A drastic change in steroid hormone levels in female octopuses after they lay eggs causes them to mutilate themselves to death, a new study has revealed.
Cephalopods, a group that includes octopuses and squids, have the largest central nervous systems and the greatest brain-to-body mass ratios among spineless creatures. However, cephalopods are unusually short-lived, unlike other big-brained animals.
After laying a clutch of eggs, mother octopuses quit eating and waste away, explain researchers, including those from the University of Chicago. By the time the eggs hatch, the mother octopuses are dead. This bizarre maternal behaviour is linked to the optic gland in octopuses, an organ similar to the pituitary gland in mammals.
Scientists found that the optic gland in maternal octopuses undergoes a massive shift in cholesterol metabolism, which results in dramatic changes in the steroid hormones produced, according to a new study published last week in the journal Current Biology.
Previous studies in other animals, including humans, have shown that changes in cholesterol metabolism can have serious consequences on longevity and behaviour.
Researchers believe the new finding reveals important similarities in the functions of these steroid molecules across the animal kingdom, in soft-bodied cephalopods and vertebrates alike.
“We know cholesterol is important from a dietary perspective, and within different signaling systems in the body too,” said Z Yan Wang, assistant professor of psychology and biology at the University of Washington in the US, and lead author of the study.
“It’s involved in everything from the flexibility of cell membranes to production of stress hormones, but it was a big surprise to see it play a part in this life cycle process as well,” Dr Wang said.
With earlier studies pointing to the role of the optic gland in female octopuses in their self-destructive behaviour, researchers analysed the chemicals produced by this organ in maternal octopuses.
Their new study found three different pathways involved in increasing steroid hormones after reproduction.
One of these pathways produces pregnenolone and progesterone – two steroids commonly associated with pregnancy – while another produces maternal cholestanoids or intermediate components for bile acids. The third pathway produces increased levels of 7-dehydrocholesterol (7-DHC), a precursor to cholesterol.
Scientists found that the maternal optic gland undergoes dramatic changes to produce more pregnenolone and progesterone, maternal cholestanoids, and 7-DHC during the stages of decline.
“There are two major pathways for creating cholesterol that are known from studies in rodents, and now there’s evidence from our study that those pathways are probably present in octopuses as well. It was really exciting to see the similarity across such different animals,” Dr Wang said.
In humans, elevated levels of 7-DHC is a “hallmark” of a genetic disorder called Smith-Lemli-Opitz syndrome (SLOS), which is caused by a mutation in the enzyme that converts 7-DHC to cholesterol, scientists say.
Children with the disorder suffer from severe developmental and behavioral consequences, including repetitive self-injury reminiscent of octopus end-of-life behaviors.
Based on the findings, researchers suspect disruption of this cholesterol production process in octopuses has grave consequences, just as it does in other animals.
“What’s striking is that they go through this progression of changes where they seem to go crazy right before they die. Maybe that’s two processes, maybe it’s three or four,” Clifton Ragsdale, another co-author of the study, noted.
“Now, we have at least three apparently independent pathways to steroid hormones that could account for the multiplicity of effects that these animals show,” Dr Ragsdale added.
In future research, scientists hope to understand how the lesser Pacific striped octopus (Ocotopus chierchiae) doesn’t self-destruct after breeding like the animals in the latest study.
Scientists hope to examine the striped octopus’s optic glands and compare them to the latest results to find how it avoids the tragic octopus death spiral.