A Comb Jelly With a Tripartite Gut

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

Authors:

Presnell et al

Abstract:

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.

Extinct Cambrian Comb Jellies Had Skeletons

A vanished history of skeletonization in Cambrian comb jellies

Authors:

Ou et al

Abstract:

Ctenophores are traditionally regarded as “lower” metazoans, sharing with cnidarians a diploblastic grade of organization. Unlike cnidarians, where skeletonization (biomineralization and sclerotization) evolved repeatedly among ecologically important taxa (for example, scleractinians and octocorals), living ctenophores are characteristically soft-bodied animals. We report six sclerotized and armored ctenophores from the early Cambrian period. They have diagnostic ctenophore features (for example, an octamerous symmetry, oral-aboral axis, aboral sense organ, and octaradially arranged ctene rows). Unlike most modern counterparts, however, they lack tentacles, have a sclerotized framework, and have eight pairs of ctene rows. They are resolved as a monophyletic group (Scleroctenophora new class) within the ctenophores. This clade reveals a cryptic history and sheds new light on the early evolution of this basal animal phylum. Skeletonization also occurs in some other Cambrian animal groups whose extant members are exclusively soft-bodied, suggesting the ecological importance of skeletonization in the Cambrian explosion.