The blue-lined octopus (Hapalochlaena fasciata) is only a few centimeters long, yet it may be one of the most dangerous marine animals on Earth.
That’s because, as part of the blue-ringed octopus genus, it’s heavily armed – not just with eight arms, but also with tetrodotoxin (TTX), a potent neurotoxin.
The mantles of female blue-lined octopuses can grow about as big as a golf ball, roughly twice the size of males. This size discrepancy can be life-threatening for a male, whose instinct to mate puts him in danger of being eaten by a much larger female.
According to a new study, however, male blue-lined octopuses have an effective survival tactic. Before attempting to mate, they deliver a precise bite near the female’s aorta, injecting TTX and temporarily paralyzing her.
Many octopus species are sexually dimorphic, with big females who sometimes cannibalize their tiny mates, the study’s authors note. This may favor the survival of males with bodies or behaviors that prevent them from being eaten.
Males of some species have evolved an extra-long hectocotylus, a specialized arm for transferring spermatophores, to keep themselves at a distance from females during mating. Others just detach their hectocotylus, sacrificing it so they can flee.
But male blue-lined octopuses don’t have an elongated mating arm, nor can they shed theirs. They have been seen trying to wrap females in their small arms while mounting them from behind, the authors write, but given the females’ relative strength, it’s unlikely that alone would save them.
Both sexes have TTX in their saliva, which they use to thwart predators and immobilize prey. While males may only grow to half the size of females, however, their posterior salivary glands – where symbiotic bacteria produce TTX – are three times heavier than those of similar-sized females, the researchers report.
This disproportionate enlargement of males’ salivary glands “likely co-evolves between demand for protection/predation and mating,” they write.
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To learn the males’ survival trick, the researchers placed six pairs of male and female blue-lined octopuses in tanks and observed their mating behavior.
All female octopuses “quickly succumbed to the males while clasped on the mantle,” the researchers write. Pairs remained stationary for about an hour.
Breathing rates while mating varied dramatically by sex. Males started at 20 to 25 mantle contractions per minute when resting, then increased to between 35 and 45 contractions per minute during copulation, the researchers report.
Females, on the other hand, experienced a sharp drop in breathing rate when mating began, the study found. They then stopped breathing altogether after about eight minutes, although none died.
While in this condition, females turned pale and their pupils constricted, losing all reflexes to bright flashes of light. It looked like loss of nerve control, the researchers explain, which happens in tetrodotoxin envenomation.
“Once the females were immobilized, the males successfully copulated, and mating ended when the females regained control of their arms and pushed the males off,” the researchers write.
Afterward, the researchers found one or two small, swollen lumps at the backs of the female octopuses’ heads, near the aorta. An open wound was visible at the site of swelling, they write, “providing physical evidence of biting.”
One of the males apparently bit too far to the left and missed his mate’s aorta, the researchers note. This female was still immobilized, they report, but she awoke after 35 minutes, earlier than all the others.
“Therefore, despite not having any direct measurements of TTX in the body of females after male biting behavior, these findings strongly suggest the co-option of the venom for mating in this species,” they write.
“It also suggests a co-evolutionary arms race between the sexes, whereby a cannibalizing large female is counteracted in males through the use of venom.”
Since none of the females died during mating, it seems they may have some level of resistance to tetrodotoxin.
A variety of animals accumulate this bacterial toxin in their bodies, including mollusks, fish, and amphibians. More in-depth studies are needed to investigate the different ways animals are using it, the researchers write.
The study was published in Current Biology.
