Those Simple, Complicated Jellies

A bunch of zoo (well, aquarium) shots today:

Lion's Mane Jelly (Cyanea capillata), Shedd Aquarium, 1/20/2013 

This is a Lion's Mane Jelly.

Moon Jelly (Aurelia aurita), Denver Aquarium, 12/30/2013

This one's a Moon Jelly.

Blue Blubber Jelly (Catostlyus mosaicus), Shedd Aquarium, 1/20/2013

A bit different, this one's a Blue Blubber Jelly.

The next two are both Sea Nettles:



Northeast Pacific Sea Nettle (Chrysaora fuscescens), Shedd Aquarium, 1/20/2013

Japanese Sea Nettle (C. pacifica), Denver Aquarium, 12/30/2013

Jellies are members of the phylum Cnidaria, characterized by radial symmetry, a diploblastic (two embryonic tissue layers -- you had three) organization, and stinging cells in the tentacles. It's hard to imagine animals more distinct from, say, mammals, and yet...

 Simplified phylogeny of the Animalia 

 

There are more than 10,000 known Cnidarians (that number includes corals, hydras, and sea anemones as well as jellies), and who knows how many unknown species. (Species counts of marine animals are always a bare minimum, given how little attention we pay to most of the oceans.) As might be expected, phylogenies of the animal kingdom consistently show them branching off from near the base of the tree (only the sponges split off earlier). Given their very simple structure and their position at the base of the tree, you might think that they would have very simple genomes. But you'd be wrong -- they actually have more genes than Drosophila melanogaster, the world-famous fruit flies that formed the basis for so much of our understanding of genetics. (1) In addition to sharing versions of most of our basic developmental signaling genes (Wnt, hox), they share genes otherwise only known in some protists, and one otherwise only known from chordates. (2) All of this indicates that a lot of animal evolution has involved losing genes from an ancestrally very complex system -- not what most people would have predicted just 20 years ago!

Despite conserving so many ancient genes, they do have some unique features: no other animal phylum shows radial symmetry (Echinoderms show a modified bilateral symmetry that closely resembles radial), and their stinging cells appear to be an entirely novel adaptation. This really shouldn't surprise us, since that early date for their split indicates that they have been evolving on their own for around 600 million years -- if they hadn't evolved unique features over that length of time, then we would have some serious explaining to do. And yet, it's so easy to treat these "simple" animals as "primitive" and assume that they can serve as a stand-in for those earliest animals.

Some creationists like to throw around the question "If we came from monkeys, why are there still monkeys?" Here we can recast the question as "If we came from cnidarians, why are there still jellyfish?" In both cases, the answer is that we shared a common ancestor millions of years ago, our ancestral line split from theirs, and both lines have been evolving their own unique features ever since. Cnidarians aren't primitive proto-metazoans, and Chimpanzees aren't primitive proto-humans; they're both vibrant, living creatures still evolving in an ever-changing environment. Treating them as anything else is bound to trip us up when we're thinking about life here on our Earth.

(1) Guder, C., Philipp, I., Lengfeld, T., Watanabe, H., Hobmayer, B., & Holstein, T. W. (2006). The Wnt code: cnidarians signal the way. Oncogene, 25(57), 7450-7460.

(2) Technau, U., Rudd, S., Maxwell, P., Gordon, P. M., Saina, M., Grasso, L. C., ... & Miller, D. J. (2005). Maintenance of ancestral complexity and non-metazoan genes in two basal cnidarians. TRENDS in Genetics, 21(12), 633-639.