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.

Comb Jellies Are More Basal Than Sponges?

Animals evolved gradually, from the lowly sponge to the menagerie of tentacled, winged and brainy creatures that inhabit Earth today. This idea makes such intuitive sense that biologists are now stunned by genome-sequencing data suggesting that the sponges were preceded by complex marine predators called comb jellies.

Although they are gelatinous like jellyfish, comb jellies form their own phylum, known as ctenophores. Trees of life typically root the comb jellies' lineage between the group containing jellyfish and sea anemones and the one containing animals with heads and rears — which include slugs, flies and humans. Comb jellies paddle through the sea with iridescent cilia and snare prey with sticky tentacles. They are much more complex than sponges — they have nerves, muscles, tissue layers and light sensors, all of which the sponges lack.

“It’s just wild to imagine” that comb jellies evolved before sponges, says Billie Swalla, a developmental biologist at the University of Washington in Seattle and a leading member of the team sequencing the genome of the comb jelly Pleurobrachia bachei. But the team is suggesting just that, in results they presented at the annual meeting of the Society for Integrative and Comparative Biology, held on 3–7 January in San Francisco, California.

Despite comb jellies' complexity, DNA sequences in the Pleurobrachia genome place them at the base of the animal tree of life, announced Swalla's colleague Leonid Moroz, a neurobiologist at the University of Florida in Gainesville. Another team presented results from genome sequencing for the comb jelly Mnemiopsis leidyi, and found that the phylum lands either below, or as close to the base as, sponges on the tree.

“We’ve always thought that predator–prey interactions and sensory adaptations evolved long after the origin of sponges,” Swalla says. “Now we need to imagine early life as a sponge, ctenophore and everything in between.” Because millions of species have gone extinct since animals appeared some 542 million years ago, Swalla says, the ancestor of all animals might look different from modern comb jellies and sponges.

Gene families, cell-signalling networks and patterns of gene expression in comb jellies support ancient origins as well. For example, Moroz and his team found that comb jellies grow their nerves with unique sets of genes. “These are aliens,” Moroz jokes. He suggests that comb jellies might be descendants of Ediacaran organisms, mysterious organisms that appear in the fossil record before animals. Indeed, in 2011, palaeontologists claimed that one of these 580-million-year-old fossils resembled comb jellies1.

Andy Baxevanis, a comparative biologist at the US National Human Genome Research Institute in Bethesda, Maryland, and a leader on the Mnemiopsis genome project, says that comb jellies are the only animals that lack certain genes crucial to producing microRNA — short RNA chains that help to regulate gene expression. Moreover, he points out, sponges and comb jellies lack other gene families that all other animals possess2, 3.

If comb jellies evolved before sponges, the sponges probably lost some of their ancestors' complexity. Alternatively, says Sally Leys, a biologist at the University of Alberta in Edmonton, sponges may have complexity that scientists have yet to appreciate. “A lot of sponges are more exquisite than a lump on a rock,” she says.

This is an incredible conjecture.  I'll wait to see if more research backs this up.  Extraordinary claims and all that.  If so...the world is far, far weirder and the story of the evolution of the animal kingdom is far more complicated than we ever expected.

A quick thought is to look for pseudogenes in sponges that are relics from something basal to a sponge and the ctenophores.