Ghost crab track Paralympics could provide insight into the development of robots and prosthetics

By: Ksenija Paziuk Ghost crab track Paralympics could provide insight into the future development of robots and prosthetics. Common along the sandy beaches of the Atlantic coast are Atlantic ghost crabs. These little guys may only be 5cm wide and kind of resemble Mr. Krabs from the Spongebob cartoons, but they’re also some of the fastest invertebrates on land! These little monsters eat everything from insects to the eggs and hatchlings of turtles and shorebirds. To defend themselves, these crabs voluntarily amputate their legs (a form of defence called autotomy); this confuses predators, which gives the crabs time to escape. With 10 legs, you wouldn’t think ghost crabs would notice a couple of missing limbs, but a group of researchers set out to test how missing specific leg pairs affects the crab’s walking and running abilities. Janne Pfeiffenberger and his team from Temple University, Philadelphia, thought certain legs might be more important for walking and running than others, so they designed a ‘Crab Paralympic Games’ to test this idea.

From the beaches of Cape May National Wildlife Refuge in New Jersey, researchers selected 85 ghost crabs to be track stars in the first ever ‘Crab Paralympics.’ Researchers amputated either the 1st, 2nd, 3rd, or 4th limb pair of these crabs, to see how Paralympian's would compare to the typical, 10-legged Olympian crab. The track stars made their debut on the big screen as they raced on a 350 cm track in their natural environment, with all trials being recorded for broadcasting. By watching the clips back researchers discovered that the crabs didn’t actually use their 4th leg pair when running, and that they lifted them up when moving quickly. With the exception of crabs missing this leg pair, Pfeiffenberger and his team discovered the Paraolympians were generally at a disadvantage compared to 10-legged crabs. Removing any other leg pair resulted in a 25% decrease in running speed!

The researchers then examined how the step pattern of crabs changes, depending on which legs were removed. Crabs with missing legs became less stable, which caused them to take smaller and more frequent steps - similar to us trying to stay balanced while walking on ice. Pfeiffenberger and his team believe that understanding this shift in balance could be beneficial to scientists and engineers, helping them to develop stable robots. So if you ever see crab-looking robots that can’t be knocked over taking over the world, you can blame Pfeiffenberger and his team.

The lessons learned from the crabs with missing legs can also be applied to other animals with 8-walking legs (like spiders and scorpions), allowing us to understand how they might be affected by limb loss. Using different walking patterns also forces ghost crabs to rely on alternative muscles. Understanding muscle gain/loss in crabs after losing legs could be applied to many organisms, including dogs and humans! Beyond muscle development, this study can also help veterinarians and pet owners realize how pets might respond to amputation and allow us to make their post-amputation life much more comfortable. So not only are Atlantic ghost crabs fast-running, baby-eating, 360° eyeball-swivelling monsters who choose to amputate their legs rather than get eaten by predators, they’re also helping scientists develop robots and understand amputation in a variety of animals! Pfeiffenberger, J., & Tonia Hsieh, S. (2021). Autonomy-induced effects on the locomotor performance of the ghost crab Ocypode quadrata. Journal of Experimental Biology. 224, jeb233536