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The octopus, this cephalopod with extraordinary abilities, is one of the most intelligent creatures in the animal kingdom. However, a very strange behavior of self-destruction only allows him (with a few exceptions) a short longevity, and his suicide seems very precisely programmed after his mating. According to the discovery resulting from a new study, the alteration of the metabolism of cholesterol, induced by the optic gland of the cephalopod, would be at the origin of this tragic phenomenon. This gland would in particular undergo a massive change in the female octopus after a successful mating, by producing steroid hormones inducing its suicide. According to the researchers, a similar hormonal alteration in mammals (including humans) could induce the same suicidal behavior as in octopus.
Soft-bodied cephalopods such as the octopus have the largest and most complex central nervous systems, as well as the largest brain-to-body mass ratios, of all invertebrates. This notably gives the octopus impressive faculties, which have always fascinated scientists. For example, it can change color to blend perfectly into its environment or imitate the shape of a poisonous animal to fool its predators. He is even able to wield tools and solve complex cognitive puzzles.
He is also able to regenerate his severed limbs, these powerful tentacles that he cleverly wields to make him an excellent predator, in addition to his hunting arsenal (venom, suction cups, beak, etc.). One would then be tempted to think that there is not much that could overcome this exceptional animal, but strangely, its average longevity is very short. While some primates and corvids live for decades, shallow-water octopuses, such as the California two-spotted octopus (Octopus bimaculoides), live only one year on average.
Like many other cephalopods, the octopus reproduces only once in its life, after which its life cycle ends. After laying her eggs, the female octopus notably begins a process of self-destruction, where she begins to fast and self-mutilate, preserving just enough strength to incubate her eggs until hatching, before succumbing to her poor state of health.
According to the new study published in the journal Current Biology, females bred in captivity even seem to intentionally speed up this process, mutilating themselves and frantically twisting their tentacles. ” What’s striking is that they go through this progression of change where they seem to go crazy just before they die. “Said in a press release Clifton Ragsdale, professor of neurobiology at the University of Chicago and one of the authors of the study. Moreover, this “programmed suicide” would also be observed in males, which when they reach the age of about a year or a year and a half, also stop feeding and end up dying, shortly after their females.
Co-led by the University of Washington and Illinois, the new research builds on earlier research that demonstrated the involvement of the octopus’ optic gland in its reproduction and lifespan. This gland is notably its main neuroendocrine center, whose functional equivalent in vertebrates is the hypophysis (or pituitary gland). In the female octopus, it was for the first time discovered that after mating, the gland changes in such a way as to alter cholesterol metabolism, leading to dramatic changes in the steroid hormones produced. Which could explain his suicidal behavior.
Three biochemical pathways involved
In 1977, Brandeis University psychologist Jerome Wodinsky demonstrated that once their optic glands were removed, mother Caribbean two-spotted octopuses (Octopus briareus) abandoned their eggs and began feeding again. They then lived several months longer than those still provided with their glands. Already at the time, scientists understood that the gland probably secreted “self-destructive” hormones, but the metabolic mechanisms inducing the phenomenon were not yet fully understood.
To arrive at their findings, the researchers in the new study then sequenced the activated and deactivated genes in the optic gland cells of mother octopuses (Octopus bimaculoides), at different stages of their declines. They then observed that when the cephalopods began to fast, there were higher levels of activity in the genes that metabolize cholesterol and produce steroids.
Cholesterol is indeed involved in many vital physiological processes in animals, such as the flexibility of the cell membrane and the production of stress hormones, ” but it was a big surprise to see it also play a role in this life cycle process says (referring to octopus self-destruction) Z. Yan Wang, assistant professor of psychology and biology at the University of Washington, and lead author of the study.
The research group further discovered three different biochemical pathways related to post-breeding increases in steroid hormones. The first produces pregnenolone and progesterone, two steroids commonly associated with pregnancy. The other produces maternal cholestanoids or intermediate compounds to make bile acids, and the third results in increased levels of 7-dehydrocholesterol (or 7-DHC), a precursor to cholesterol.
Some of these pathways are also activated to produce cholesterol in mice and other mammals. In humans, a high level of 7-DHC is toxic. This is notably a phenomenon noticed in Smith-Lemli-Opitz syndrome (a genetic disorder caused by a mutation in the enzyme that converts 7-DHC to cholesterol). Children with this condition suffer from severe developmental and behavioral problems, repeatedly self-harming. This behavior is very similar to that seen in octopus, according to the research group.