In the name of survival, starfish sever their own body parts to escape predators. They will also eventually regenerate those lost limbs, but how this biological process works has remained a mystery. Now, a team of scientists have detected a type of neurohormone that is responsible for triggering limb-cutting in the common sea star (Asterias rubens). The findings are detailed in a study published August 29 in the journal Current Biology.
What is autotomy?
Autotomy is the ability of an animal to detach a body part–and still survive–as a way to get away from an animal trying to eat it. Lizards can shed their tails to escape and newts can regrow limbs, eyes, jaws, spinal cords and more. Starfish–or sea stars–will cut off one or more of their arms and grow back the limbs lost to autotomy over time.
“The tissues in the region where autotomy occurs are pre-adapted to facilitate rapid regeneration,” Maurice R. Elphick, a study co-author and biologist at Queen Mary University of London (QMUL), tells Popular Science. “It’s as if starfish are built to expect that regeneration following autotomy is going to happen at some stage in their life, i.e. they are going to be attacked by a predator or their arms are going to get trapped under a rock and they’re going to need to escape.”
[Related: The sea star’s whole body is a head.]
According to Elphick, when scientists collect starfish along the beach, they often find many organisms that have clearly lost an arm or two at some point in their lives.
“You can see when autotomy has occurred because the regenerated arm is smaller than the other arms and it probably never quite attains the same size as the original arms,” says Elphick.
While autotomy was first scientifically described over three decades ago, the molecules and hormones behind it have remained elusive. Understanding what is really behind this process at a molecular level could have implications for regenerative medicine in the future.
Stars, they’re just like us (sort of)
Initially, the team on this new study was looking into a neurohormone in mammals called cholecystokinin (CCK). They were curious at how a similar neurohormone in starfish works.
“One of its biological actions [in humans] is to stimulate contraction of the gallbladder and that contraction of the gallbladder forces bile into the intestine and this reflects its role in facilitating digestion of food,” says Elphick.
CCK also inhibits feeding behavior. When the stomach becomes full and food is passed into the intestine, CCK is one of the hormones that triggers a signal to the brain that it’s time to stop eating.
Given CCK’s role in mammal feeding behavior, Elphick and study co-author Ana Tinoco of the QMUL and University of Cadiz in Spain were looking at how a cholecystokinin-type neurohormone called ArSK/CCK1 works when starfish eat. While examining the neurohormone in the lab, they observed that it actually triggered autonomy in some starfish, but not all.
“In some animals just one arm was lost, but in a couple of animals four of the five arms were autotomized,” says Elphick.
They found that at a molecular level, cholecystokinin-type neurohormones work similarly in both starfish and humans. They bind to a specific receptor protein on the cell surface of target cells. The binding then triggers a signal that ultimately changes some of the properties of the target cells.
[Related: Antlion larvae can ‘play dead’ for more than an hour–then, things get random.]
One of these changes is muscle contraction. In humans, it causes the gallbladder to contract. For starfish, the contractions triggered by these neurohormones appear to be related to autotomy.
“We have evidence that the process of autotomy is facilitated by the release of this peptide, which we think then causes a muscle at the base of the starfish arm to contract,” says Elphick. “This then facilitates detachment of the arm and closure of the wound after autotomy has occurred.”
The team is still determining the finer details about how this process works, but have zeroed in this area for further study.
Future treatments
While the way that cholecystokinin-type neurohormones work in starfish and humans are very similar, both organisms have evolved to use them differently. However, the similarities mean that further study could advance our understanding of human tissue regeneration and better treatments for limb injuries.
“The process of autotomy is complicated, as are many processes in biology, and is likely to be regulated by multiple factors and we’d love to identify other molecules that are involved in regulating the process of autonomy in starfish,” says Elphick.
[Related: The blueprints for early organs may be hiding in sea stars.]
It also offers a lesson in how scientists can start with one question or project that evolves into something entirely different.
“I think an important take home message from this study is the importance of just being able to do experiments where you don’t always know what the outcome is going to be and you don’t actually have a hypothesis,” says Elphick. “You just make observations and then occasionally you’ll discover something that’s really intriguing and interesting”