In a new study, some juvenile sea spiders were able to regrow amputated body parts, which was previously assumed to be impossible in these marine arthropods.

Sea spiders possess a remarkable, previously unknown ability: They can regrow their rear ends.

In a series of experiments, scientists discovered that juveniles from the sea spider species Pycnogonum litorale were able to fully regenerate a number of amputated body parts from their lower body, including hind limbs, parts of their guts, reproductive organs and even their anuses.

Sea spiders, which belong to the class Pycnogonida, are a group of around 1,300 marine arthropods with eight legs. While they look similar to terrestrial spiders they are only very distantly related to them. Other arthropods, such as spiders, centipedes and crabs, can also regenerate body parts, enabling them to escape predators that have taken a bite out of them. However, it had long been assumed that sea spiders didn’t possess this ability because scientists had never observed the animals doing it, and because sea spiders have evolved hard exoskeletons to protect them from predators, which suggested they might not need any other form of defense.

In a new study, published Jan. 23 in the journal Evolution(opens in new tab), researchers tested this assumption by amputating body parts from 23 juvenile and 23 adult P. litorale sea spiders. The adults were unable to regenerate any of the lost body parts, but surprisingly a majority of the juveniles eventually regrew the missing parts.

“We were the first to show that this is possible,” Gerhard Scholtz(opens in new tab), a zoologist at the Humboldt University of Berlin in Germany, told French news agency AFP(opens in new tab). “Nobody had expected this.”

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