From crabs to lizards, some animals can regrow their lost tentacles, tails, or claws after losing them when attacked by predators. While impressive, sea spiders can take things to the next level, capable to regrow certain body parts after amputation — and this could open the door to new research on regeneration.

Despite their presence throughout the ocean, sea spiders are largely understudied. There are about 1,500 known species of them out there, especially in the polar regions. They can be found in tide pools and on the floor of the deep sea. They are mostly legs (usually eight) and don’t have lungs, obtaining oxygen through their exoskeleton.

So far, nothing that out of the ordinary. But this is where things start to change.

Researchers from the Humboldt University in Berlin found these creatures can regenerate complete parts of their bottom halves, such as muscles, reproductive organs and the anus, or live without them. “Nobody had expected this,” Gerhard Scholtz of Humboldt University, senior author of the study, said in a statement.

Looking into sea spiders

In their study, Scholtz and the team of researchers amputated different hind limbs and posterior parts of a group of 23 immature and adult sea spiders Pycnogonum litorale. While there wasn’t regeneration of body parts in adults, some stayed alive two years later. Meanwhile, the juvenile spiders had a complete or near-complete regeneration of body parts.

This included the musculature, parts of reproductive organs, the hindgut, and the anus. Over 90% of the sea spiders survived long-term, and 16 of the young ones molted at least once, the researchers found. They also observed regrowth of the posterior in 14 of the young sea spiders, while none of the adult specimens molted or regenerated.

Regeneration capabilities change throughout the animal kingdom, with many creatures able to fully regenerate at least some limbs. Flatworms, for example, can regrow their body just from a few cells. Last year researchers could regrow a lost leg from an African clawed frog, unable to naturally regenerate its limbs, using drugs.

Vertebrates, including humans, have almost no regeneration capability, with some exceptions. Lizards can regrow their tails, for example. And humans can regrow their liver to full size after being halved, with children also capable of regrowing the tips of their fingers. However, the loss of a limb can’t be restored naturally in any mammal.

The researchers said their findings could lead to further research in the field. There are many different species that can be tested as they did with sea spiders, Scholtz said in a statement. Their next step could be to discover the mechanism behind the regrowth, he said, finding out what happens on the cellular and molecular level.

“Perhaps there are stem cells involved which are undifferentiated cells that can assume new shape and fate?” Scholtz said. “In the end, maybe the mechanisms we detect in arthropods may help medical treatments of limb loss or finger loss and so on in humans. This is always the hope.”

The study was published in the journal PNAS.