by Matthew Cobb
Ctenophores (“teen-o-forz”), or ‘comb jellies’ are some of the most beautiful organisms in the ocean. They are generally transparent, and biradially symmetrical, so they have infinite planes of rotational symmetry. As if that wasn’t enough, they have beautiful ‘combs’ along their edges, which shimmer in the light as they move the animal around, using simple muscle cells. This gives you an idea of how beautiful they are:
Their prey, however, may not find them so beautiful, and their eating habits are anything but elegant. Here’s a video of ctenophores (Bereo ovata, apparently) just swallowing other ctenophores (Mnemiopsis macrydi). The mouth on the first snarf looks so human-like I wondered if it wasn’t CGI, but it isn’t. It’s just life in all its gory glory:
Ctenophores are not jellyfish. They don’t sting and they are never colonial organisms. Strikingly, they have a peripheral nerve net and can respond to gravity and perhaps light. This video filmed in the Arctic sea off Svalbard in Norway shows that they can separate the wheat from the chaff when it comes to eating their prey:
True facts about the ctenophores:
• 150-200 species
• Found at a range of depths
• Grow from 2mm – 2m in size
• They have a mouth at one end, and an apical sense organ at the other
• They have a blind gut (which you can see on these videos)
• They eat zooplankton, small fish, and even ctenophres
• They are selfing hermaphrodites (ie they do it with themselves)
• The fertilized eggs are released into the water and develop into mini-adults in 24 hours
In 2008, a paper in Nature suggested that the ctenophores might be on a different branch of life from the rest of the animal kingdom. However, they authors themselves said:
The placement of ctenophores (comb jellies) as the sister group to all other sampled metazoans is strongly supported in all our analyses. This result, which has not been postulated before, should be viewed as provisional until more data are considered from placozoans and additional sponges. If corroborated by further analyses, it would have major implications for early animal evolution, indicating either that sponges have been greatly simplified or that the complex morphology of ctenophores has arisen independently from that of other metazoans.
Nature 452, 745-749(10 April 2008)
An initial study of the first ctenophore genome to be sequenced Mnemiopsis leidyi (a close relative of the victim in the video above) suggested that sponges and ctenophores first branched off from the rest of the animal kingdom (Ryan et al, 2010):
h/t @hannahjwaters on Twitter
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I thought Jeebus said, “The smallest seed is the mustard seed.” The seed of these ctenophores gotta be a lot lot smaller than that. How would Jeebus not KNOW that!!?? And with so many iterations of ctenophores? (<great word, but I'll never remember it)
Matthew Cobb and by extension,Jerry, thanks again for uplifting our level of biological knowledge. The real world…fantastic!!
I guess if Dick Cheney insulted a ctenophore it wouldn’t have much of an effect. In fact, he might be thanked for the suggestion.
Would an intelligent ctenophore make a distinction between ordinary reproduction and masturbation?
Uh, no, that’s not what the term means, and it’s not what the animals are. If you look at the videos or the picture on top, you can see that, if you’re strict, there’s only one plane of symmetry (hence the bi- part, instead of just “radial symmetry”), and if you’re less strict, there are about 4 that intersect along the long axis of the animal.
Why is that striking? The same is true of cnidarians (the “nettling” animals: corals, jellyfish, cube jellies and the like).
Bottom Diagram:-
Can someone translate the info in the three rectangles down the left side please?
Second the request.
The rectangles containing “TALE”, various including “ANTP”, and more various including “Hox/ ParaHox”?
I don’t know what the “TALE” is.
ANTP is an abbreviation for a gene involved in the development of limbs and limb-like structures, “ANTENNEPEDIA”. IIRC it’s effect was first appreciated from Drosophila mutants with messed up or absent ANTennae, giving one half of the name, and something going on with their legs, “PEDIA” (feet, in painful Latin), giving the other half of the name.
“Hox” genes are involved in managing the development of body structures along the body axis, resulting in among other features, “segmentation”. The majority of animal forms on Earth are segmented – look at the repeated structures of a worm, an insect, or the spine of a vertebrate : all segmentation, and all controlled by Hox (or the derived “Parahox”) genes.
