It’s widely thought that sea squirts (also known as tunicates or ascidians), once they’ve found the place where they'll spend the rest of their lives, have no further need of their brain and eat it.
The punchline is, “It’s rather like getting tenure.”
The facts should never get in the way of a great joke, but the truth is more complicated. The swimming tadpoles are only about a millimeter long, and there are only a few hundred neurons in the entire tadpole (Meinertzhagen and Okamura 2001), of which the “brain” is only a small part. Tadpoles have miniaturized brains.
Sea squirt larvae do undergo metamorphosis into a adult with a small brains, but it's not the vestigial little thing that the “eat your own brain” story suggests. “In fact, adult ascidians have perfectly good brains, an order of magnitude larger than those of their larvae, and their behaviour is as finely adapted to sessility as that of the larvae to motility” (Mackie and Burighel, 2005).
We’ve learned a lot about how brains work from invertebrates, and their complexity is often underrated.
Meinertzhagen IA, Okamura Y. 2001. The larval ascidian nervous system: the chordate brain from its small beginnings. Trends in Neurosciences 24(7): 401-410. http://dx.doi.org/10.1016/S0166-2236(00)01851-8
Mackie GO, Burighel P. 2005. The nervous system in adult tunicates: current research directions. Canadian Journal of Zoology 83(1): 151-183. http://pubs.nrc-cnrc.gc.ca/cgi-bin/rp/rp2_abst_e?cjz_z04-177_
26 May 2008
Sea squirt myths busted
I bust another invertebrate neurobiology myth on the All In The Mind blog that crops up in a post about cars, of all things. What follows is a slightly edited version of my comment on the post.