The octopus is one of the subjects favored by scientists. They have complex and large nervous systems, are very intelligent and good at escaping. They can instantly change body color, skin texture, and body shape, and can even regrow wrists after they’ve been lost.
However, the behavior of female octopuses after breeding, both in the wild and in captivity, has been elusive. Octopuses are single-breeding animals, that is, they reproduce only once in their lives. The female octopus stops eating four days after laying her eggs, and dies before the eggs hatch. Female octopuses in captivity also seem to hasten their own demise—they slam into the walls of their tanks, bite off their own skin, and even devour their own tentacles. Male octopuses aren’t much better — females usually kill and eat their mates, and if they escape, they die after a few months.
In 1977, a biologist noticed that if he removed the optic gland (which is similar to the pituitary gland in most land animals and gets its name because it sits between the eyes) of a female Caribbean two-spot octopus, The female octopus will give up taking care of the eggs, start eating again, and even mate again, “living” for a few more months. The biologist and other researchers concluded at the time that the optic glands of the octopus must secrete some kind of “suicide” hormone. However, they don’t know what hormone it is or how the optic gland works.
normal female octopus
octopuses gathering to lay eggs
In 2015, a team in the United States determined the genome of the California two-spot octopus (also known as the California two-spot octopus). This is the first time that the genome of the cephalopod to which the octopus belongs has been fully sequenced. In the new study, the team focused on the odd behavior of female octopuses after spawning.
Non-mating adult female octopuses are active predators, spending a lot of time outside their dens hunting prey such as fiddler crabs. Scientists divide the octopus spawning period into four stages.
During the first stage, early spawning, the female octopus stays in the nest like a hen guarding the eggs, touching them and spitting streams of water on them (to provide oxygen). In the second stage, that is, within 4 days after laying eggs, although the female octopuses will eat, they seldom leave the eggs, and just sit back and wait to prey on crabs that are too close to them. After 4 days, the third stage, the female octopus stops eating completely. This stage will last for eight to ten days. In the fourth stage, the female octopus rapidly depletes and becomes very ugly. During this time, the female octopus is listless and often stays away from her eggs and bumps against the walls or corners of the breeding tank. They began to bite their tentacles hard, or coiled the tentacles around the mantle. At this time, the octopus looked unrecognizable and ugly. Their skin turns pale and their muscles lose their color. What is even more incredible to witnesses is that the female octopus looks like it is dismembering and destroying itself.
egg guard octopus
Female octopus entangles itself before dying
In the new study, scientists used genetic sequencing tools to detect a variety of molecular signals produced by the optic glands of breeding female octopuses. They found that the behavior of female octopuses during all four stages of reproductive life was associated with these signals, proving that the optic glands do indeed determine the eventual death of female octopuses. This is the first time that the molecular mechanism behind the strange behavior of breeding female octopuses eventually “killing themselves” has been revealed.
In the study, the scientists collected the optic glands of octopuses at various stages of breeding and sequenced the RNA transcriptome. RNA carries DNA’s instructions for making proteins, so sequencing RNA transcriptomes is a good way to understand what’s going on in cells and what genes are doing over time.
During the non-mating phase, when female octopuses are more active in hunting and feeding, they produce large amounts of neuropeptides (small protein molecules used by neurons to communicate with each other and are involved in feeding behavior in many animals). After mating, the number of neuropeptides dropped dramatically.
As the octopuses began to fast and starve, the activity of genes responsible for the production of three substances — catecholamines (a type of neurotransmitter), steroids that metabolize cholesterol, and insulin-like growth factors — increased. This suggested increased activity related to metabolism in the octopus, but it also surprised scientists because it was the first time the optic gland had been found to be involved in activities other than reproduction.
How do changes in these molecules and signals lead to different behavioral changes? It is still unclear. Early in the breeding season, female octopuses continue to feed but are not actively foraging. This may suggest that neuropeptide molecules, while keeping the octopuses fed, also reduce the activity of the muscles they use to move. Under the action of other hormones, the octopus becomes more egg-protective. This way, they can use as much energy as possible to reproduce their offspring.
Why an animal as intelligent as an octopus ended up like this is still debated in the scientific community. However, there is a serious phenomenon of “fish eating fish” in octopus groups, so there is a speculation that octopuses are biologically destined to “exterminate themselves”, possibly to prevent female octopuses from eating their offspring. Additionally, octopuses have a tendency to grow unrestricted. So removing hungry adults can stop the octopus ecosystem from being dominated by a few cranky giant octopus individuals.
However, some scientists point out that while even dolphins in aquariums can appear to “suicide” in depression (before they appear to be happy), humans cannot impose their views on animals. After all, humans can reproduce multiple times, and the vast majority of humans live beyond reproductive age, which is very different from octopuses in this respect. If the purpose of survival is to pass on one’s own genes, then the octopus’ “suicide” is not so weird or terrifying.
Octopuses shouldn’t behave like human values
How does the optic gland cause the octopus to “suicide”?
In the latest study, scientists found that changes in various biochemical pathways after mating in octopuses may be related to the “self-extermination” of octopuses, and one of the changes led to a precursor of cholesterol – 7-dehydrocholesterol (7-DHC ) content increases.
Scientists now know that cholesterol is not only important in diet, but also plays an important role in several signaling systems in the body. Cholesterol is involved in everything from the elasticity of cell membranes to the production of stress hormones, but for the first time it has been found to be involved in the life cycle, which has surprised scientists.
The scientists examined the optic glands and optic lobes of breeding and non-breeding California blue spot octopuses and identified three chemical pathways that lead to an increase in steroid hormones after breeding. One of them produces the pregnancy hormone. The other two produce 7-DHC and bile acid intermediates, respectively, which scientists had not previously known to participate in the reproductive strategy of one reproduction (or one-on-one birth).
Cholesterol may have played a role in octopus ‘suicide’ behavior
Animals with larger brains tend to live longer, so the “premature suicide” phenomenon of larger-brained octopuses is puzzling. It stands to reason that breeding offspring is a very happy thing, but why did the octopus “die” after this blessing? It now appears that at least three independent steroid pathways may explain a series of behaviors in octopuses before “suicide”.
Some of these cholesterol-producing pathways also exist in mammals and rodents. In humans, abnormally high 7-DHC levels can be toxic and a sign of a genetic disease. A mutation in the enzyme that converts 7-DHC to cholesterol can lead patients to be less protective of themselves. High levels of 7-DHC are toxic and even fatal to octopuses as they are to humans. This is intriguing, since the two animals are very different from an evolutionary perspective.