by Greg Miller
Cephalopods—octopuses, squid, and their relatives—ruled the seas in the Cambrian era, some 500 million years ago. But their world changed in a big way with the Cambrian Explosion, a rapid diversification of life on Earth that included the origin of fish. Suddenly, cephalopods had new opportunities—delicious fish!—and their first serious competition and potential predators. They had to get smart in a hurry.
So it’s no wonder then that modern cephalopods have the most complex brains of any invertebrates. An octopus brain (lower, right) has 50 to 75 lobes and at least as many neurons (about 100 million) as a mouse brain (lower, left). And that’s not counting the smaller "brains" in each arm and the still smaller "brains" (ganglia, technically) associated with each sucker.
Although the octopus brain rivals the size and complexity of many vertebrate brains, its architecture differs dramatically. “Short of martians showing up and offering themselves up to science, cephalopods are the only example outside of vertebrates of how to build a complex, clever brain,” says neuroscientist Cliff Ragsdale of the University of Chicago in Illinois. For that reason, Ragsdale says, these creatures have much to teach us about brain evolution.
Ragsdale and his postdoctoral fellow Shuichi Shigeno are investigating whether the neural circuits that control movement, memory, and other functions in an octopus brain work the same way as do the analogous circuits in other animals. The British zoologist J. Z. Young did early work on octopuses in the 1960s, mapping out their brain anatomy and ablating individual lobes to investigate their contributions to behavior. But as of yet, Ragsdale says, no one has brought modern molecular methods to bear on these questions.
That’s what Shigeno has begun doing. At the Society for Neuroscience annual meeting in Chicago earlier this week, he presented preliminary findings from a series of experiments that investigated gene expression in the developing brain of Octopus bimaculoides, a small octopus that inhabits mud flats along the southern California coast. Shigeno found that several genes with well-known roles in patterning “lower” brain regions such as the brain stem and spinal cord in mice are expressed in a similar pattern in the developing octopus brain.
Genes involved in the development of “higher” brain regions in mice, on the other hand, showed more diverse patterns in the octopus brain. Genes expressed in the mouse hypothalamus, an area that regulates sleep and appetite, among other functions, also showed up in a lobe of the octopus brain thought to perform analogous tasks. But genes expressed in the mouse hippocampus and cerebral cortex, although also expressed in the octopus brain, had a distribution that bore little resemblance to their distribution in mice. To Shigeno, the findings suggest that lower levels of the octopus brain may be wired much like the analogous circuits in the vertebrate brain, but higher levels may show more divergence from the vertebrate wiring diagram.
Ragsdale agrees, but he cautions that such studies of gene expression tell only part of the story. He and Shigeno already have other experiments in the works to map out neural circuits in the octopus brain more directly and to study the neurochemistry of neurons in areas such as the hypothalamus analog to see if they secrete the same signaling peptides as do hypothalamic neurons in other animals. “We have a lot to learn about these animals,” Ragsdale says.
Photo credits: Cliff Ragsdale and Suichi Shigeno