After a female octopus lays a clutch of eggs, she does what any new parent would: carefully watch over her progeny. She stays with them in her den, protects them from predators and blows water over them to keep them oxygenated, writes Nicholas Bakalar for the New York Times.
But then her behavior turns bizarre; she stops eating and begins to self-mutilate, tearing off her skin or even eating her own arms. She’s dead before the eggs can hatch.
Scientists have known that the animal’s optic glands are responsible for this behavior. When the glands are removed, the octopuses resume eating and live months longer. But just how these glands trigger the animal’s gruesome death has been a mystery.
Now, in a new study published in Current Biology, researchers describe changes to a series of biochemical pathways that happen after mating and may be responsible for the animal’s self-destruction. One of these changes leads to an increase in 7-dehydrocholesterol (7-DHC), a precursor to cholesterol.
“We know cholesterol is important from a dietary perspective, and within different signaling systems in the body too,” says Z. Yan Wang, a professor of psychology and biology at the University of Washington and lead author of the study, in a statement. “It’s involved in everything from the flexibility of cell membranes to production of stress hormones, but it was a big surprise to see it play a part in this life cycle process as well.”
The authors examined optic glands and optic lobes in both mated and unmated female California blue-spot octopuses, writes Christa Lesté-Lasserre for New Scientist. In their paper, the scientists describe three chemical pathways that increase steroid hormones after reproduction. The first pathway produces pregnancy hormones. The other two produce 7-DHC and bile acid intermediates, neither of which were previously known to be involved in semelparity—the reproductive strategy in which an animal has just one reproductive episode before dying.
“This is an elegant and original study that addresses a longstanding question in the reproduction and programmed deaths of most octopuses,” Roger T. Hanlon, a senior scientist at the Marine Biological Laboratory in Woods Hole, Massachusetts, who was not involved in the study, tells the New York Times’ Nicholas Bakalar.
The self-mutilating behavior is particularly odd because larger-brained animals, like octopuses, tend to live longer, per New Scientist.
“What's striking is that they go through this progression of changes where they seem to go crazy right before they die,” Clifton Ragsdale, a professor of neurobiology at the University of Chicago and coauthor, says in a statement. “Maybe that's two processes, maybe it's three or four. Now, we have at least three apparently independent pathways to steroid hormones that could account for the multiplicity of effects that these animals show.”
Some of these cholesterol-producing pathways exist in mammals and rodents. In humans, higher-than-normal levels of 7-DHC are toxic, and a sign of a genetic disorder called Smith-Lemli-Opitz syndrome (SLOS). A mutation in the enzyme that converts 7-DHC to cholesterol causes the disorder, per a statement, and can lead to self-injurious behavior in children.
“The important parallel here is that what we see in humans, as well as in octopuses, is that high levels of 7-DHC are associated with lethality and toxicity,” Wang tells New Scientist. “And that, to me, is really interesting, just because of how evolutionarily divergent these two animals are.”