Southern Fried Scientist decided to feature a week of surreal science related to the oceans. I take this opportunity to be a lazy blogger and repost this piece (slightly rewritten) from May 2008.
Adult sea squirts (also known as tunicates or ascidians) are sessile animals. As adults, they really don't move. But if anyone has heard about sea squirts, they’ve probably hear that little sea squirts start life as smart little tadpoles, searching this way and that for a place to land. Once they’ve found the place where they'll spend the rest of their lives, they go through a metamorphosis into the immobile adult.
But as they have no further need of their brain, they 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.
Mackie GO, Burighel P. 2005. The nervous system in adult tunicates: current research directions. Canadian Journal of Zoology 83(1): 151-183. DOI: 10.1139/z04-177
Meinertzhagen IA, Okamura Y. 2001. The larval ascidian nervous system: the chordate brain from its small beginnings Trends in Neurosciences 24(7). 401-410. DOI: 10.1016/S0166-2236(00)01851-8