Many species of animals die after breeding. But in octopus mothers, this decrease is particularly worrying: In most species, as an octopus’s eggs are about to hatch, it stops eating. She then leaves her protective conversation over her offspring and turns to self-destruction. She can hit herself on a rock, tear her own skin, and even eat pieces of her hands.
Now, researchers have discovered the chemicals that seem to control this deadly frenzy. After an octopus lays eggs, there are changes in the production and use of cholesterol in its body, which in turn increases the production of steroid hormones – a biochemical change that will condemn it. Some of the changes may suggest processes that explain invertebrate longevity in general, said Z. Yan Wang, an assistant professor of psychology and biology at the University of Washington.
“Now that we have these pathways, we are really interested in associating them with individual behaviors or even individual differences in the way animals express these behaviors,” Wang told Live Science.
Scheduled to die
Even as an undergraduate student studying English, Wang sparked interest in female reproduction, she said. When he pursued a master’s degree in science, he kept this interest and was impressed by the dramatic deaths of octopus mothers after they laid their eggs. No one knows the purpose of the behavior. Theories include the idea that dramatic death signs remove predators from the eggs or that the mother’s body releases nutrients into the water that feeds the eggs. Most likely, Wang said, the die-off protects babies from the older generation. The octopuses are cannibals, he said, and if the bigger octopuses got stuck, they could end up eating all of each other’s little ones.
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A 1977 study by Jerome Wodinsky, a psychologist at Brandeis University, found that the mechanism behind this self-destruction was in the optic glands, a set of glands near the octopus’s eyes that are roughly equivalent to the human pituitary gland. If the nerves in the optic gland were cut, Wodinsky found, the mother octopus would abandon her eggs, start eating again and live for another four to six months. This is an impressive life extension for creatures that only live about a year.
But no one knew what the optic gland did to control this self-inflicted cataract.
“From the beginning, I was very willing to do the experiments we described in the document we just published, which is essentially the juicing of the optic gland and then the identification of the ingredients of this juice,” Wang said.
Wang and her colleagues analyzed the chemicals produced in the optic glands of two-point octopuses in California (Octopus bimaculoides) after they laid eggs. In 2018, a genetic analysis of the same kind showed that after spawning, the genes in the optic glands that produce steroid hormones (which are made, in part, with cholesterol components) began to exceed. With this study as a guide, the scientists focused on steroids and related chemicals produced by the optic glands in two-point octopuses.
They found three distinct chemical shifts that occurred when the octopus’s mother laid her eggs. The first was the increase in progenolone and progesterone, two hormones associated with reproduction in many creatures (in humans, progesterone increases during ovulation and during early pregnancy). The second shifts were more amazing. Octopus mothers began to produce higher levels of a cholesterol building block called 7-dehydrocholesterol or 7-DHC. Humans produce 7-DHC during the production process cholesterol also, but do not keep any in their systems for long; the compound is toxic. In fact, babies born with the Smith-Lemli-Opitz Syndrome genetic disorder cannot clear 7-DHC. The result is mental disability, behavioral problems including self-injury and physical abnormalities such as extra fingers and toes and cleft palate.
Finally, the optic glands also began to produce more components for bile acids, which are acids produced by the liver in humans and other animals. Octopuses do not have the same type of bile acids as mammals, but they are obviously the building blocks of these bile acids.
“It suggests that this is a brand new class of signaling molecules in the octopus,” Wang said.
The components of bile acid are interesting, Wang said, because a similar set of acids has been shown to control the worm’s lifespan. Caenorhabditis elegans, which is commonly used in scientific research due to its simplicity. The components of bile acid may be important in controlling longevity in invertebrate species, Wang said.
Octopuses are difficult to study in captivity because they require a lot of space and perfect conditions to grow to sexual maturity and reproduce. Wang and other octopus researchers have now found a way to preserve the smallest striped octopus in the Pacific (Octopus chierchiae) live and reproduced in the laboratory. Unlike most other octopus species, Pacific striped octopuses can mate several times and lay many eggs. They do not self-destruct as their eggs prepare to hatch, making them perfect specimens for studying the origins of morbid behavior.
“I’m really, really excited to study the dynamics of the optic gland in this genre,” Wang said.
The researchers published their findings in the May 12 issue of the journal Current Biology.
Originally published in Live Science.