Bad-Ass Comb Jellies

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Drawing of Pleurobrachia from Eugene Kozloff’s Invertebrate Zoology, showing anal pore on top.

Jellyfish biologists gathered at a meeting called Ctenopalooza in Florida a couple weeks ago, and dumped some big news on the jellyfish world. And in the words of my 11-year old daughter, it left me thinking, “What the poop?”

Back when I first learned about comb jellies in the 80’s they were lumped together with your standard jellyfish in a big group called the coelenterates, which roughly translates to hole inside. Both these types of animals were thought to have a single hole to their bellies, which they used to bring food in and excrete waste out.

Through the years, comb jellies were cleanly severed from jellyfish on the tree of life for lots of reasons. Their microscopic weaponry is really different. Their reproduction is different. Their muscle and nervous systems are very different. Their DNA is dissimilar. And their digestion is too.

As reported in Science, at Ctenopalooza researcher William Browne of the University of Miami in Florida showed footage of red-glowing genetically-modified fish passing through the branched guts of a comb jelly called Pleurobrachia. After two or three hours, tiny pores opened up on the animals’ back ends and undigested bits of glowing poop emerged.

Pretty cool. But not quite as new as Science might have you believe.

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My underlines in Kos’s book circa 1990 show that I once read about the existence of a comb jelly’s anal pores. (What I remembered about them is a different story.)

My Invertebrate Biology textbook written in 1990 reported that comb jellies have tiny openings on their back end called anal pores. It even pointed those holes out on a diagram of Pleurobrachia.

The author Eugene Kozloff wrote, “In terms of their development, these pores are not true anuses, but fluid and indigestible residues do leave the body through them.”

So a quarter of a century ago, comb jellies’ poop was already in textbooks.

Still, the video does put lots of cool questions on the table–not because we suddenly know that comb jellies poop–but because of what we know now about jellyfish evolution.

Recent DNA studies suggest that the comb jellies are the oldest of the animals. So does that mean the regular jellyfish lost the ability to poop after inheriting it from the comb jellies? Or did the comb jellies develop the through-gut later after they diverged from the jellyfish? Did complete guts evolve just once, or many times? And do our own complete guts have any connection to those of the comb jellies?

The technology to video and track glowing food can certainly help answer those kinds of questions. So, you can bet your ass there’s much more to be learned about jellyfish poop. And, holy crap, our notions about how digestion evolved might just be up-ended by comb jellies’ rear-ends.

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