No backside, no problem for some young sea spiders.

The creatures can regenerate nearly complete parts of their bottom halves — including muscles, reproductive organs and the anus — or make do without them, researchers report January 23 in Proceedings of the National Academy of Sciences.

The ability to regrow body parts isn’t super common, but some species manage to pull it off. Some sea slug heads can craft an entirely new body (SN: 3/8/21). Sea spiders and some other arthropods — a group of invertebrates with an exoskeleton — can regrow parts of their legs. But researchers thought new legs were the extent of any arthropod’s powers, perhaps because tough exteriors somehow stop them from regenerating other body parts.

This image shows a juvenile sea spider after the last quarter of its body, including two legs and the anal tubercle, were amputated. The anal tubercle contains the animal’s hindgut and its anus. G. Brenneis et al/PNAS 2023

After the first molt, the anal tubercle and two legs are beginning to reform, shown as short stubs attached to a new body segment (center, bottom) at the animal’s back end. G. Brenneis et al/PNAS 2023

The sea spider’s anal tubercle and legs are taking shape after the animal has shed its exoskeleton a second time. G. Brenneis et al/PNAS 2023

After the third molt, the animal looks normal again. G. Brenneis et al/PNAS 2023

A mishap first clued evolutionary biologist Georg Brenneis in that sea spiders (Pycnogonum litorale) might be able handle more complex repairs too. He accidentally injured one young specimen that he was working on in the lab with forceps. “It wasn’t dead, it was moving, so I just kept it,” says Brenneis, of the University of Vienna. Several months later, the sea spider had an extra leg instead of a scar, he and evolutionary biologist Gerhard Scholtz of Humbolt University of Berlin reported in 2016 in The Science of Nature.

In the new study, most of the 19 young spiders recovered and regrew missing muscles and other parts of their lower halves after amputation, though the regeneration wasn’t always perfect. Some juveniles sported six or seven legs instead of eight.

None of four adults regenerated. That may be because adults no longer shed their skin as they grow, suggesting that regeneration and molting are somehow linked, Brenneis says. Two young sea spiders also didn’t regenerate at all. The animals survived with only four legs and without an anus. Instead of pooping, the pair regurgitated waste out of their mouths.

This young sea spider has three small stubs (center, bottom) that, if left untouched, would develop into the animal’s last two body segments, which would include two leg pairs and its anal tubercle. G. Brenneis et al/PNAS 2023

This image shows the same sea spider, but after its developing hind body — the three stubs from the previous photo — has been removed. G. Brenneis et al/PNAS 2023

After the first molt, there are no signs of regrowth. The animal has four legs instead of eight. G. Brenneis et al/PNAS 2023

After the third molt, the animal still has only four legs and no anal tubercle. Because it doesn’t have an anus, the animal regurgitated its waste through its mouth. G. Brenneis et al/PNAS 2023

Next up is figuring out whether other arthropods also regenerate more than scientists thought, and how sea spiders do it, Brenneis says. “I would like to see how it works.”