Octopuses react to the popular mood-altering drug called MDMA or "ecstasy" in the same way as we do, according to a study which unveils an evolutionary link between the social behaviours of the sea creatures and humans.
Scientists from the Johns Hopkins University School of Medicine in the US studied the genome of a kind of octopus not known for its friendliness toward its peers, and tested its behavioural reaction to ecstasy.
They found preliminary evidence of an evolutionary link between the social behaviours of the octopuses and humans, species separated by 500 million years on the evolutionary tree.
The findings, published in the journal Current Biology, may open opportunities for accurately studying the impact of psychiatric drug therapies in many animals distantly related to people.
"The brains of octopuses are more similar to those of snails than humans, but our studies add to evidence that they can exhibit some of the same behaviours that we can," said Gul Dolen, an assistant professor at the Johns Hopkins University.
"What our studies suggest is that certain brain chemicals, or neurotransmitters, that send signals between neurons required for these social behaviours are evolutionarily conserved," Dolen said.
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Octopuses, said Dolen, are well-known to be clever creatures. They can trick prey to come into their clutches, and Dolen said there is some evidence they also learn by observation and have episodic memory.
The gelatinous invertebrates (animals without backbones) are further notorious for escaping from their tank, eating other animals' food, eluding caretakers and sneaking around.
However, most octopuses are asocial animals and avoid others, including other octopuses.
Because of some of their behaviours, Dolen still thought there may be a link between the genetics that guide social behaviour in them and humans.
One place to look was in the genomics that guide neurotransmitters, the signals that neurons pass between each other to communicate.
Dolen and Eric Edsinger, a research fellow at the Marine Biological Laboratory in the US, took a closer look at the genomic sequence of Octopus bimaculoides, commonly referred to as the California two-spot octopus.
Specifically, in the gene regions that control how neurons hook neurotransmitters to their membrane, researchers found that octopuses and humans had nearly identical genomic codes for the transporter that binds the neurotransmitter serotonin to the neuron's membrane.
Serotonin is a well-known regulator of mood and closely linked to certain kinds of depression.
The serotonin-binding transporter is also known to be the place where the drug MDMA binds to brain cells and alters mood.
The researchers set out to see how octopuses react to the drug, which also produces pro-social behaviours in humans, mice and other vertebrates.
Dolen designed an experiment with three connected water chambers: one empty, one with a plastic action figure under a cage and one with a female or male laboratory-bred octopus under a cage.
Four male and female octopuses were exposed to MDMA by putting them into a beaker containing a liquefied version of the drug, which is absorbed by the octopuses through their gills.
Then, they were placed in the experimental chambers for 30 minutes. All four tended to spend more time in the chamber where a male octopus was caged than the other two chambers.
"It's not just quantitatively more time, but qualitative. They tended to hug the cage and put their mouth parts on the cage," said Dolen.
"This is very similar to how humans react to MDMA; they touch each other frequently," Dolen said.
Under normal conditions, without MDMA, five male and female octopuses avoided only male, caged octopuses.
The experiments suggest that the brain circuits guiding social behaviour in octopuses are present in normal conditions, but may be suppressed by natural or other circumstances.