Scientists discover how starfish get ‘legless’

When most animals are attacked by predators, the encounter is often fatal. But some animals have evolved an intriguing strategy to survive such attacks - they relinquish a part of their body to the predator so that they can escape and survive. This process whereby an appendage is automously detached from the rest of the body is known as autotomy. The ability to 'autotomise' appendages has evolved in several animal types, with tail autotomy in some lizard species being a well-known example. Others include autotomy of legs in crabs and autotomy of arms in starfish. However, very little is known about the mechanisms that enable some animals to rapidly shed a body appendage in this way.

Reporting in the journal Current Biology, scientists at Queen Mary University of London have discovered a neurohormone that triggers arm autotomy in starfish. Whilst testing the effects of different neurohormones in the common European starfish Asterias rubens, they made the surprising observation that injection of one neurohormone caused animals to shed one or more of their arms. Remarkably, in some animals four arms were shed, leaving just one arm attached to the central body region containing the stomach, which is essential for survival. The neurohormone that was found to act as an 'autotomy-promoting factor' in starfish is a small protein (a neuropeptide) that is related to the human hormone cholecystokinin or CCK. In humans, CCK acts as a satiety hormone, stopping us from eating more when we feel full up, as well as mediating physiological responses to stressful situations. Previously reported research by the same research team at Queen Mary University of London found that a CCK-type neurohormone inhibits feeding in starfish, consistent with the role of CCK as a satiety hormone in humans. The new research suggests that starfish CCK may be released as a generalised response to the stress associated with being attacked by a predator. So if starfish are busy feeding on their favourite food, mussels, they stop feeding and try to escape but if one of their arms is caught by the predator they let go of it and live to feed again, albeit with one less arm! However, starfish also have remarkable powers of regeneration so having lost an arm they can, over a period of several weeks and months, grow a new one to restore their original complement of five arms.

The scientists at Queen Mary University of London were also interested in finding out how the CCK-type neurohormone triggers arm autotomy in starfish. Dr Ana Tinoco, a member of the London-based research group who is now working at the University of Cadiz in Spain, explained how they addressed this question. "We discovered that the CCK-type neurohormone is present in nerve fibres in a special muscle that is located at the base of each arm and which is known as the tourniquet muscle. As its name implies, when starfish autotomise an arm the tourniquet muscle contracts and assists in enabling the arm to fall off, whilst also closing off the wound after an arm is shed. When we tested the CCK-type neurohormone for its effects on muscle in starfish we found that it caused contraction. Therefore, we think that it triggers arm autotomy in starfish, at least in part, by being released by nerve fibres in the tourniquet muscle and causing the muscle to contract. However, our experiments indicate that it may be one of a several neurohormones that work together to control the whole process of arm autotomy in starfish, which also involves breaking of ligaments that connect different parts of the skeleton as the base of the arm."

Maurice Elphick who led the research project, commented on the broader significance of the findings of this study. "We think our discovery is fascinating because the ability of starfish to shed their arms is such an amazing biological phenomenon. But it is also important physiologically, because ours is the first study to identify a neurohormone that triggers autotomy in animals. We hope that this will provide the foundations of further research so that we can learn more about how autotomy works, not only starfish but also in other animals. We would like to know what is it that enables some animals to autotomise and then regenerate arms or legs, whilst most animals can't do this. Understanding the mechanisms of autotomy in animals may be useful in the field of regenerative medicine. If we can understand how some animals are able to lose and then regenerate limbs automously, that knowledge may be helpful in improving recovery from traumatic limb loss when it occurs in humans or domestic animals."

The research was funded by the BBSRC and Leverhulme Trust.