Most octopus species live for a year. But the death of octopus mothers after they reproduce has long been a scientific spectacle.
Why exactly octopus mothers engage in a form of self-harm that leads to death just after they reproduce remains a bit of a mystery. But a study published Thursday in the journal Current Biology uses the California two-point octopus as a model to help explain the physiology of this strange behavior.
Z. Yan Wang, an assistant professor of psychology and biology at the University of Washington and an author of the study, explained that the female of the species goes through three reproductive stages.
After parturition, the mother lays her eggs and handles them with care. She takes each egg one by one and threads them carefully into long strands. Then she cements them to the wall of her burrow and stays there, blowing water over the eggs to keep them oxygenated and fiercely protect them from predators.
But then she stops eating. She begins to spend a lot of time away from the eggs. She loses color and muscle tone; her eyes are damaged. Many mothers begin to injure themselves. Some rub against the gravel of the seabed and scars their skin; others use their suction cups to create lesions along their bodies. In some cases, they even eat their own arms.
Scientists have known for some time that the octopus’ reproductive behavior, including death, is controlled by the animal’s two eye glands, which function like the pituitary gland in vertebrates, secreting hormones and other products that regulate various bodily processes. (The glands are called “optic” because of their location between the animal’s eyes. They have nothing to do with vision.) If both glands are surgically removed, the female leaves her brood, starts feeding again, grows and has a longer life span .
The new study describes specific chemical pathways produced by the optic glands that control this reproductive behavior.
One pathway, they found, generates pregnenolone and progesterone, which isn’t surprising since these compounds are produced by many other animals to aid reproduction.
Another produces the precursors of bile acids that promote the absorption of dietary fats, and a third makes 7-dehydrocholesterol, or 7-DHC. 7-DHC is also generated in many vertebrates. In humans, it has several functions, including an essential role in the production of cholesterol and vitamin D. But elevated levels of 7-DHC are toxic and have been linked to conditions such as Smith-Lemli-Opitz syndrome, a rare hereditary disease. characterized by severe intellectual, developmental, and behavioral problems. In octopuses, Dr. Wang and her colleagues believe that 7-DHC may be the essential factor in causing the self-injurious behavior that leads to death.
Roger T. Hanlon, a senior scientist at the Marine Biological Laboratory in Woods Hole, Mass., who was not involved in the study, said that “this is an elegant and original study that answers a long-standing question in the reproduction and programmed mortality of most octopuses.”
dr. Wang said that “the most exciting thing for us was to see this parallel between octopuses, other invertebrates, and even humans.” She added that it was “remarkable to see this shared use of the same molecules in animals that are very distant from each other.”
The molecules may be the same, but death, she said, is very different. In general, we regard the death of a human being as a failure, of organ systems or of function.
“But with an octopus, that’s not true,” said Dr. Cheek. “The system should be doing this.”