The cephalopod’s genome reveals how the creatures evolved intelligence to rival the brightest vertebrates.
We humans think we’re so fancy with our opposable thumbs and capacity for complex thought. But imagine life as an octopus … camera-like eyes, camouflage tricks worthy of Harry Potter, and not two but eight arms – that happen to be decked out with suckers that possess the sense of taste. And not only that, but those arms? They can execute cognitive tasks even when dismembered.
And on top of all that razzmatazz, octocpuses (yes, "octopuses") have brains clever enough to navigate super complicated mazes and open jars filled with treats.
The octopus is like no other creature on this planet. How did these incredible animals evolve so spectacularly from their mollusk brethren? Scientists have now analyzed the DNA sequence of the California two-spot octopus (Octopus bimaculoides) and found an unusually large genome. It helps explain a lot.
“It’s the first sequenced genome from something like an alien,” says neurobiologist Clifton Ragsdale of the University of Chicago in Illinois, who co-led the genetic analysis, along with researchers from the University of Chicago, the University of California, Berkeley, the University of Heidelberg in Germany and the Okinawa Institute of Science and Technology in Japan.
“It’s important for us to know the genome, because it gives us insights into how the sophisticated cognitive skills of octopuses evolved,” says neurobiologist Benny Hochner who has studied octopus neurophysiology for 20 years.
As it turns out, the octopus genome is almost as large as a human’s and actually contains more protein-coding genes: 33,000, compared with fewer than 25,000 in humans.
Mostly this bonus comes from the expansion of a few specific gene families, Ragsdale says.
One of the most remarkable gene groups is the protocadherins, which regulate the development of neurons and the short-range interactions between them. The octopus has 168 of these genes – more than twice as many as mammals. This resonates with the creature’s unusually large brain and the organ’s even-stranger anatomy. Of the octopus's half a billion neurons — six times the number in a mouse – two-thirds spill out from its head through its arms, without the involvement of long-range fibers such as those in vertebrate spinal cords.
A gene family that is involved in development, the zinc-finger transcription factors, is also highly expanded in octopuses. At around 1,800 genes, it is the second-largest gene family to be discovered in an animal, after the elephant’s 2,000 olfactory-receptor genes.
Not surprisingly, the sequencing also revealed hundreds of other genes specific to the octopus and highly expressed in particular tissues. For example, the suckers express a unique set of genes that are similar to those that encode receptors for the neurotransmitter acetylcholine. This may be what gives the octopus the spectacular characteristic of being able to taste with its suckers.
The researchers identified six genes for the skin proteins known as reflections. As their names suggests, these alter the way light reflects from the octopus allowing for the appearance of different colors, one of the tricks an octopus uses – along with changing its texture, pattern or brightness – in their mind-blowing ability to camouflage.
When considering the creature’s extraordinary learning and memory capabilities, electrophysiologists had predicted that the genome might contain systems that allow tissues to rapidly modify proteins to change their function; this was also proven to be the case.
The octopus’s position in the Mollusca phylum illustrates evolution at its most spectacular, Hochner says.
“Very simple mollusks like the clam – they just sit in the mud, filtering food," he observes. "And then we have the magnificent octopus, which left its shell and developed the most-elaborate behaviors in water.”