Self-decapitating sea slugs regrow their bodies – hearts and all

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6949039/

From what little I understand from it, it doesn't seem they have the mechanism explained yet, though they hint that it's possible the immune system may have been co-opted for this. Relevant paragraph from "Discussion" section below:

Kleptoprotein in bioluminescent fish was unexpected because ingested proteins are usually decomposed in digestive systems, including pyloric caeca in fishes, into amino acids or oligopeptides and absorbed through the gut wall as nutrients. This means that the original structure and enzymatic activity of ingested proteins are typically not maintained during their passage through digestive tracts. However, the phenomenon of protein uptake without full digestion has been reported in some vertebrate immune systems. For example, M cells in mammalian intestinal epithelia play an important role for the immune system by taking macromolecules or even microbes into the cell as antigens via pinocytosis (24). Similar antigen-sampling functions were reported in the intestinal epithelium of cyprinid fishes, and the presence of M cell–like cells was reported in the intestine of a salmonid fish (25–27). While this mechanism for protein uptake is not selective and the protein is not used for its original function, these immune systems may have been co-opted to serve a kleptoprotein role in Parapriacanthus bioluminescence during evolution. We expect that the reason why ostracod luciferase was exploited for kleptoprotein bioluminescence of Parapriacanthus is due partly to its high proteolytic resistance and highly stable nature (fig. S9). Kleptoprotein bioluminescence using the ostracod luciferase might have also evolved in parallel in some other teleost lineages, e.g., the apogonid fishes Jaydia ellioti and Rhabdamia cypselura, which use vargulin for their luminescence and whose light organs are connected to the digestive tract (16, 17), as in Parapriacanthus.