on these results we propose that digestive-associated tissues functioned as excretory sites before the evolution of specialized organs in nephrozoans. We conclude that diffusion was likely the ancestral mode of excretion, whilst the emergence of a compact, multiple-layered bilaterian body plan necessitated the evolution of active transport excretory mechanisms that was later recruited into the specialized excretory organs.