Many marine invertebrates, such as jellyfish, are able to regenerate lost body parts, an important skill as it has been observed that between a third and a half of all of these creatures show signs of injury. Whether it be by being eaten by a turtle or hit by a boat, jellyfish get injured; since avoidance of injury isn't possible, repair is necessary. This paper studied how moon jellies (Aurelia aurita), which are unable to regenerate body parts in their free swimming adult form, are able to survive traumatic injury and thrive.
The researchers studied young, free swimming moon jellies called ephyrae, that have eight "arms" that drive swimming through contraction, but lack the tissue that forms the "bell" that connects the arms. These ephyrae had varying numbers of arms cut off, and their ability to survive was studied over time. It was found that the jellies would resume a symmetrical shape over time; for instance, a suddenly three armed jelly would make a triangular shape, while a four armed would generate a square. This occurred because the arms were physically rearranged to reform symmetry. This did not occur by having cells divide between the arms, pushing them apart, or by having cells die along the cut zone, pulling arms together, but rather through mechanical forces generated by swimming. In the asymmetrical jellies, swimming contractions would cause a bulge along the cut zone due to the lack of symmetry. The researchers noted this phenomenon is similar to squeezing an inflated ball at one end: you create a bulge at the other end, which relaxes when the squeeze is released. Since the relaxation that occurs after a contraction in jellyfish swimming is due to elastic retraction, like a rubber band snapping back after being pulled, there is a pulling force on the contracted arms. This force helps pull the arms around, slowly moving them into a symmetrical shape with each swimming stroke (see https://www.youtube.com/watch?v=VpWf74BkAbE&feature=youtu.be. for video model). This reacquisition of symmetry could be accelerated by stimulating muscle contractions, and could be inhibited by exposing cut jellies to muscle relaxants. This study shows how moon jellies are able to survive as a species in an environment where they inevitably are injured, and how they have naturally developed a system of survival that is as energy efficient as possible.
Abrams et al., 2015. Self-repairing symmetry in jellyfish through mechanically driven reorginization. Proc Natl Acad Sci USA, 112: E3365-E3373.