A farewell to arms: using X-ray synchrotron imaging to investigate autotomy in brittle stars

Autotomy, the self-induced loss of a body structure, occurs in every living class of echinoderms and is related to the remarkable regeneration capabilities of the group. It is particularly prevalent in brittle stars (Class Ophiuroidea). Autotomy is facilitated by mutable collagenous tissue, which undergoes nervous system-mediated changes in tensile stiffness, tensile strength, and viscosity. The previous investigations of autotomy have been based on observations of the external surface, surgical manipulation of internal structures, or data on the morphology of structures post-autotomy. We used fast phase-contrast X-ray synchrotron imaging to visualize full autotomy events in vivo in the arms of specimens of the brittle star Ophioderma brevispina. This method requires no chemical or surgical manipulation and enabled us to identify several key stages in the autotomy process. We used this methodology to observe critical changes within the internal structure of the arm as it transitions from a functional mechanical apparatus to a dysfunctional disarticulated state. This method is the first in which the full intersegmental plane of the arm can be observed during autotomy. It can be applied to visualize autotomy and motion in vivo in other brittle star taxa, as well as in other groups such as asteroids and arthropods.