Some young sea spiders can regrow their rear ends

The creatures seem to lose the ability when they stop shedding skin as adults

A microscope image of an adult sea spider made a full recovery after its back half was amputated.

The adult sea spider (Pycnogonum litorale) in this microscope image made a full recovery after its back half was amputated, showing that arthropods can regrow more body parts than scientists realized.

G. Brenneis

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.

  1. A microscope image of a juvenile sea spider with the last quarter of its body, including two legs and the anal tubercle, were amputated.
  2. A microscope image of a juvenile sea spider after the first molt shown as short stubs attached to a new body segment at the animal’s back end.
  3. A microscope image of a juvenile sea spider as its new anal tubercle and legs start taking shape.
  4. A microscope image of a juvenile sea spider with its anal tubercle and legs fully reformed.

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.

  1. A microscope image of a young sea spider with three small stubs at the bottom of its body.
  2. A microscope image of the same young sea spider without the three stubs.
  3. A microscope image of the young sea spider with four legs.
  4. A microscope image of a young sea spider with four legs spread out.

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.”

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