Big brains and bones go hand-in-hand in biology, but the octopus is an exception. For the first time, scientists have fully sequenced the octopus genome, revealing clues about how these invertebrates became so intelligent.
With sophisticated eyes similar to our own, large brains, color-changing skin and eight prehensile arms covered in chemical-sensing suckers, octopuses are a strange patchwork of traits. It turns out that their genetics are just as eclectic, according to a paper in the journal Nature describing the genome of the two-spot octopus, Octopus bimaculoides.
"With a few notable exceptions, the octopus basically has a normal invertebrate genome that's just been completely rearranged, like it's been put into a blender and mixed," said co-lead author Caroline Albertin of the University of Chicago in a statement.
One of those notable exceptions is the presence of a large family of genes key to brain development that were previously only ever observed in vertebrates. In particular, a family of genes involved in short-rage interactions between neurons was much larger than in any other animals. Octopuses lack myelin coatings on their neurons, which are critical for long-range interactions — so it appears that they compensated for this by beefing up the efficiency of the short-range interactions between their neurons.
The entire octopus genome is nearly as large as that of a human in terms of base pairs – the sequence of "letters" that make up the genetic code – and some octopus species have even larger genomes. Since many components of the genome are not actually genes, this species of octopus actually has more genes than humans do — even though its full genome is smaller.
Having a full octopus genome to explore has provided some answers about these bizarre creatures, but many questions remain.
"The octopus appears to be utterly different from all other animals, even other mollusks, with its eight prehensile arms, its large brain and its clever problem-solving capabilities," said co-senior author Clifton Ragsdale of the University of Chicago in a statement. "The late British zoologist Martin Wells said the octopus is an alien. In this sense, then, our paper describes the first sequenced genome from an alien."
It won't be the last. The Cephalopod Sequencing Consortium is already working on sequencing the genomes of other cephalopods such as cuttlefish, squids and nautiluses.
