For many organisms, getting a limb caught in a predator’s jaws would usually mean death. But not starfish. They rip off the limb, leaving the predator something to chew on while they crawl away. But how do starfish manage to do this?
Starfish and several other animals (including lizards and salamanders) have the ability to autonomically chop off limbs when under attack. Until now, the biological mechanisms behind this phenomenon in starfish were poorly understood. An international team of researchers led by Maurice Elphick, Professor of Animal Physiology and Neuroscience at Queen Mary, University of London, has discovered that a neurohormone released by starfish is the main reason they chop off limbs that get caught in the jaws of a predator.
So how does this neurohormone (specifically a neuropeptide) enable the starfish to escape? When the starfish becomes stressed from an attack by a predator, this hormone is released, stimulating the muscles at the base of the animal’s arms, causing them to break off.
The researchers confirmed that this neuropeptide “acts as an autotomy promoter in sea stars and is the first neuropeptide identified as regulating autotomy in animals.” study It was recently published in Current Biology.
Hold on
Elphick’s team investigated how a neuropeptide known as ArSK/CCK1 promotes autonomy in European starfish. Asterias RubensArSK/CCK1 is already known to inhibit feeding behavior. A. Rubens By contracting the stomach, muscle contractions are involved in limb loss, and the researchers found that their ability to induce contractions goes beyond feeding.
The starfish underwent an experiment simulating a predator’s jaws biting into one of its arms. Clamps were placed on one of three sections of one of its arms – at the end, middle, or base, where autotomy is known to occur (also known as the autotomy plane). The starfish was then suspended by these clamps over a glass bowl of seawater. In the first half of the experiment, the starfish was left to react naturally, while in the second half, they were injected with ArSK/CCK1.
Without injections, autotomy was primarily observed in animals whose arms were clamped closest to the autotomy surface. Clamping the middle or end of the arm elicited less of a response from starfish.
In the second half of the experiment, the previously used clamping technique was combined with injections of ArSK/CCK1; for comparison, some animals were injected with the related neuropeptide ArSK/CCK2. Remarkably, 85% of animals injected with ArSK/CCK1 clamped in the middle of the arm or close to the autotomy plane showed autotomy, and some animals autotomized an additional arm; this only occurred in about 27% of animals injected with ArSK/CCK2.
Let go
Although ArSK/CCK1 proved to be the most effective chemical trigger of autotomy, its activity at the autotomy surface depends on specific aspects of the starfish anatomy.
Like all echinoderms, starfish have an endoskeleton made up of tiny bones, or ossicles, connected by muscles and collagen fibers. This skeleton allows the starfish to position and move. Two unique features found only in the autotomy allow this structure to break. Beneath the skin of the autotomy are areas where the collagen fiber bundles separate, making them more susceptible to breaking. The second feature is a band of muscle close to the area of the collagen fiber bundles. This muscle, known as the tourniquet muscle, helps tighten the arm to prevent it from falling off in a threatened situation.
By analyzing tissue from a starfish’s arm during autotomy, the scientists were able to gain new insight into this process. Immediately after the starfish’s arm is grabbed by a predator, ArSK/CCK1 tells the nerves of the hemostatic muscles to tighten the area immediately adjacent to the autotomy surface. While this is happening, collagen in the body wall in that area softens and breaks, as do the muscles and ligaments that hold the ossicles together. It is now believed that ArSK/CCK1 is also involved in softening this tissue in preparation for rupture.
After a starfish autotomizes a limb, it eventually regenerates, as do other animals that can use autotomy to their advantage (even lizards regenerate their tails). In the future, understanding why some animals have the ability to regenerate may explain why humans did not evolve the ability, or why some of our ancestors lost it. Elphick acknowledged that there may still be unidentified factors that work in tandem with ArSK/CCK1, but further insights may one day provide a clearer understanding of the process.
“Autotomy is an important adaptation for survival that has evolved in several animal taxa,” the team said in the same paper. study“[and] The findings of this study provide important insights into the neural mechanisms controlling this remarkable biological process.”
Current Biology, 2024. DOI: 10.1016/j.cub.2024.08.003