So, what I’m inferring from that diagram is that Porifera and Ctenophora have the ANTP gene (family), but the Cnidaria, Placozoa and other phyla of multi-celledsy animals (“Metazoa”) have also developed the Hox/ Parahox mechanism.
Which is surprising. Previously to this work, on gross morphology grounds, the Cnidaria and Ctenophora were tentatively grouped together as being primarily radially symmetrical (echinoids are secondarily radially symmetrical ; their larvae are bilaterally symmetrical) while Placozoa are just plain asymmetrical and everything else (including echinoids) has a bilateral symmetry.
Looking from a different direction though, there are hints to the phylogeny shown. The Cnidaria have a characteristic of forming a stack of sessile (welded to the sea floor) “medusoid” larvae, which then individually separate from the top of the stack and disperse to the four currents and their individual fates. That’s a very “segmented” lifestyle.
Where the ANTP genes get expressed in Placozoa, I don’t know. They’re decidedly short in apparent anatomy to express anything in! The Ctenophores however do characteristically have a couple of long filamentous tentacles, armed with stinging cells, where I guess that their ANTP is expressed.
Thank you!
a few inaccuracies:
All of the acronyms and abbreviations in the boxes represents a family of genes, all from the large homeobox superfamily. These are developmental switches, and the Hox genes (one family) are indeed involved with anterior-posterior organization of bodies and the proximal-distal organization of appendages, but this is true in all bilaterians, not just the segmented ones.
The relatively few homeobox genes in cnidarians organize the oral-aboral axis. The most basal group of cnidarians completely lacks medusae.
Some ctenophores have feeding tentacles, but most do not, and those that do lack stinging cells but instead have sticky ‘glue cells’.
All adult echinoderms, not just echinoids, are basically (but not quite) secondarily (penta-)radially symmetrical, except for some sea cucumbers which seem to be re-evolving bilateral symmetry.
Corrections accepted.
My biology comes from looking at fossils, so it’s a bit rough and ready.
I’m trying to remember where the Chancellorids are placed phylogenetically ; ISTR that they were an offshoot or sister group of the Echinodermata which are asymmetrical. But that’s all getting a bit messy.
Oops. I forgot to mention that ctenophores, unlike cnidarians, don’t quite have a blind gut: they have “anal pores” at the end that’s opposite the mouth.
The bottom diagram is a phylogenetic tree, with the ancestor at the left end and time progressing toward the right. Each box lists genes that evolved somewhere along the internodes shown.
That’s now how I’d put it (and I’m a phylogeneticist myself). Phylogenetic trees don’t have a trunk. The paper found that there are three branches (sponges, ctenophores, everything else) and can’t resolve which two of these are each other’s closest relatives.
For me, the “everything else” branch is a bit of a surprise; I didn’t expect it to contain the placozoans. But there are other papers that support other arrangements… for instance, I’m told there are some recent ones that, likewise based on molecular data, find Ctenophora and Cnidaria as sister-groups – the classical idea from the 19th century. *shrug*
Nice, Matthew.
Note that the shimmering ‘combs’ are plates of cilia fused together and beating together. Organelles, not “simple muscle cells”
“..they have beautiful ‘combs’ …they move the animal around, using simple muscle cells”
‘Grow from 2mm – 2m in size’
2 metres? That thing could swallow people!
the long ones are the belt-like “Venus’s girdles” that couldn’t swallow much of anything.
Ah. It said ‘size’ not ‘length’ so I just imagined it scaled-up. I’m really rather disappointed, a 2-metre Bereo would be amazing.
Its surprisingly difficult in the clips to guess which one is going to eat the other.
Incidentally, prior to this post I had never even heard of Placozoa. It feels as though a new continent has been discovered and I’ve only just noticed.
It’s as if a new continent was discovered, and it turned out to be The Seychelles.
I’m curious to know how they digest such large prey items. Do they secrete enzymes into the ‘stomach’, or what?
yes
“The mouth on the first snarf looks so human-like I wondered if it wasn’t CGI”–it even blew a raspberry at the camera for good measure! Wonderful videos.
‘biradial’ means 2 planes of symmetry, not infinite, not one.