Humans have always envied the ability to regenerate limbs, tails, or other body parts found in some groups of animals. However, typically, the parts that can regrow are only a small fraction of the body, such as legs or tails.
It is not hard to see the value of leaving behind a body part when trying to quickly escape a predator, or simply abandoning an injured limb to grow a new one. Earthworms, Mexican axolotls, salamanders, and certain lizards have long been known for this ability. Even large animals like crocodiles have recently been discovered to possess remarkable regenerative powers.
However, scientists have now uncovered one of the most extreme examples ever found – sea slugs – that voluntarily detach their heads and then regenerate their entire bodies from there.
Recently, scientists announced the discovery of two species of sacoglossan sea slugs that possess even more extraordinary capabilities.
The head of the sea slug can still move normally.
Specifically, these two species can regenerate an entirely new body complete with a heart and other internal organs. The newly discovered sea slugs can utilize the photosynthetic capabilities of chloroplasts that they incorporate from algae in their diet, allowing them to survive long enough to undergo the regeneration process.
“We were very surprised to see their heads moving after being severed. Initially, I thought it would soon die without a heart and other vital organs, but the reality was completely different; it regenerated its entire body,” said researcher Sayaka Mitoh from Nara Women’s University in Japan.
Mitoh is a doctoral student in Yoichi Yusa’s laboratory. The Yusa lab raises sea slugs from eggs to study their life history traits. On one occasion, Mitoh observed something unexpected: a sacoglossan slug moving around without its body. Scientists even witnessed one slug performing this feat twice.
Image of the sea slug’s head detaching and regenerating a new body.
According to the researchers’ report, the head, separated from the heart and old body, will begin to move immediately after detachment. Within a few days, the wound at the back of the head heals. The heads of relatively young slugs begin to eat algae within a few hours. They start regenerating a heart within a week. The entire regeneration process is completed in about three weeks.
Heads of older individuals that do not consume algae will die after about 10 days. In both cases, the slug’s body that was removed does not regenerate a new head. However, headless bodies can still move and respond to touch for several days or even months.
Researchers Mitoh and Yusa are uncertain why sea slugs can perform this feat. However, Mitoh speculates that there must be stem cell-like cells in the severed neck capable of regenerating the body.
One possibility is that it helps eliminate internal parasites that inhibit reproduction. The researchers also do not know what immediate signal prompts sea slugs to discard the rest of their bodies. These are areas that require further research in the future.
The mentioned species of sea slugs are also considered unique in that they incorporate chloroplasts from algae consumed into their bodies, a practice known as kleptoplasty. This provides the animals with the ability to supply energy to their bodies through photosynthesis. Scientists believe this ability may help sea slugs survive after self-amputating a part of their body long enough to regenerate a new one.
Mitoh added: “Since the detached bodies often remain active for many months, we can study the mechanisms and functions of kleptoplasty using living organs, tissues, or even cells. Such studies are almost entirely lacking, as most research on kleptoplasty in sacoglossans has been conducted at the genetic or individual level.”
