As in vertebrates, octopus brains contain high levels of regulatory RNA molecules, the paper – available as a preprint at bioRxiv and yet to be peer-reviewed – finds. This could help them develop lots of different neurons and greater neuronal complexity.
“We show that the major RNA innovation of soft-bodied cephalopods is a massive expansion of the microRNA gene repertoire,” the authors write.
“The only comparable [microRNA] expansions happened, strikingly, in vertebrates. Thus, we propose that [microRNAs] are intimately linked to the evolution of complex animal brains.”
MicroRNA (miRNA) – not to be confused with the similarly named mRNA – don’t code for proteins, leading to the somewhat unflattering nickname “junk DNA”. Far from useless, they regulate gene expression by binding to mRNA and preventing it from being translated to protein.
A high number of miRNA allows organisms greater control over gene expression. A cephalopod with lots of miRNA – the study found 51 have been conserved in octopus and squid since their lineages diverged 300 million years ago – therefore may be able to generate more types of neuron, which could explain the complexity of their brains.
The team studied 18 tissues of the common octopus, initially speculating that RNA editing may underpin their impressive cognitive abilities. However, they found no significant differences between octopus and other invertebrates, although they concede some editing may be relevant in individual cases.
Instead, the teeny tiny miRNA emerged as a factor potentially contributing to octopus brain complexity. The authors also examined miRNA expression patterns, finding that they are expressed most strongly in nervous tissues and are present in the brains of developing octopus hatchlings.
“Together, our data suggest that indeed novel [soft-bodied cephalopod] miRNAs contribute to the development of the octopus brain.”
Octopus are renowned for their smarts – although not all of them are quite so clever. The size and texture of their brains can vary, suggesting they might not all possess the same guile.
However, those that do can be found putting their intelligence to good (read: antisocial) use, punching fish, throwing debris, and displaying some impressive escape artistry. They are capable of many other complex behaviors, including potentially feeling physical and emotional pain and maybe even dreaming. As a result, they are now recognized as sentient beings in the UK, alongside squid and lobsters.
This intelligence is unusual among invertebrates – and now, we may have a better understanding of what might underpin it.