Octopuses are doomed to be orphans from an early age. After a female octopus lays her eggs, she stops eating and begins to self-harm, ripping off her skin and biting off the tips of her tentacles.
By the time a young octopus wriggles out of its egg, its mother is already dead. A few months later, his father will also die.
The short and grim life of the octopus has long fascinated scientists. In 1944, researchers theorized that mating somehow hit a molecular “self-destruct” button within sea creatures.
It’s taken almost 80 years, but this vague assumption is finally taking shape. Researchers recently found that mating appears to alter several critical cholesterol-based biochemical pathways into various hormones in female octopuses.
“We know cholesterol is important from a dietary standpoint, and in different signaling systems in the body as well,” says molecular biologist Z. Yan Wang, who conducted the research at the University of Chicago.
“It’s involved in everything from the flexibility of cell membranes to the production of stress hormones, but it was a big surprise to see it play a role in this life cycle process as well.”
In humans, some cholesterol precursors are toxic at high levels. Genetic disorders that increase cholesterol metabolism can therefore lead to serious developmental and behavioral problems, including repetitive self-harm and eating disorders. Severe cases can even be life-threatening.
The symptoms are eerily reminiscent of female octopuses in their final days, suggesting researchers might be onto something.
It’s taken years to get here, and in large part, it’s because of an underappreciated little organ found in octopus and squid.
In 1977, researchers discovered that the optic gland somehow played a role in the programmed death of an octopus.
This organ is similar to the pituitary organ in humans. It is found between the eyes of the octopus and is linked to the sexual development and aging of cephalopods. When removed from a female octopus, the creature lives for several months after laying its eggs.
In 2018, scientists took this knowledge and sequenced RNA from two optic glands of two female octopuses at different stages of decline.
As an octopus neared death, the authors noticed higher levels of activity in several genes that control sex hormones, insulin-like hormones, and cholesterol metabolism.
Now, a few years later, some of the same researchers have directly analyzed the molecules secreted by this organ in mated females and unmated females.
After mating, it seems that the optic gland really secretes more sex hormones, insulin-like hormones and cholesterol precursors.
These three molecules could ultimately contribute to signaling systems that trigger death. Or maybe it’s just the buildup of these molecules in the octopus’ body that’s lethal, as it is in humans.
While the optic gland has previously been linked to the production of sex hormones in cephalopods, the other two pathways have only recently been identified in the “self-destruct” sequence.
In the future, Wang and his colleagues hope to look further “downstream,” to see what other molecules are part of this oddly timed death.
“What is striking is that [octopuses] go through this progression of changes where they seem to go crazy right before they die,” says neurobiologist Clifton Ragsdale from the University of Chicago.
“It might be two processes, maybe three or four. Now we have at least three seemingly independent pathways to steroid hormones that could account for the multiplicity of effects these animals show.”
Wang says she’s particularly pleased that two of the pathways identified by her team are known from other rodent studies.
“[N]Now our study shows that these pathways are probably also present in octopuses,” says Wang.
“It was really exciting to see the similarity between such different animals.”
The study was published in Current biology.