Lauren Vandepas: "Behold, comb jelly poo!"

The ctenophore (comb jelly) species used in this study 
were the lobate ctenophore Mnemiopsis leidyi 
and the cydippid Pleurobrachi bachei
Does everybody poop? That an animal poops may seem like a no-brainer – food goes in, gets digested, and whatever wasn’t digested comes out, right? There’s a mouth for food intake and a, you know, a butt, that lets out food waste. A lot of animals don’t have a set up as “complicated” at this; scattered throughout the animal kingdom, there are lineages that don’t have a secondary digestive opening (an anus, or a BUTT). In these animals, food is ingested through the mouth, processed by the gut, and whatever hasn’t been digested gets expelled back through the mouth. The anus may not seem like the end-all (ha!) innovation during animal evolution, but it’s actually a pretty big deal. Some evolutionary biologists consider the emergence of the through-gut (a digestive tract featuring a mouth and an anus) to be one of the drivers of animal body plan diversification – perhaps allowing for more efficient food processing, or providing a stable sort of scaffold to build on.

Bilateral symmetry, i.e. having a left-half and a right-half of the body – was another animal innovation that arose after animals had been around for millions of years already. On the oldest branches of the animal tree of life the four non-bilaterian phyla, which split off from the rest of the animal tree prior to the appearance of animals with bilateral symmetry, process food in vastly different ways. Cnidarians (the phylum that includes sea anemones and jellyfish) have a single opening that serves as both a mouth and anus, called a “blind gut”. Sponges and placozoans (tiny animals that look like amoebas) don’t have guts at all. Comb jellies, or ctenophores, have been kind of a question mark as far as gut classification – in the mid-1800s some scientists reported seeing ctenophores expelling some kind of waste from two pores located opposite the mouth, though this observation was contentious in the field even back then. More recently, the literature about ctenophore digestion has been pretty mixed, with some biologists concluding that ctenophores have a blind gut similar to cnidarians, while others have been unsure of how to classify the ctenophore digestive tract. Maybe it’s a through-gut, maybe it’s something in between a through-gut and a blind gut, who knows –  ctenophores are weird. (They are really, really weird).
Two possible hypotheses of through-gut evolution. The top phylogenetic tree shows that a functional through-gut may have been developed in multiple lineages independently. The bottom tree shows is a hypothesis of a single emergence of an animal through-gut in the animal ancestor that gave rise to the ctenophore lineage, with subsequent losses of the through-gut in cnidarians, sponges, and placozoans. We currently don't know which of these, or neither of them, is how the through-gut actually arose.

In a study published in Current Biology this year, my colleagues and I demonstrated that comb jellies do, in fact, definitely poop. They have an anus! They’re even overachieving in the anus department – they have two anal pores. At the University of Miami Professor Bill Browne and his grad student, Jason Presnell, saw ctenophore pooping in action while they were observing their animals. While taking a larval invertebrate class in Panama, my course mini-project on how ctenophore larvae feed went in an unexpected direction when I watched the larvae expel digested shrimp through their anal pores. What. We discussed these odd observations we’d made, and after seeing how varied the literature was with respect to what the heck ctenophores are using their aboral pores for, we decided to tackle the mysterious “poo” problem.
Diagram showing ctenophore digestion in three parts:
ingestion of prey, break-down and distribution of digestible food,
and expulsion of undigested wastes.

Bill, Jason, and I wanted to show that two different ctenophore species that aren’t closely related use their anal pores in the same way – as anuses. We fed the jellies fluorescent shrimp or fish and watched the food pass through the ctenophores’ guts and out the anal pores. Behold, comb jelly poo! Ctenophores have a mouth on one end and anal pores at their other end – a through-gut! Demonstrating that through-guts aren’t just for bilaterian animals raised some interesting questions about when through-guts appeared during animal evolution. It’s already widely thought that this trait arose separately several times in different lineages within bilaterians. Ctenophores split off way before bilaterians evolved – do they have the same type of through-gut as bilaterians? Are ctenophore through-guts built in the same ways as bilaterians’ during embryonic development? Is this another independent evolution of the through-gut? We don’t yet have an answer for how homologous the ctenophore through-gut is to other animals’, but we’re looking at what genes ctenophores use to form their gut during development, and examining their digestive cells for clues.

Contrary to the popular belief that “Everybody Poops”, not all animals can – but ctenophores definitely do.

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