<p class="first" id="d8754877e71">Human and octopus lineages are separated by over
500 million years of evolution [1,
2] and show divergent anatomical patterns of brain organization [3, 4]. Despite these
differences, growing evidence suggests that ancient neurotransmitter systems are shared
across vertebrate and invertebrate species and in many cases enable overlapping functions
[5]. Sociality is widespread across the animal kingdom, with numerous examples in
both invertebrate (e.g., bees, ants, termites, and shrimps) and vertebrate (e.g.,
fishes, birds, rodents, and primates) lineages [6]. Serotonin is an evolutionarily
ancient molecule [7] that has been implicated in regulating both invertebrate [8]
and vertebrate [9] social behaviors, raising the possibility that this neurotransmitter's
prosocial functions may be conserved across evolution. Members of the order Octopoda
are predominantly asocial and solitary [10]. Although at this time it is unknown whether
serotonergic signaling systems are functionally conserved in octopuses, ethological
studies indicate that agonistic behaviors are suspended during mating [11-13], suggesting
that neural mechanisms subserving social behaviors exist in octopuses but are suppressed
outside the reproductive period. Here we provide evidence that, as in humans, the
phenethylamine (+/-)-3,4-methylendioxymethamphetamine (MDMA) enhances acute prosocial
behaviors in Octopus bimaculoides. This finding is paralleled by the evolutionary
conservation of the serotonin transporter (SERT, encoded by the Slc6A4 gene) binding
site of MDMA in the O. bimaculoides genome. Taken together, these data provide evidence
that the neural mechanisms subserving social behaviors exist in O. bimaculoides and
indicate that the role of serotonergic neurotransmission in regulating social behaviors
is evolutionarily conserved.
</p>