The Presence of a Functionally Tripartite Through-Gut in Ctenophora Has Implications for Metazoan Character Trait Evolution

The current paradigm of gut evolution assumes that non-bilaterian metazoan lineages either lack a gut (Porifera and Placozoa) or have a sac-like gut (Ctenophora and Cnidaria) and that a through-gut originated within Bilateria [1–8]. An important group for understanding early metazoan evolution is Ctenophora (comb jellies), which diverged very early from the animal stem lineage [9–13]. The perception that ctenophores possess a sac-like blind gut with only one major opening remains a commonly held misconception [4, 5, 7, 14, 15]. Despite descriptions of the ctenophore digestive system dating to Agassiz [16] that identify two openings of the digestive system opposite of the mouth—called “excretory pores” by Chun [17], referred to as an “anus” by Main [18], and coined “anal pores” by Hyman [19]—contradictory reports, particularly prominent in recent literature, posit that waste products are primarily expelled via the mouth [4, 5, 7, 14, 19–23]. Here we demonstrate that ctenophores possess a unidirectional, functionally tripartite through-gut and provide an updated interpretation for the evolution of the metazoan through-gut. Our results resolve lingering questions regarding the functional anatomy of the ctenophore gut and long-standing misconceptions about waste removal in ctenophores. Moreover, our results present an intriguing evolutionary quandary that stands in stark contrast to the current paradigm of gut evolution: either (1) the through-gut has its origins very early in the metazoan stem lineage or (2) the ctenophore lineage has converged on an arrangement of organs functionally similar to the bilaterian through-gut.