The Crazy Cucumber from the Sea! And its Even Crazier Nervous System!!

So, I know the Great and Powerful Oz PZ likes his cephalopods. And I know cephalopods are really cool, because they can be very intelligent using an apparently simpler nervous system than we vertebrates have. But I think my fellow vertebrates are ignoring our crazy cousins here. I’m speaking of course about the echinoderms, which are (among the bilateria) very closely related to us. We are ignoring the amazing phenomenon that is our fellow deuterostome clade. What is so amazing about these creatures with so few sensory organs, let alone anything approaching sentience?

They are pentaradial, man, that’s what!

No joke, that’s really cool. They start off life with a body plan of bilateral symmetry, like any other balaterian, but by the time they are adults, they’ve switched to a body plan of five-fold radial symmetry. Their nervous systems consist primarily of five radial nerves extending from a nerve ring around the oral region. That is a crazy switch. If you’re curious, here is what some researchers in Japan did to investigate this, using sea cucumbers. I found this so cool, I wrote a 1500 word paper on the topic. Also, the paper counted as the take home exam PZ gave us.

Basically, sea cucumbers, or holothurians, are thought to be a great model organism for echinoderms, because their second larval stage is similar to the larval echinoderms found in the fossil record. It has what the researchers call a “barrel” shape to it, with four or five ciliary band nerve tracts that each run the circumference of the body. It’s called the doliolaria, and it is not present in most modern echinoderms.

Nervous system layouts in the auricularia and doliolaria stages

Shifting of the nerve tracts between larval stages results in the connected ciliary rings.

The researchers cultured stocks of the holothurian Stichopus japonicusand ran immunological assays on whole larvae at different points in development. Between the first larval stage (auricularia) and doliolaria, the bilateral nerve tracts shift to form the ciliary bands, but the connections between the bands remain functional. It’s quite interesting, but aside from the main attraction.

What is really interesting is how the adult nervous system forms. What happened in this study was that eventually, the larval nervous system ceased to be immunoreactive. This happened gradually beginning from the posterior end. Even before this started, however, and within the doliolarial form, a new nerve ring began to form that matched the shape of the oral ciliary band. Interestingly enough, however, this new nerve ring, which quickly began developing the radial processes typical of the adult nervous system, demonstrated a different immunoreactivity than any part of the larval nervous system.

The antibodies used were of two sera known to react in general to echinoderm neural cells. Another marker used was an antiserotonin antibody, to detect serotonergic ganglia typical of the echinoderm larval stage. One set of nerve tracts, the larval form, reacted to the 1E11 antiserum, and the anterior ganglia were bound by serotonin antiserum. The new ring, however, bound the 1E9 antiserum, and neither of the other two, giving evidence that it is a completely different structure.

A, C-F) 1E9 antiserum binds to the newly forming adult nervous system. B) Anti-serotonin binds to anterior band of larval nervous system.

Now you can see why echinoderms are so amazing. It appears that when they are transitioning into juvenile or adult form, they grow an entirely new nervous system. That’s right, they don’t stop at rearranging their existing nervous system or expanding on it, it appears they may just use it as a scaffold for a new one altogether. In fact, none of the larval nervous system is known to be present in the adult form. It’s an astonishing metamorphosis, and I had no idea until this week how dynamic these creatures were.

Works Cited

Nakano, H., Murabe, N., Amemiya, S., Nakajima, Y. 2006. Nervous system development of the sea cucumber Stichopus japonicas. Dev. Biol. 292(1): 205-212.

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