Octopuses are known for their high intelligence and amazing ability to change colors. But for the females of these cephalopods, life ends early and tragically: after laying their eggs, they starve to death. Researchers have now discovered the reason for this strange behavior. The culprit is apparently a drastic change in the messenger substances released by the animals’ visual glands, including a form of cholesterol, as the analyzes have now revealed. This blood fat serves as a precursor to sex hormones, but it acts like a kind of “suicide” hormone in female octopus, causing them to stop eating and can even cause self-mutilation.
Octopuses have the largest brains of any invertebrate, and are considered particularly sophisticated in other ways as well. But unlike other animals with complex nervous systems, they are unusually short-lived: many of these cephalopods live just a little over a year. After the females have laid their eggs, they guard them at the risk of their lives and sacrifice themselves in the process: they stop eating and literally starve to death. In captivity, some female octopus have even been observed to engage in self-destructive behavior and self-mutilation. Even before their young animals hatch, the octopus mothers are usually dead. Scientists received the first indications of what triggers this self-destructive behavior in the cephalopods as early as the 1970s: If they removed the ocular glands from the Caribbean two-spotted octopus (Octopus hummelincki) after laying their eggs, they left them Mother octopus laid eggs for themselves, continued to eat normally, and survived the reproduction.
What is happening in the octopus gland?
Even then, these experiments suggested that the optic gland, an organ with similar functions to the human pituitary gland, played a key role in the self-destructive behavior of female octopus. It was also known that the visual gland of the octopus, like the human pituitary gland, primarily produces sex hormones. It was therefore to be assumed that these hormones are also responsible for the strange change in behavior of the animals. “However, which signaling substances underlie this important change was previously unknown,” explain Z. Yan Wang from the University of Washington in Seattle and her colleagues. In order to clarify this question, they have now for the first time examined in detail which messenger substances are produced in the visual glands of female Californian two-spotted octopuses (Octopus bimaculoides) before mating and after oviposition. To do this, they analyzed both the gene activity in the octopus’s visual glands and the substances released.
The analyzes revealed that three different metabolic pathways in the octopus ocular gland change when the eggs are laid. The first produces two steroid hormones, pregnenolone and progesterone, which are typically associated with pregnancy and egg production, respectively. The second metabolic pathway increasingly releases precursors of bile acids and the third produces increased amounts of 7-dehydrocholesterol (7-DHC), a precursor of the blood fat cholesterol. While the changes in pregnancy hormones were expected, the changes in bile acid and cholesterol precursors were not. “Neither of these two substances was previously known to be involved in semelparity,” explain Wang and her colleagues. Life strategies in which a living being reproduces only once in a lifetime are referred to as semelparity.
Cholesterol Precursors as a “Suicide” Signal?
However, the results suggest that the cholesterol precursor 7-DHC in particular could play an important role in the switch to self-destructive behavior in female octopus. Evidence of this is provided, among other things, by a genetic disease in humans. The precursor substance 7-dehydrocholesterol, which also occurs in humans, is normally quickly converted into cholesterol by an enzyme and is toxic to us. However, children with Smith-Lemli-Opitz syndrome lack this enzyme and the toxic 7-DHC accumulates in them. As a result, they develop severe developmental disorders and behavioral problems – including self-injurious behavior as in the female octopus. “In the octopus, the 7-DHC could trigger the production of signaling factors that ultimately lead to death, or the accumulation of the 7-DHC itself is lethal, as in humans,” Wang and her colleagues speculate.
The team could not only have clarified the cause of the self-destructive end of the octopus females – their results also provide exciting insights into the different modes of action and effects of cholesterol and its metabolic pathways – a molecule that is important in both invertebrates and humans. “We know that cholesterol is important in relation to nutrition and that it also plays a role in various signaling systems in the body,” explains Wang. “It’s involved in everything from the flexibility of the cell membrane to the production of stress hormones. Still, it was a big surprise that it also plays a role in life-cycle processes like in the octopus.”
Source: Z. Yan Wang (University of Washington, Seattle) et al., Current Biology, doi: 10.1016/j.cub.2022.04.043